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Erschienen in:
Sensing by Acoustic Biosignals Kapitel lesen Erstes Kapitel lesen

2015 | OriginalPaper | Buchkapitel

5. Sensing by Optic Biosignals

verfasst von : Eugenijus Kaniusas

Erschienen in: Biomedical Signals and Sensors II

Verlag: Springer Berlin Heidelberg

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Abstract

https://static-content.springer.com/image/chp%3A10.1007%2F978-3-662-45106-9_5/MediaObjects/218259_1_En_5_Figa_HTML.gif
After the interface between physiologic mechanisms and the resultant biosignals has been examined (Volume I), the subsequent interface between optic biosignals and the associated sensing technology is discussed here. In the genesis of optic biosignals—induced biosignals—an artificial light is coupled into biological tissue. The resulting transmitted light intensity is strongly governed by the light absorption and scattering in tissue. The light absorption, for instance, is modulated by blood oxygenation and local pulsatile blood volume. Consequently, the transmitted light intensity reflects multiple physiologic parameters—which are vital for the assessment of cardiorespiratory pathologies and the state of health—and comprises the optic biosignal. The genesis of optic biosignals is considered from a strategic point of view. In particular, the introduced common frame of hybrid biosignals comprises both the biosignal formation path from the biosignal source at the physiological level to biosignal propagation in the body, and the biosignal sensing path from the biosignal transmission in the sensor applied on the body up to its conversion to an electric signal. Namely, the optical sensor is comprised of a light source on the skin to generate the incident light and couple it into tissue, and a distant light sink to detect the resulting transmitted light. The transilluminated region can be approximated as an arrangement of tissue layers and blood vessels. If an arterial vessel is considered with a blood pressure pulse propagating along the vessel, then there is a local pulsatile change in the arterial radius. Provided that blood in vessels absorbs the light to a larger extent compared to the tissues surrounding these vessels, it is clear that the transmitted light intensity temporarily decreases for increasing arterial radius in the transilluminated region. Thus, the propagating light is modulated by diverse physiological phenomena. A certain portion of light leaves the body and is detected by the light sink applied on the skin. The sink converts the transmitted light intensity into the electric signal. It is highly instructive from an engineering and clinical point of view how light interacts with biological tissues. Discussed phenomena teach a lot about the physics of light (as engineering sciences), and, on the other hand, biology and physiology (as live sciences). Basic and application-related issues are covered in depth. In fact, these issues should remain strong because these stand the test of time and mine knowledge of great value. Obviously, the highly interdisciplinary nature of optic biosignals and biomedical sensors is a challenge. However, it is a rewarding challenge after it has been coped with in a strategic way, as offered here. The chapter is intended to have the presence to answer intriguing “Aha!” questions.

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Vorheriges Kapitel Sensing by Acoustic Biosignals
Fußnoten
1
As a curiosity concerning the light source and light sink, the discovery of the elementary concept of human vision is worth to be narrated shortly (Splinter 2007). It was a great challenge for many scientists over the centuries to find out if the human eye emits a “fire” seizing ambient objects with its rays and provides man with the sense of vision, or the eye captures light emitted by ambient sources. Aristotle (384 BC–322 BC), an ancient Greek philosopher and scientist, proposed the first hypothesis that eyes capture light. However, even many centuries later—e.g., as supported by Galen of Pergamum (around 129–200), a Greek philosopher and physician—the optic nerve was seen as a duct along which rays from the brain are emitted. It was not until the 17th century that Johannes Kepler (1571–1630), a German astronomer, provided a theory of retinal image formation.
 
2
Light is electromagnetic radiation which behaves as a wave when it propagates through space (Sect. 6). In particular, electromagnetic radiation is associated with electric and magnetic fields which oscillate in-phase perpendicular to each other and perpendicular to the propagation direction of the wave; known as transverse electromagnetic wave (Sect. 6). If the spatial curvature of waveforms is so small that they appear to be planar, the idealization of the wave propagation as plane waves is commonly used; compare Sect. 6. An important aspect of light is its emission and absorption by accelerated charged particles (such as electrons or polarized molecules); compare Sect. 5.1.2.
An (uncharged) quantum of light is known as photon. The photon can be seen as a minute (discrete) energy packet of light, whereas the packet’s energy (i.e., the photon energy) depends on the frequency of the wave (5.3).
 
3
A (perfect) black-body is an object which absorbs all radiation incident upon it at all wavelengths and from all angles of incidence while none of the radiation is reflected. That is, the surface of the black-body appears colourless and black; compare Footnote 21. All radiation absorbed by the black-body is re-emitted. Interestingly, this thermal radiation by the black-body does not depend on the type of radiation which is incident on it but is characteristic of this black-body only.
In particular, the black-body (in its thermodynamic equilibrium with its environment) emits a broad continuous spectrum of electromagnetic radiation according to Planck’s law (Footnote 9). The brightness varies with the absolute temperature ϑ and the wavelength at which it is observed. When ϑ increases, the overall radiated energy increases disproportionally (energy proportional to ϑ 4) while the peak of the emission spectrum moves to shorter wavelengths (the peak wavelength is proportional to 1/ϑ).
For instance, the wavelength of maximal radiation of the sun is about 483 nm (visible radiation) which corresponds to thermal black-body radiation with ϑ of 6,000 K. On the other hand, the human skin with ϑ of 30 °C (Fig. 3.​21a) emits maximal radiation at about 9.6 µm (infrared radiation invisible to the human eye). In general, warm biological bodies emit infrared radiation.
 
4
The electron structure of atoms should shortly be reviewed. All electrons in the orbit of an atom occupy the lowest energy states possible, i.e., from the lowest energy state (with the strongest binding of electrons to the positive nucleus) upwards to higher energy levels depending on the total number of available electrons. It is referred to as the ground state in which electrons are spatially closest to the nucleus.
In the case of molecules (built out of atoms) the involved molecules, atoms, nuclei, and electrons (from outer shells) interact with each other, so that energy states of interacting atoms are completely different (modified) in comparison to those of isolated atoms. Likewise, molecules and atoms in solid bodies, liquids, and dense gasses—as well as in biological tissues—can not be considered as isolated, single units in their (photon-related) excitations and local (phonon-related) mechanical motions. Additional quantized energy levels are introduced because of (quantum mechanically) interacting electrons and atoms, and, on the other hand, vibration of atoms of a molecule to each other and rotation of a molecule as a whole (Fig. 5.3).
As illustrated in Fig. 5.4, possible energy states of a molecule—in contrast to those of isolated atoms—are usually composed of
  • numerous electron states because of interacting electrons and atoms (Fig. 5.4a). These states, in turn, are split into
  • vibration energy sub-states because of atom vibration to each other (Fig. 5.4b). These vibration sub-states, in turn, are split into
  • rotation energy sub-states because of molecule rotation as a whole (Fig. 5.4b).
Electron states occur in typical distances of about 1–10 eV. Numerous and closely aligned vibration sub-states arise in typical distances of about 0.1 eV while rotation sub-states arise in typical distances of about 0.001 eV (Giancoli 2006). The discrete energy levels of an isolated atom progressively mutate into energy bands with almost continuous energy levels when numerous atoms are brought together to form a bulk; compare Footnote 6 and Fig. 5.4. In short, molecules show spectra of multiple bands while isolated atoms show spectra of multiple lines.
In fact, the available number of discrete electron states within an isolated atom is much less than within a crystal or molecule including this particular atom. This number of states increases proportionally with the (usually large) number of neighbouring atoms which interact with this particular atom. Likewise, a single discrete energy state within an isolated atom is said to be split into numerous states when this particular atom is embedded within a crystal.
It is interesting to note that widening of single spectral lines also occurs, i.e., widening of individual energy states and sub-states. This is mainly due to a limited lifetime of excited states and sub-states, as well as due to interactions and collisions among involved atoms and among involved molecules. In addition, rotation and vibration sub-states are damped in their motions because of interactions among closely aligned atoms and molecules, which also contributes to the widening of individual spectral lines.
Lastly, the Doppler effect Doppler effect contributes to the widening of spectral lines because energy states of moving molecules virtually change with respect to the motionless laboratory system when these molecules move relative to the ambient electromagnetic field (or light field); see Footnote 214 in Sect. 3. That is, moving molecules experience higher or lower field frequencies when they move towards the light source or away from the source, respectively, and thus molecules experience the correspondingly higher or lower energy of light, in line with (5.3) (especially in gases).
 
5
For comparison, the valence band and conduction band (Footnote 6) overlap in conductors. The overlap essentially yields free valence electrons which energy can easily be increased through their acceleration (e.g., by applied electric fields) and so electrons can easily occupy the conduction band and contribute to the electric current through conductor. The energy gap between the valence and conduction band is very large in insulators (5–10 eV) so that at ordinary room temperatures no electron (with the kinetic/thermal energy of about 0.04 eV) can reach the conduction band. For comparison, the energy gap is relatively narrow in semiconductors (about 1 eV).
 
6
The valence band refers to the highest energy levels (states) within the electron shell of atoms, which still are completely filled with electrons. In contrast, the conduction band refers to available energy levels (above the valence band), in which electrons are already unbound from their individual atoms and can freely move within the atomic lattice of the material. For instance, such unbound movements contribute to the charge transport and thus to the electric current through the material. It should be stressed that the valence and conduction band are composed of closely aligned (quantized) energy levels, so that a single band appears as a continuum; see Footnote 4.
Just as an electron at one energy level in a particular atom may elevate to another empty energy level within the electron shell, so an electron can change from one energy level in a given energy band to another level in the same energy band or even to another energy band. In the latter case, the electron crosses the energy gap of forbidden energies provided that both bands do not overlap; compare Footnote 5.
 
7
The boundary between the n-type and p-type semiconductor forms the pn junction (Fig. 5.5a).
  • The n-type semiconductor carries an excess of valence electrons. For instance, an impurity atom with 5 valence electrons is embedded into the silicon lattice, so that 4 valence electrons of the impurity form 4 covalent bonds with the neighbouring silicon atoms and the remaining valence electron of the impurity remains weakly bound. This remaining valence electron can be easily liberated at room temperatures and is able to carry an electric current, like electrons in metallic conductors (Footnote 5). Likewise, the n-type semiconductor yields surplus free negative charge carriers (or current carriers).
  • The p-type semiconductor lacks valence electrons. For instance, an impurity atom with 3 valence electrons is embedded into the silicon lattice, so that all 3 valence electrons of the impurity form 3 covalent bonds with the neighbouring silicon atoms and the remaining unsatisfied bond of the silicon atom creates a deficient valence electron. This unsatisfied bond can attract an electron from the neighbouring (silicon) bonds at room temperatures, thus leaving a hole at the original position of the attracted electron. This hole can again attract another electron from the neighbouring (silicon) bonds to restore this unsatisfied bond. Consequently, the hole can move around the silicon crystal and is able to carry an electric (positive) charge and electric current. Likewise, the p-type semiconductor yields deficient negative charge or surplus free positive charge carriers (or current carriers), known as electron holes.
It should be noted that the n-type and p-type semiconductors are electrically neutral (uncharged) as a bulk substance in their ground state. In terms of energy bands (Footnote 6), a hole is a vacant (unoccupied) energy level within a band.
 
8
The quantised nature of light means that increasing the intensity of light increases the number of photons in the light beam but not the photon’s energy. The individual energy of photons can only be changed by varying the light frequency f (5.3).
 
9
Max Karl Ernst Ludwig Planck (1858–1947) was a German physicist who originated quantum theory and proposed a revolutionary idea that energy emitted by a resonator can only take on discrete values or quanta.
 
10
For the sake of completeness, it should be pointed out that the interaction of
  • uncharged radiation such as light (i.e., flow of uncharged photons, Footnote 2) with a material is relatively weak in comparison with the interaction of
  • charged radiation such as electron beam (i.e., flow of negatively charged electrons) with a material.
This is because most materials, especially biological samples, are composed of polar or charged components (e.g., water molecules are polar, compare Footnote 12 in Sect. 2) so that the charged radiation interacts more likely with such electrically non-neutral components than does the uncharged radiation (Fig. 2.​4).
 
11
It should be noted that the geometry-related damping—as described in section “Specific Issues” in Sect. 4.​1.​2.​1—is of minor relevance for the establishment of the induced optic biosignal (Fig. 5.1). This is due to the fact that both conditions of far field, i.e., the inequality r ≫ 2 ⋅ λ (with r as the light propagation distance from the light source in the range of at least a few millimeters) and the light wave propagation as a plane wave, usually apply for light in tissue. Likewise, the decay ΔI (< 0) of the light intensity I along the finite propagation distance Δr = r 2 − r 1 (with r 2 > r 1, compare Fig. 4.​21) can be neglected because ΔI ∝ (1/r 2 2  − 1/r 1 2 ) and ΔI → 0 when r → ∞; compare Footnote 25 in Sect. 4.
 
12
Usually transitions between energy levels involve the emission of light photons (a transition towards a lower energy level, Sect. 5.1.1.1) or the absorption of light photons (a transition towards a higher energy level, Sect. 5.2.1.3); compare Figs. 5.3 and 5.5. It should be noted that the emission wavelength equals (approximately) the absorption wavelength given isolated excitable molecules and/or atoms without mutual interactions.
 
13
An interesting example is given by glass (HyperPhysics 2012). In fact, it is opaque to infrared light (vibrational modes of the glass atoms are excited), transparent to visible light (no available energy levels above the ground state where electrons reside), and again opaque to ultraviolet light (available energy levels of electrons above the ground state); compare Fig. 5.4a.
 
14
In extreme cases, excessive heat can even lead to a thermally induced denaturisation of proteins, known as photocoagulation (Footnote 17).
 
15
Photochemical reactions are chemical reactions initiated in tissue by the absorption of light; compare Sect. 5.2.1.4. In consequence of the absorption, transient excited states of molecules and/or atoms can be created (including all rotation, vibration, and electron elevation), which then trigger specific chemical reactions. For instance, (visible) light with λ of less than about 600 nm is already able to resolve strong chemical bonds such as covalent and ionic bonds (with the binding energy 1–5 eV); compare typical binding energies from Sect. 5.1.1.1 and (5.3).
Specific chemical reactions in biological tissues can also be initiated after a special light-absorbing substance was injected. For instance, light with a particular λ may ultimately lead to death of unwanted or mutated cells which selectively retained the light-absorbing substance (this therapy is known as photodynamic therapy).
 
16
The (lowest) ionization energy W I is equal to the binding energy of an (outer) electron in an isolated atom or molecule. The level of W I increases progressively as the atom loses (emits) electrons, i.e., electrons from orbitals closer to the nucleus experience greater forces of attraction and thus require progressively higher W I.
 
17
Considering electrobiological interactions, it is interesting to observe that the induced heat ΔW in tissue is
  • accumulated within tissue, and, on the other hand,
  • actively transported away from the local site of the light absorption to maintain homeostasis (Pfützner 2003); compare Sect. 3.​1.​5 and Sect. 6.
The accumulated heat increases primarily the tissue temperature ϑ by Δϑ. As the time t passes, the local blood perfusion increases and transports a part of the induced heat away from the impact site (the local region of the light absorption). Thus, the regulatory vasodilation of vessels in tissue (Footnote 130 in Sect. 2)—with their contact area A penetrating the tissue volume—performs important thermal regulatory functions, preventing overheating of the tissue and balancing its local ϑ.
The sum of the accumulated heat (proportional to Δϑ) and the transported heat (proportional to the product ϑ ⋅ Δt) can be expressed as
$$ \Delta W = m \cdot c \cdot\Delta \vartheta \; + \;\alpha \cdot A \cdot \vartheta \cdot\Delta t $$
or, as a differential equation,
$$ \frac{{{\text{d}}W}}{{{\text{d}}t}} = m \cdot c \cdot \frac{{{\text{d}}\vartheta }}{{{\text{d}}t}} + \alpha \cdot A \cdot \vartheta\, ,$$
where m is the tissue mass, c the specific heat capacity, and α represents a measure of the blood flow velocity in (regulatory) dilated vessels. In fact, increasing both A and α facilitates the thermal regulation in tissue, whereas the resulting time constant τ of regulatory processes is determined by the following ratio, to give
$$ \tau = \frac{m \cdot c}{\alpha \cdot A}\, . $$
For instance, a step-wise supply of the (heat-inducing) power P ⋅ ε(t) (= dW/dt ⋅ ε(t)) into biological tissue with ε(t) as the Heaviside step function (i.e., ε(t) = 0 for t < 0 and ε(t) = 1 for t ≥ 0) yields the exponential increase of ϑ in tissue according to
$$ \vartheta (t) = \frac{P}{\alpha \cdot A} \cdot \left( {1 - e^{ - t/\tau } } \right) \cdot \varepsilon (t)\, ; $$
compare Footnote 29 in Sect. 2 and the corresponding figure in Sect. 6. This step response ϑ(t) as a response of tissue (in its zero initial state in terms of the tissue temperature, i.e., ϑ(0) = 37 °C) to a step input P ⋅ ε(t) can be easily derived from the above differential equation. The tissue is considered here as a system with a single input P ⋅ ε(t) and a single output ϑ(t).
Please note that the initial rate dϑ/dt of the temperature increase in tissue is proportional to P; for details see Sect. 6. In particular, the resulting dϑ/dt is inversely proportional to c of the exposed tissue. That is, the higher c, the lower is the resulting rate dϑ/dt because more thermal energy can be uptaken by tissue with higher c (for a given change in ϑ). For instance, this initial rate dϑ/dt is less in muscle than in fat because the heat capacity c of muscle is larger than of fat (3,600 vs. 2,000 W ⋅ s/(K ⋅ kg) according to Pfützner (2003)). This rate dϑ/dt decreases as t increases because the inert thermoregulatory responses come progressively into action, i.e., the local slope of the exponential increase decreases with increasing t.
In other words, the thermoregulatory response such as vasodilation causes the temperature rise dϑ/dt to be nonlinear. The temperature rise is linear only before vasodilation increases the blood flow to a level high enough to reduce the rate of the temperature rise; this linear range may last up to a few minutes after a hyperthermia treatment has been started (Furse 2009). Given a continuous power supply into tissue and following a time interval of a few τ (e.g., after 5 ⋅ τ), the blood flow reaches its steady-state response, the temperature rise converges to zero, and the temperature itself plateaus at a steady-state level of the final temperature ϑ (t → ∞) = P/(α ⋅A); see the last Equation from above.
 
18
Beer-Lambert absorption law is named after August Beer (1825–1863), a German mathematician and chemist, and Johann Heinrich Lambert (1728–1777), see Footnote 31. The works of Beer stated that the absorptive capacity is proportional to the concentration of the absorbing substance. In addition, the earlier works of Lambert stated that the intensity decay of light with the thickness of the sample is proportional to the intensity of the incident light. Both statements yield Beer-Lambert absorption law; compare with its derivation in Footnote 19.
 
19
The rationale of the exponential law in the absorption of light deserves a short notice (compare (4.​7) and (5.4)). In fact, it is assumed that the light intensity drop dI (< 0) over the propagation distance dr is proportional to the absorption coefficient µ A of light and to the light intensity I itself in the volume element with the depth dr (Fig. 5.6), to give:
$$ {\text{d}}I = - \mu_{\text{A}} \cdot I \cdot {\text{d}}r\quad {\text{or}}\quad \frac{{{\text{d}}I}}{I} = - \mu_{\text{A}} \cdot {\text{d}}r\, . $$
Please note that the light is absorbed at a constant rate, i.e., dI depends on the total I, which, in fact, constitutes the origin of the exponential law. Upon integration from the incident light intensity I 0 to the (decreased) transmitted light intensity I after the distance r (on the left side of the above equation) and a simultaneous integration from the initial depth r = 0 to the final depth r (on the right side of the above equation), the differential equation from above yields the exponential law of the light absorption from (5.4).
 
20
If more than one absorber with µ A 1 and µ A 2 is present, the effective product µ A ⋅ r (5.4) is the sum of the corresponding products of each absorber, i.e., µ A ⋅ r = µ A 1 ⋅ r + µ A 2 ⋅ r. In analogy, two absorbers connected in series—such as a blood vessel with µ A 1 embedded within (otherwise bloodless) tissue with µ A 2 (≪ µ A 1 , Table 5.1) in accordance with Fig. 5.1a—yield the effective product
$$ \mu_{\text{A}} \cdot r = \mu_{\text{A}}^{1} \cdot r_{1} + \mu_{\text{A}}^{2} \cdot r_{2}\, . $$
Here r 1 is the path length of light through the blood volume in the vessel and r 2 the path length through the tissue volume surrounding the vessel. In the case of the embedded blood vessel, it can be approximated µ A ⋅ r ≈ µ A 1 ⋅ r 1, which means that blood in tissue or pulsatile extensions of this blood vessel determines the total light absorption of perfused tissue. Likewise, the product µ A ⋅ r (or µ A 1 ⋅ r 1) increases temporarily from (local) diastole to (local) systole because the systolic r 1 is larger than diastolic r 1 (see section “Cardiac Activity” in Sect. 5.1.2.3 and Fig. 5.14c).
 
21
The chromophore is a chemical group, e.g., a group of atoms or molecules, that causes the chromophore’s coloured appearance. The chromophore absorbs light at certain wavelengths which depend on available energy states of this chemical group. Namely, if the energy possessed by photons falls within the energy difference between two available energy states (energy gap) the photons are absorbed (compare Footnote 4 and Fig. 5.3). The remaining wavelengths are transmitted or reflected, which gives rise to a specific observed color (Footnotes 3 and 23).
 
22
In fact, the particular choice of the applied wavelengths depends on a particular goal of the biomedical optics. If the light has to permeate a heterogeneous biological tissue then the aforementioned optical window for red and near-infrared light should be used, where the total light absorption is relatively low; compare Fig. 5.7 with the indicated colours of light.
On the other hand, if some components of this heterogeneous tissue should be contrasted with other components, e.g., blood vessels should be contrasted with the surrounding tissue (i.e., with a sort of the (average) tissue, almost bloodless tissue, see Table 5.1), for the unaided human eye then an optical contrast between vessels and the surrounding tissue should be aimed at Kaniusas (2011). Thus, spectral regions of light should be found where the light absorption and scattering by blood vessels differ as strongly as possible from the light absorption and scattering by the surrounding tissue.
Such optical contrast, for instance, is provided by blue and green light, so that the corresponding µ A of blood dominates strongly with respect to µ A of the different tissues (almost bloodless tissues); compare Fig. 5.7. In contrast, red light is absorbed by blood to a lesser degree; i.e., red light penetrates both bloodless and blood-rich tissues with little losses and produces practically no optical contrast between blood vessels and the surrounding tissue. Considering the light scattering, it should be noted that the scattering strength does not vary as strongly as the absorption strength (i.e., the size of µ A) over λ. Therefore, the scattering phenomena can hardly be applied for the aforementioned contrast enhancement by optimizing the colour of the incident light.
 
23
When hemoglobin in blood is oxygenated (S rises), blood absorbs red light to a lesser degree than when oxygen is depleted in blood (S drops); see Fig. 5.8. Therefore, a relatively large amount of red light is reflected back to the observer when oxyhemoglobin dominates, i.e., arterialoxygenated blood looks reddish. On the contrary, dominating deoxyhemoglobin in venous deoxygenated blood lets venous blood to appear bluish.
 
24
In fact, besides the absorption of light (see section “Volume Effects” in Sect. 5.1.2.2), the interaction of the uncharged radiation (Footnote 10) with a material includes Krieger (2004)
  • elastic scattering (see Footnote 25) and
  • inelastic scattering.
In particular, the absorption of the uncharged radiation can be based on
  • complete absorption of a photon (see section “Volume Effects” in Sect. 5.1.2.2 and Fig. 5.3b) by either
    • motions of molecules and/or atoms, i.e., rotation and/or vibration of molecules and/or atoms, or
    • excitation of atoms, i.e., elevation of an electron within the electron shell of atom, or even
    • ionization of atoms, i.e., removal of an electron out of atom. In addition,
  • partial absorption of a photon can also take place, known as inelastic scattering. In particular,
    • considering Raman effect, the incident photon excites rotational and vibrational motions of polarizable molecules. It yields scattered photons with less energy (and larger λ, (5.1) and (5.3)) by the amount of the transition energy (or the energy gap, see section “Volume Effects” in Sect. 5.1.2.2).
    • In the case of Compton effect, the incident photon knocks an electron out of an atom (usually a weakly bound electron from an outer shell). Another photon is simultaneously emitted (or scattered) with less energy by the amount of the bound electron energy (i.e., of the electron removed out of the atom).
    • (Quasi) elastic scattering may also take place, in which the spectrum of the scattered light is Doppler -broadened in comparison with the spectrum of the incident light. This is because translational and rotational motions of (optically anisotropic) molecules shift the frequency of the scattered light depending on their motion velocity; compare Footnote 214 in Sect. 3. For instance, the effective λ of the scattered light is shortened when the scatter (or the molecule) moves towards the light sink. Correspondingly, the frequency of the scattered light increases while the light propagation velocity remains constant; compare (5.1).
However, in the case of near-infrared and visible light, the ionization of atoms and the inelastic scattering are almost irrelevant because this light has a relatively low quantum energy (≪ 1 keV, (5.3)).
 
25
Elastic scattering of the light wave occurs when charged particles in a medium (e.g., electrons in molecules) are forced to oscillatory motions by the electric field of the incident light wave or, in other words, by photons hitting the electron shell and inducing forced oscillations of electrons in it. These accelerated motions start to emit light of the same frequency as the incident wave; compare Footnote 2. The respective (re)radiated patterns from (numerous) oscillating particles superimpose and yield a “single” source of the scattered light. Likewise, the photon energy of the scattered photon is not changed in the course of the elastic scattering.
In particular, the incident electric field of light forces electric dipoles in the dielectric medium (induced and permanent dipoles, see Sect. 6) to align and alternate in synchrony with this incident field. In the conducting medium, its free charge carriers (such as free electrons or ions) oscillate back and forth at the same frequency as the incident field. Likewise, oscillating charged particles act as small antennas, reradiating the incident wave that becomes the scattered wave.
 
26
Since there are many scattering particles (e.g., in the highly heterogeneous biological tissue), the individual scattered waves from each particle combine to form the entire scattered wave at the observation point. Provided that the scattering particles are randomly located in tissue, the individual scattered waves have random phases at the observation point; i.e., the distance light travelled from each particle to the observation point is random. It is instructive that the power density—proportional to the square of the resulting electric field magnitude, see Sect. 6—of the entire scattered wave at the observation point is equal to the sum of the power densities scattered from each particle. It is known as incoherent light scattering.
 
27
John William Strutt Rayleigh (1842–1919) was an English physicist who made significant contributions in the fields of optics and acoustics. He explained the concept of wavelength-dependent light scattering as well as the sunlight interaction with the upper atmosphere yielding blue coloured sky (Footnote 29).
 
28
Gustav Adolf Feodor Wilhelm Ludwig Mie (1868–1957) was a German physicist who described the scattering of light by particles whose size is comparable to the wavelength of light.
 
29
The blue colour of the sky at daylight is caused by the wavelength dependent Rayleigh scattering off molecules of the air. The blue end of the visible light is scattered more effectively in the direction down to the earth, i.e., in the direction which is almost perpendicular to the direction of the sun’s incident light. In other words, the isotropic Rayleigh scattering (Fig. 5.11a) dominates towards an observer on the earth, who looks from overhead, not directly into the sun. In contrast, the anisotropic Mie scattering (Fig. 5.11b) is mainly directed forward in the direction of the sun’s incident light and thus does not reach this observer. Consequently, the blue of the sky is less saturated when the observer looks closer to the sun because the forward directed Mie scattering then starts to dominate. Mie scattering is less strongly wavelength dependent than Rayleigh scattering and thus scatters in the whole range of the visible light, not only at its blue end. Likewise, relatively large water droplets in a cloud scatter in the whole visible range, which causes the cloud to appear white; compare Footnote 30.
 
30
The observation that the scattering efficiency increases with the particle size in both Rayleigh and Mie scattering can be observed in cataract, i.e., cloudy vision and glare, especially in aged persons. Here special proteins aggregate in the lens (damage of lens’s proteins due to their unfolding and denaturation, see Footnote 18 in Sect. 2), which results in the local density variations leading to local variations in the index of refraction. Thus scattering centers are built in the lens, which scatter light out of its normal path to the retina. Blurred and dimmed images result. In addition, halos around point-like light sources can be also seen as a consequence of the forward scattering; compare Footnote 29.
 
31
Johann Heinrich Lambert (1728–1777) was a Swiss German physicist, mathematician, astronomer, and philosopher who, among others, pioneered work in photometry and proved the irrationality of π (= 3.14159 …, i.e., the ratio of a circle’s circumference to its diameter); compare Footnote 18.
 
32
In the skin, Rayleigh scattering from small-scale structures dominates below 650 nm while Mie scattering from large-scale structures dominates above 650 nm (Jacques 1998). The epidermis of the skin is mainly composed of keratin fibers which behave like collagen fibers in the dermis (Jacques 1998). The thinness of the epidermis reduces its scattering relevance for visible and near-infrared applications involving the light diffusion. Thus the scattering in the dermis alone can be used to describe the skin scattering in general terms. In the dermis, Rayleigh scattering is induced by the microstructure within collagen fibers and other small cellular structures, whereas Mie scattering is induced by the relatively large and elongated collagen fibers (see section “Inhomogeneity Effects” in Sect. 5.1.2.2).
 
33
Provided that the light scattering dominates over its absorption and the light propagates mainly in the forward direction, i.e., µ S ≫ µ A, µ S, ≫ µ A, and g is close to 1, a description of photon movements with a single (relatively) large step 1/µ S′ is equivalent to another description with many (relatively) small steps 1/µ S (Jacques 2002); please note that µ S′ < µ S if g > 0 (5.7). A large step with 1/µ S′ involves only isotropic deflection while each small step involves anisotropic deflection. This situation with the scattering as the dominant tissue-photon interaction, known as diffusion regime, usually applies to biological tissues exposed to visible light and near-infrared light (Table 5.1). In addition, the diffusion regime justifies the use of µ S′ (5.7).
 
34
For the transmission of a collimated light through a thin tissue layer the total absorption coefficient µ T = µ A + µ S can also be defined. Here the total absorption of the incident light intensity is governed by (5.4) with µ T instead of µ A. For thicker tissue layers, the collimated light transmittance can also be described by an exponential law; however, multiple scattering should be accounted for (Tuchin 2005).
 
35
The reflection factor depends on both the angle of incidence and the plane of light polarization, i.e., the direction of the electric field \( \vec{E} \) can be parallel or perpendicular to the plane of incidence. In particular, (5.11) describes the reflectivity at normal incidence (φ 1 = 0 in Fig. 5.13) and the perpendicular polarization (see \( \vec{E} \) in Fig. 5.13) only.
 
36
When light reflects from a medium of higher n (n 2 > n 1 and Γ O < 0, (5.11)), the reflected light experiences a phase shift of 180° on the boundary (light with the perpendicular polarization only, Footnote 35). Otherwise, there is no phase shift when light reflects from a medium of lower n (n 2 < n 1 and Γ O > 0), as also illustrated in Fig. 5.13. Such phase shifts play an important role in the light interference which, for instance, manifests colourfully in thin soap films.
Similar behaviour was already observed in Fig. 4.​26 where reflected body sounds also experience a phase shift of 180°. This is because inner body sounds bounce back into tissue from the tissue-air boundary, whereas the air shows a lower sound propagation velocity (v A < v T in Fig. 4.​26) and thus a higher index of “acoustical” refraction (compare (5.2)).
 
37
In the cutaneous tissue, slight dilation and contraction of arterioles and capillaries during each pressure pulse contribute to the local changes in the light absorption. Consequently, a high density of arterioles and capillaries is required near the surface of the skin—as found e.g., in the fingertip—to attain large pulsatile changes in the absorption.
It seams that arterio-venous-anastomoses (i.e., shunts between arterioles and venules, see Fig. 3.​22b), venules, and veins contribute to the pulsatile volume in the cutaneous circulation (Kim 1986). The shunting of a (high pressure) arterial pulse via open arterio-venous-anastomoses generates a pulsatile volume in the (low pressure) local venous side, where changes in the vascular venous volume are much greater than those in the arterial bed. Here, it should be recalled that venous compliance is greater than arterial compliance (Sect. 2.​5.​1). Thus the same variation in the blood pressure yields a larger change in the venous volume than arterial volume; likewise, the volume pulse is amplified while the pressure pulse passes from stiff arterioles, through anastomoses, to compliant venules. The pulsatile venous volume can be expected only in tissue regions close to arterio-venous-anastomoses, i.e., veins located more proximally do not show venous pulses because the shunted arterial pulses become damped with increasing propagation distance (see pulse propagation in Sect. 2.​5.​2.​3). To give an experimental example, the optical pulsatile deflection amplitude (comparable to s S,D in Fig. 5.15a) was shown to be proportional to the venous pulse pressure within the finger as the recording site (Kim 1986).
The arterio-venous-anastomoses are sympathetically controlled, i.e., sympathetic denervation opens these anastomoses. Consequently, the (reflex) vasoconstriction or stress-related reactions tend to reduce the optical pulsatile deflection amplitude because of closed anastomoses (closed shunts) and reduced pulsatile changes in the local venous volume. Finally, it should be noted that the arterio-venous-anastomoses are abundant in the fingers—to facilitate their thermoregulatory functions (Sect. 3.​1.​5)—in contrast to the earlobe; see Footnote 53 for more details.
 
38
In fact, alternating (pulsatile) components in I(t) can be isolated (relatively) easily from direct (non-pulsatile) components. For instance, the alternating intensity I AC(t) can be extracted out of the total intensity I(t) (5.13) with a high-pass filter, whereas the direct intensity I DC(t) can be extracted with a low-pass filter; compare Figs. 5.26 and 5.27.
 
39
It should be noted that the arterial radius reaches its maximum value in synchrony with the maximum pressure in the vessel in the course of its pulsatile widening. However, the maximum pressure can be expected closely before the end of the local systole (in peripheral arterial vessels, Footnote 40) because systole ends with the dicrotic notch and not with the maximum pressure; compare Fig. 2.​38. The latter applies only for the reflectionless propagation of pulses (Sect. 2.​5.​2.​3).
 
40
The terms systole (ventricular contraction and ejection) and diastole (ventricular relaxation and filling), strictly speaking, are related to the activity of heart ventricles (Sect. 2.​4.​2). For instance, these terms are useful to interpret the pulse of the blood pressure leaving the heart along the aorta (Fig. 2.​32b). In the case of distal vessels or even peripheral arterial vessels in a finger—as depicted in Fig. 5.14c—the terms systole and diastole can be used only conditionally.
In particular, the pressure pulse arrives at a peripheral site with a certain time delay (known as pulse arrival time, Sect. 3.​1.​3.​1) which depends on both the distance from the heart to the peripheral site and the velocity of the propagating pulse (3.​6). The local blood surge in the periphery—or the local systole—is delayed with respect to the ventricular systole in the heart; likewise, the decreasing blood volume in the periphery after the blood surge—or the local diastole—is also delayed with respect to the ventricular diastole.
 
41
First oximeters to assess S of arterial blood had only one wavelength in the red region of spectrum (Zourabian 2000). As illustrated in Fig. 5.8, µ A of oxyhemoglobin in this spectral region is markedly different from µ A of deoxyhemoglobin. However, the single wavelength application prevented this oximeter to assess the total hemoglobin concentration and thus to compensate for possible changes in the total hemoglobin concentration (Sect. 3.​1.​4). A reference measurement (or a calibration procedure) was necessary on bloodless tissue without the primary chromophore (see section “Volume Effects” in Sect. 5.1.2.2), e.g., on tissue made temporarily bloodless by its squeezing (see below).
Later oximeters with two wavelengths were introduced. They used one red wavelength usually around 660 nm as a measure for oxyhemoglobin and a second near-infrared wavelength at around 800 nm as a reference (at the isosbestic point, Fig. 5.8).
Today’s oximeters use the second wavelength at higher near-infrared wavelengths at around 890 nm (see section “Light Wavelength” in Sect. 5.2.1.2). Here µ A of oxyhemoglobin and µ A of deoxyhemoglobin show reverse courses over the wavelength with respect to the isosbestic point. That is, µ A at red light decreases with increasing oxygenation of hemoglobin while µ A at near-infrared light correspondingly increases (Fig. 5.19). This reverse behaviour is explicitly used in the assessment of S (see section “Blood Oxygenation” in Sect. 5.1.2.3). In addition, the pulsatile nature of the transmitted light intensity is also used today in the estimation of S in order to focus on oxygenation of pulsatile arterial blood only (Footnote 49). The latter technology is known as pulse oximetry.
As already noted, early oximeters used a calibration procedure, in which
  • tissue was compressed to eliminate blood (Kamat 2002). The light absorbance by bloodless tissue (∝ I DC, compare with the compartmental model of tissue, Fig. 5.14b) was used as a baseline to isolate arterial blood and to estimate S of this arterial blood (especially from I − I DC, see 5.13). In addition,
  • pneumatic cuffs were introduced to measure increase in the light intensity when, for instance, the transilluminated ear was squeezed (compare sections “Motion Artefacts” in Sect. 5.1.2.3 and “Contacting Force and Skin Temperature” in Sect. 5.2.1.2). It was also customary to
  • heat tissues (e.g., earlobe) in order to filter out absorption effects due to venous and capillary blood. The (local) skin heating is known to produce (local) vasodilation of vascular beds under the skin surface, in the course of which the pulsatile component of the transmitted light intensity increases (Mendelson 1988); compare Fig. 3.​21. For instance, the latter authors observed a five-fold increase in the pulsatile ratio \(\mathcal{R}\) (5.17) by increasing the local skin temperature from 34 to 45 °C, whereas the optical sensor was applied on the forearm and operated in the reflectance mode (Fig. 5.22b).
 
42
The hematocrit is the volume percentage of blood occupied by the (packed) red blood cells.
 
43
Strictly speaking, the relationship between S and R—as given in (5.21)—would become linear only if c 2 = 0. This is because the linearity requires that two mathematical properties, namely, homogeneity and additivity, are satisfied. In fact, they are satisfied only if c 2 = 0.
 
44
The dissimilar propagation distances of red and near-infrared photons (before they enter the light sink) occur due to the wavelength dependence of µ A and µ S′. For instance, for low values of S and red light, the relevant µ A of blood is relatively high (Fig. 5.8) which determines strong absorption of red light. In consequence, the average propagation distance of red light (photons) in tissue is decreased for low S. It reduces the sensitivity of the corresponding I to (local) absorbance changes in the deeper layers of tissue relative to the sensitivity measured with near-infrared light (Schmitt 1991). For this reason, the (local) slope of S versus R curve—as shown in Fig. 5.18—increases for low S (< 80 %), thereby decreasing the effective resolution of ΔS for a given ΔR. For high S (> 80 %), the reverse is true yielding relatively high resolution of S. Figure 5.20 illustrates decreased propagation distance and decreased probing depth of red light for low S in comparison with near-infrared light. Similar observations can be derived from Fig. 5.17 while comparing light paths during expiration; e.g., red light for low S is confined to shallow depths.
In contrast to the photon diffusion theory, both propagation distances at the two wavelengths are assumed to be equal and do not depend on optical properties of tissue in the model of Beer-Lambert absorption law (5.4).
 
45
A reliable testing and calibration of oximeters is an important issue. Usually, in-vivo empirical calibrations on volunteers are performed, having the volunteers to inspire hypoxic gas and analysing their arterial blood gas samples as reference. The reduced partial pressure of oxygen in the inspired air (Footnote 219 in Sect. 3) decreases the resulting blood oxygenation level; such calibration is usually performed in a stepwise manner from S = 100 % down to only 70 % (Venema 2012). Also animal tissues with active oxygenation can be used for calibration purposes (Zonios 2004). In-vitro calibrations can be carried out by the use of heparinised blood samples (Reichelt 2008).
Besides blood as calibrating media, phantom media (artificial media) can also be used for testing and calibration of oximeters. For instance, scattering and absorption properties of tissue can be experimentally approximated by using suspended polystyrene microspheres, e.g., with 1 µm diameter, the approximate size of biological cells (Mourant 1998; Kumar 1997). Calibrated intralipid solutions, special rubber and resin can also be used to mimic scattering tissues, whereas an ink is added to simulate chromophores in absorbing tissue (Benaron 2005). In addition, patented solutions exist to imitate oxygenation changes of blood using electrically-controlled light absorption based on artificial media such as liquid crystals (Winter 2002).
 
46
The accuracy of oximeter is low in patients with decreased blood volume (see above) and diminished peripheral pulsation (essential to calculate R, (5.18)). In fact, the accuracy of the optical sensor applied on a peripheral site is low in patients who need them most, such as
  • patients with hypotension,
  • cold extremities (hypothermia, compare Fig. 5.28), or
  • peripheral vasoconstriction.
High sympathetic tone of such patients reduces significantly vascular pulsations in the periphery; compare Footnote 37. For instance, continuous intraoperative assessment of blood oxygenation during anaesthesia shows a relatively low accuracy (or requires special measures to reach an acceptable accuracy) because of—in part—poor tissue perfusion due to (intended) hypothermia and/or hypotension.
 
47
Monte Carlo simulation is based on modelling of possible photon trajectories, i.e., random walk of photons, from the light source through biological tissue to the light sink (compare Fig. 5.20). The histories of individual photons are simulated as they undergo multiple absorption and scattering events. Each photon is followed until it disappears in tissue or reaches the light sink. Probability distributions are used to describe propagation of photons. Around 1 million photons are usually used in such simulations (Meglinski 2002).
 
48
For instance, an optical sensor in the reflectance mode (Fig. 5.22b) can be used as a motion sensor (Asada 2003); see text for details. The output of such sensor can act as a motion (noise) reference for a motion (noise) cancellation filter, which is supposed to recover the true motion-free (artefact-free) optic biosignal (compare Fig. 5.21c) from a corrupted optic biosignal (Fig. 5.21b).
  • It should be recalled that the reflectance mode is more sensitive to motions than the transmittance mode (see section “General Issues” in Sect. 5.2.1.2).
  • The source-sink distance of the motion sensor must be relatively short in order to get a relatively small penetration depth of light (5.26) and short light paths. It ensures that only peripheral superficial tissues are probed by light, i.e., those tissues which are easily affected by motions (see above).
  • The application site of the motion sensor should be distant from arteries and close to veins instead.
  • The employed wavelength of light should be selected in the red region of the spectrum such that the propagating light is highly sensitive to motion-displaced venous blood, i.e., sensitive to deoxygenated blood, namely, deoxyhemoglobin; compare a relatively high µ A applicable for red light and deoxygenated blood in Fig. 5.8.
 
49
The pulse oximetry solved many problems, such as low accuracy in the oxygenation assessment due to significant light absorption by tissue components other than blood (Fig. 5.14b), inherent to oximetry in the past and is the method used today (Wukitsch 1988); compare Footnote 41. The predecessor of the pulse oximetry did not use the pulsatile nature of the transmitted light intensity and operated in terms of the spectrometry.
 
50
In fact, the banana-shaped region of the penetrated tissue delimits the pathway of photons which detection probability is the highest at the light sink (pathway A in Fig. 5.9b). Consequently, other pathways show lower detection probabilities (pathway B in Fig. 5.9b). In other words, if a propagating photon remains within this banana-shaped region during its random walk through tissue, it will be detected by the sink. However, if a photon is scattered out of this banana-shaped region (pathway C in Fig. 5.9b), it is likely that this photon will no longer be accessible for the sink.
 
51
It should be noted that near-infrared light would be more appropriate to assess the pulsatile fraction \( \mathcal{R} \) (or the blood perfusion index) as compared with red light. This is because the light absorption changes minimally for near-infrared light within the normal range of blood oxygenation, as compared with red light; see Fig. 5.8.
 
52
In the case that high sensitivity of the transmitted light to scattering variations in tissue is required—variations in µ S′ may also have diagnostic value—the authors in (Kumar 1997) suggest optimal distances r R for the reflectance mode (Fig. 5.22b). Namely,
  • if both µ A and µ S′ of tissue are small, the latter sensitivity has its maximum value when the distance r R is small; i.e., the registered reflectance increases as µ S′ increases, see text. In contrast,
  • if both µ A and µ S′ are large, this sensitivity has its maximum value when the distance r R is large; i.e., the registered reflectance decreases as µ S′ increases, see text.
 
53
Significant differences can be observed in optic biosignals recorded from the finger or earlobe, as typical application regions of the optical sensor (Fig. 5.22); compare Footnote 55. The reason for this is that the fingertip includes a large number of arterio-venous anastomoses (for the thermoregulatory control, see Sect. 3.​1.​5), whereas such anastomoses lack in the ear (Middleton 2011); see Footnote 37. Since arterio-venous anastomoses are sympathetically innervated (controlled), i.e., the sympathetic activation closes (blood) shunts between arterioles and venules (Fig. 3.​22b), the transmitted light intensity from the finger sensor can be expected to reflect the sympathetic vascular tone but not from the earlobe sensor. In particular, pulsatile changes of the transmitted intensity that arise in the course of pulsatile changes of the (local) blood volume reflect this sympathetic tone. Namely, with increasing sympathetic tone in the finger, the distensibility and radius of anastomoses decrease. Consequently, the local pulsatile blood volume decreases as well as the deflection amplitude of the pulsatile changes in the transmitted intensity (comparable to the deflection s S,D in Fig. 5.15a, Sect. 5.1.2.3). Obviously, the pulsatile blood volume is strongly affected by the distensibility of the arterial vessel; in general, the pulsatile volume is a product of the vessel’s compliance and the pulsatile pressure within the vessel (see 5.14, 2.​23, and Sect. 2.​5.​1). For instance, the deflection amplitude decreases by about 48 and 2 % with the optical sensor applied on the finger and ear, respectively, in response to the vasoconstrictive cold stimulus (Awad 2001); compare Fig. 5.28.
The ear is relatively immune to vasoconstrictive effects of the sympathetic system, thereby minimizing the effects of local peripheral vasoconstriction on the measurements of blood oxygenation with the optical sensor on the ear (see section “Specific Issues” in Sect. 5.1.2.3). The pulsatile blood volume in the ear is
  • less affected by the vessel’s distensibility and
  • mainly responds to pulsatile changes of the (central) blood pressure (according to 2.​23) and to changes in the systemic circulation (Awad 2001).
In addition, the optic biosignal from the earlobe was reported to be rather insensitive to changes in the contacting force (i.e., the contacting force between the optical sensor and its application site on the skin) due to movements (Middleton 2011); compare section “Contacting Force and Skin Temperature” in Sect. 5.2.1.2. That is, motion artefacts can be expected to be less dominant in the optic biosignal from the earlobe in comparison with that from the finger (see section “Motion Artefacts” in Sect. 5.1.2.3).
 
54
To give a tangible example, the authors in (Asada 2003) suggest the flanks of the finger as the application sites of the light source and sink rather than the dorsal and palmar sides of the finger. The former locations are desirable, because both flanks have a thin epidermal layer (i.e., less attenuation of the penetrating light) and, on the other hand, the lateral (digital) arteries in the finger would necessarily reside in the resulting light pathway; see Fig. 5.9a for the location of the digital arteries.
 
55
In analogy with Footnote 53—comparing the finger and earlobe—there are distinct differences in optic biosignals recorded from the finger or the chest (Kaniusas 2006). In contrast to the fingertip, the chest contains less defined composition of tissue layers under the skin surface, includes usually a thick layer of the subcutaneous adipose tissue, and, more importantly, is relatively weakly perfused by blood. Thus large variability in the resulting light probing depth as well as the (estimated) absorption and scattering properties can be expected from one subject to another; compare Fig. 5.24.
 
56
It should be noted that such a high contacting pressure—namely, close to the mean arterial pressure—can not be applied on the skin for a long period of time in order to amplify the pulsatile component (Asada 2003). It would completely squeeze capillary beds and venous vessels (see section “Motion Artefacts” in Sect. 5.1.2.3), thus limiting and impeding the supply of arterial blood and the return of venous blood.
 
57
Similar behaviour of the pulsatile amplitude in the arterial vessel was already observed in the oscillometric method for the monitoring of blood pressure (Sect. 3.​1.​3.​1). Here radial oscillations of the arterial vessel wall reach their maximum amplitude when the cuff pressure (i.e., the external pressure outside the vessel, Fig. 5.26) passes the mean arterial pressure. As the cuff pressure increases above or decreases below the mean blood pressure, volume pulsations of the arterial vessel decrease.
 
58
In contrast to thermal interactions, photochemical interactions are subjected to the principle of reciprocity. It implies that the threshold for photochemical injury is proportional to the total exposure, the product of
  • light intensity (such as brightness of the light source or irradiance of a surface) and
  • exposure duration.
For instance, blue-light retinal injury can result from viewing either a bright light for a short time or a less bright light for a longer time.
Obviously, the same principle of reciprocity applies in photography. The effect of light on the film is proportional to the product of light intensity (determined by aperture of an optical system) and exposure duration (determined by shutter speed).
 
59
The exposure to infrared light blended with visible light is less risky than the exposure to infrared light only. This is because the optical aversion response is due to visible light only.
 
60
It should be noted that the effective irradiance is the difference between the irradiance on the skin coming from external sources and the irradiance emitted from the skin itself (according to thermal black-body radiation, see Footnote 3). As noted in Sect. 3.​1.​5, the radiation emission from the human skin plays a major role at room temperature when there are no external optical sources of high temperature (≫ 30 °C, i.e., much higher than the approximate skin temperature, Fig. 3.​21a). If an external optical source is present and its radiation source’s temperature (e.g., of several hundred degrees Celsius) is much higher than 30 °C, the radiation emission from the skin can be neglected. Then the effective irradiance equals the irradiance from the external source only.
 
61
Premature ventricular contraction (or extrasystole, an additional heart beat) is a heart beat outside the regular sequence of heart beats, i.e., it is a premature heart beat before the normal heart beat was supposed to occur. In contrast to normal heart beat,
  • this extrasystole is (spontaneously or artificially via electrical stimulation, see Sect. 6) initiated in ventricles (or in the Purkinje fibers, Fig. 2.​35) by a pacemaker other than the sinoatrial node; compare Footnote 120 in Sect. 2. In addition,
  • slowed conduction of the (action) impulse in ventricles may lead to a local re-excitation of ventricles, known as (single) re-entry mechanism. Here the impulse propagates along closed pathways (i.e., loops in the heart) such that the excitation wave front returns to a previously excited tissue after a certain time delay, i.e., after the refractory period, to be more precise, after the absolute refractory period (Sect. 2.​2.​2). This is long enough to permit this re-entered tissue to regain its excitability and to become re-excited. The re-entry can be promoted not only by slowing conduction velocity but also by shortening the refractory period and also by a dilated heart. Likewise, the wavelength of the impulse (given by the product of the conduction velocity and refractory period, compare (5.1)) must be shorter than the physical length of the aforementioned loops because the excitable tissue—into which the impulse is re-entering—has to recover its excitability (Roger 2004). Interestingly, anisotropic structures of the cardiac muscle (such as regional ischemia or fibrotic regions) favour the discussed re-entrant circular excitation since anisotropy can lead to re-entrant loops. For multiple re-entry mechanisms in terms of ventricular fibrillation see Sect. 6.
It should be noted that the premature ventricular contraction may also induce ventricular fibrillation (Sect. 6), a life-threatening state. In particular, a preceding premature ventricular contraction exaggerates non-uniformity in the recovery of excitability in ventricles; compare with the re-entry mechanism from above. This non-uniformity elongates the vulnerable period (Sect. 6) and favours imminent fibrillation in response to another proceeding premature ventricular contraction during the vulnerable period. Interestingly, the vulnerable period can even extend beyond the T wave, i.e., beyond the repolarization phase of ventricles (Sect. 2.​4.​2).
The premature ventricular contraction is characterized by an abnormally deformed, widened, and strong QRS complex in the electrocardiogram while the P wave is usually missing (Sect. 2.​4.​2). The widening of the QRS complex is basically due to relatively slow propagation of the excitation through cardiac muscles as compared to (normal and fast) propagation along the conductive system in the heart (Fig. 2.​35). The deformation of the QRS complex is caused by the different pathway of the excitation wave front in ventricles as compared to the normal pathway beginning at the atrioventricular node.
In contrast to premature ventricular contraction, premature atrial contraction is an abnormal heart beat initiated in atria by a pacemaker (i.e., prematurely depolarized region in an atrium) other than the sinoatrial node. The premature atrial contraction is characterized by an abnormally shaped P wave (and usually shortened PR interval) and the normal narrow QRS complex in the electrocardiogram. This is because this premature beat is initiated outside the sinoatrial node but the excitation propagates still normally through the atrioventricular node into ventricles (Sect. 2.​4.​2).
 
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Metadaten
Titel
Sensing by Optic Biosignals
verfasst von
Eugenijus Kaniusas
Copyright-Jahr
2015
Verlag
Springer Berlin Heidelberg
Buch
Biomedical Signals and Sensors II

Print ISBN: 978-3-662-45105-2

Electronic ISBN: 978-3-662-45106-9

Copyright-Jahr: 2015

DOI
https://doi.org/10.1007/978-3-662-45106-9_5

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