Abstract
Since a biosignal can be defined as a description of a physiological phenomenon (Sect. 1), it is obvious that biosignal parameters reflect physiological parameters. For proper diagnostic or therapeutic interpretation of the derived biosignal’s parameters (or results of the biomedical signal analysis), an adequate understanding of their physiological causes is necessary. Here, definitions and basics of biosignal parameters are relevant, as well as their time-related or event-related behavior along with their mutual interdependencies.
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Notes
- 1.
“Aging seems to be the only available way to live a long live,” according to the words of Daniel Francois Esprit Auber (1782–1871).
- 2.
The activity of the sympathetic nervous system (SNS) is due to a disruption of the autonomic balance because of the body’s physical and mental activity (body accelerator); a mobilization of metabolic resources and increased energy expenditure are given during times of stress, arousal, or other external challenges, supported by elevated blood pressure and redirected blood from the intestinal reservoir toward skeletal muscles. The SNS is mediated by the release of hormones (e.g., adrenalin) induced via splanchnic nerves, the neuronal activation of beta receptors in the heart (sinoatrial node, atrioventricular node, and heart muscles), the acceleration of the slow diastolic depolarization (Footnote 111), and a decrease in cardiac refractory period, typically resulting in an increasedf C and increased contractile force of heart muscles.
- 3.
The activity of the parasympathetic nervous system (PNS) is concerned with the regulations of routine functions of the body during rest or digestion (body brake), promoting restoration and conservation of bodily energy, i.e., reducing energy expenditure. The PNS influence is mediated via release of acetylcholine by the vagus nerve toward the heart (sinoatrial node and atrioventricular node) which mainly slows down the diastolic depolarization, increases cardiac (ventricular) refractory period (Footnote 56), and thus typically decreasesf C and diminishes blood pressure (Footnote 71).
- 4.
It is worth noting that reciprocal behavior of the sympathetic and parasympathetic activities is given for tonic (basic) control of f C while both reciprocal and nonreciprocal behaviors arise for reflex (stimulus related) control of f C (Zemaityte 1997). Interestingly, in the case of concurrent sympathetic and parasympathetic activities, parasympathetic vagal stimulation seems to override sympathetic stimulation in the heart. For instance, a relative decrease inf C due to PNS stimulation (e.g., due to artificial electrical vagal stimulation) is largest when the baseline level of f C is relatively high and SNS activity is well expressed (Levy and Zieske 1969). By contrast, a relative increase inf C due to SNS stimulation is smallest when prevailing PNS activity (or level of vagal tone) is high.
- 5.
Advantageously, f C is a frequency-related parameter but not amplitude related, in contrast to, for instance, chest circumference changes induced by airflow or respiration. Thus, the influence of unavoidable body movements or external noise can be expected to be lower.
- 6.
In general, an increased activity of the PNS represents overall healthier people while decreased values reflect temporal dysfunction. An increased activation of the SNS can be attained by a 90 ∘ tilt, standing, physical activity, or stress, while an activation of the PNS can be attained by controlled respiration, compare Sect. 3.2.1 (HRV task force 1996). Furthermore, a positive stress could be assessed by an increase in both PNS and SNS, while distress (or physical exhaustion, nervous tension) is indicated by an increase in the SNS with a simultaneous decrease in the PNS (Riftine 2006). Furthermore, there is significant evidence that the SNS plays an important role in the genesis of arrhythmias, while the PNS has a protective role, decreasing the probability of arrhythmias (Bigger 2006).
- 7.
The power spectral density provides information of how signal power (i.e., variance) is distributed as a function of frequency, compare Footnote 150. In the case of the interbeat intervals, which have a physical unit of 1 ms, the spectral density has the unit of 1 ms2/Hz. Thus, the power in a particular frequency range is an integral of the power spectral density over the frequency range (compare Fig. 3.4a). It is important to note that an estimation of the power spectral density assumes a stationary sequence of interbeat intervals; obviously, this requirement is more valid for short sequences than long because different short-term, mid-term, and long-term regulatory mechanisms impact the stationarity of the sequence (compare Fig. 3.3a).
- 8.
The subject’s transition from supine to upright reflects the dynamic response of the autonomic nervous system and is typically used as a standard test in the HRV analysis ( = orthostatic test). When standing, the gravitational force yields a widening of compliant venous vessels of the legs, which reduces the available blood volume, venous return, and thus filling of the left side of the heart (compare Footnote 225). In order to counteract a decrease in all three stroke volume, cardiac output, blood pressure (see baroreflex, Sect. 3.2.2.1)
-
The peripheral resistance increases (through vasoconstriction)
-
The heart contractility increases (through neuronal control)
-
The level off C increases (through neuronal control); compare (2.31)
-
When standing the total power of the HRV is reduced (Fig. 3.4b, c)
The lower is the base level of f C (i.e., the higher is the prevailing PNS activity), the more pronounced and more rapid is the f C rise in response to standing up. By contrast, the higher is the base level of f C, i.e., the higher is the prevailing SNS activity, the less pronounced and more inert is the f C rise.
-
- 9.
A linear phenomena or linear system satisfies the principle of additivity and homogeneity. That is, the net response of the linear system caused by multiple stimuli is the sum of the responses caused by each stimulus separately, according to the so-called superposition principle. In other words, nonlinear measures (or features) describe nonlinear (output) signals produced by nonlinear (biological) systems.
- 10.
The most important requirements for a reliable HRV analysis are summarized below:
-
The sampling rate of the biosignal (compare Fig. 3.2) should be relatively high ( > 500 Hz) for a precise estimation of the instantaneous f C; i.e., significantly higher than used during standard electrocardiogram recordings ( ≤ 256 Hz)
-
Abnormal events as ectopic beats, arrhythmic events, and noise should be appropriately eliminated, because they are not governed by the sinoatrial node (Footnote 120)
-
The choice of fiducial point to asses interbeat intervals may be critical,
-
Either the P wave in the electrocardiogram (compare Fig. 1.15a) is the most appropriate fiducial point to assess the activity of the sinoatrial node but low in amplitude and difficult to detect
-
The prominent maximum during QRS complex after the P wave (Fig. 3.2a), or even
-
The centre of area under the QRS complex
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It should be noted that clinical and scientific practice relies mostly on the fiducial points based on the QRS complex.
-
- 11.
In the given case of Fig. 3.5a, the body circumference changes due to respiratory muscle activity are detected by respiratory belts positioned around the subject’s abdomen and chest. The extension and contraction of an elastic, tightly applied belt, relative to its initial length, is detected during inspiration and expiration, respectively. A piezoelectric transducer in the belt converts its mechanical tension into voltage which is amplified and shown in Fig. 3.5a.
- 12.
In the given case of Fig. 3.5c, the air flow through the mouth is registered by a mouthpiece connected to an airflow transducer. The transducer basically consists of a tube with a woven screen inside which acts as a flow-resisting object ( = flow resistance). During inspiration, the outer side of the flow resistance has a higher air pressure than the inner side (toward the mouth) while during expiration the reverse is true. The resulting pressure difference across the flow resistance reflects the nonzero air flow ( = pressure difference divided by flow resistance). The measured pressure difference is converted to voltage with the amplified and calibrated version being shown in Fig. 3.5c.
- 13.
In the given case of Fig. 3.7b, the temperature of the airflow is detected by a miniature thermistor close to the nostrils. During inspiration, the relatively cool air (compared to the subject’s body temperature) enters the nostrils and is warmed up in the body. During expiration, the warm air leaves the respiratory airways. The thermistor converts the air’s temperature into voltage with the amplified and calibrated version given in Fig. 3.7b.
- 14.
Sleep apnea affects the sleep of millions of individuals, with a minimum prevalence of about 1%, dominating in men (Saletu and Saletu-Zyhlarz 2001; Peter et al. 1995). For instance, 2% of women and 4% of men between 30 and 60 years have more than five obstructive apneas and hypopneas per sleep hour and accompanying daytime sleepiness (Lee-Chiong 2006), the latter drastically reducing quality of live (the definitions of apnea types follow in the text). Central apneas are reported to be less common than obstructive, i.e., they occur at about 10% the rate of obstructive (De Backer 1995).
- 15.
For instance, epidemiological studies have shown that 37% of males and 19% of females were snorers while 10% and 7%, respectively, represent potential apnea patients (Saletu and Saletu-Zyhlarz 2001).
- 16.
The body mass index is an anthropometric measure defined as human weight in kilograms divided by the square of height in meters. Usually index values over 30 kg/m2 indicate obesity.
- 17.
The relevance of the pulse can be best illustrated in words “…The pulse ranks first among our guides; no surgeon can despise its counsel, no physician shut his ears to its appeal…” (Mahomed 1872).
- 18.
The first measurement of the blood pressure is ascribed to Stephen Hales (1677–1761), an English physiologist. He pioneered quantitative measurements of the blood pressure in animals by a vertical glass tube with one end inserted into a horse artery. The rise in the column of blood in the tube was an estimate for the instantaneous blood pressure level (compare Footnote 223).
- 19.
It should be stressed that the advancement alone does not yield the waveform of the aortic pressure at the heart, for the pressure pulse waveform changes strongly during pulse propagation toward the periphery (section “Reflected Pulse Propagation” in Sect. 2); only the running time from the heart to the finger is compensated through the deliberate advancement in Fig. 3.10a.
- 20.
Different application and technical issues have to be considered if catheters are used. For instance, the inserted end of the catheter should be kept at the same vertical level as an external pressure sensor so that the hydrostatic pressure does not impact the blood pressure being measured. From a technical point of view, the catheter behaves as a low-pass filter with a resonant frequency (Neuman 2011). That is, a smaller diameter of the catheter [increasing catheter resistance to flow, (2.19)] or a larger length of the catheter (increasing fluid mass and inertia) would damp high-frequency components of the blood pressure waveform or, in engineering terms, would decrease the cut-off frequency of the low-pass filter.
- 21.
Nikolai Sergeyevich Korotkoff (1874–1920) was a Russian military surgeon who described the sounds heard over an artery below a compression cuff. The Korotkoff sounds arise at each blood pressure pulse and are due to the local turbulence in the blood flow passing through constricted opening of the artery, as long as the cuff pressure is in between p D and p S. That is, the artery temporally opens during systole and close during diastole. The turbulent blood flow moves not only in the axial direction of the vessel but also in other directions, including its radial direction, which causes mechanical vibrations of the vessel wall and thus generates acoustical sounds. When the cuff pressure is lower than p D, the artery remains open, the blood flow is nearly laminar (Sect. 2.5.2.2), i.e., all lamina of the blood move parallel to the axis of the vessel, radial vibrations of the wall are absent, and the vessel is silent.From the historical perspective, it is interesting to observe that Korotkoff was actually not looking for a method to measure the blood pressure (Geddes and Roeder 2009). He was rather interested in collateral blood circulation, which he evaluated by feeling the pulsations of a stenosed artery while he pressed down on the artery. The pulsations meant pulsatile flow of blood. Later, as a byproduct of this investigation, he introduced auscultated sounds from beyond the cuff on the upper arm, named as Korotkoff sounds, to estimate the blood pressure. Harold Nathan Segall (1897–1990), a Canadian physician, went on and showed that the auscultation can be substituted by palpation just beyond the cuff.
- 22.
It should be noted that Scipione Riva-Rocci (1863–1937), an Italian physician, significantly improved the mercury sphygmomanometer (Fig. 1.13) in terms of its easy and general use. He used a simple rubber tube as an inflatable cuff on the upper arm to constrict the brachial artery, a bulb to inflate the cuff, a glass manometer filled with mercury to measure the cuff pressure, and manual palpation of the radial pulse. The obliteration of the palpated pulse corresponded to mercury pressure equal to p S.
- 23.
In practice, the pressure under the cuff is not homogenous, with lower values at the edges of the cuff (Neuman 2011). Thus, while the pressure under the centre of the cuff is above p S, the pressure near the edges could be even lower than p S. In the latter case, the blood pressure pulse would open the artery under the edge region, increase the local limb volume, decrease the cuff volume to a small amount, and thus slightly increase the cuff pressure. In other words, cardiac pulsations in the cuff pressure begin at cuff pressures higher thanp S. In analogy, the pulsations in the cuff pressure do not disappear when the cuff pressure is lower thanp D because the limb volume changes by a small amount over the cardiac cycle. The volume and pressure of the cuff must follow these changes.
- 24.
Pierre-Simon Laplace (1749–1827) was a French mathematician, physicist, and astronomer who contributed significantly to mechanics, statistics, mathematics, and astronomy.
- 25.
Simeon Denis Poisson (1781–1840) was a French mathematician and physicist after whom the ratio was named that describes the quotient of the transverse strain and the axial strain of material under axial stress. Incompressible materials yield a Poisson’s ratio of 0.5.
- 26.
In the case of the pulse arrival time as τ, the use of the electrocardiogram wave as a timing signal provides data with much more scatter than are obtained when using an arterial pulse for timing (Geddes et al. 1981a, b). The reason for this is the large variability of the pulse arrival time between the R peak and the pulse wave arrival. This pulse arrival time consists of a portion of the time required for
-
Propagation of excitation over the ventricles
-
Period of isovolumetric contraction
-
Pulse transit time from the aortic valve to the artery at the measuring site, e.g., at the neck in Fig. 3.15
Of these times, the isovolumetric period is the most variable, showing a significant variability during changes of the blood pressure and the same order of magnitude if compared, for instance, to the pulse transit time between the aorta and the ear lobe (Franchi et al. 1996). Using the arterial pulse as a timing reference, in the case of the pulse transit time as τ, eliminates this variable (Geddes et al. 1981a); it implies also τ′′ < τ′ in Fig. 3.15. In addition, if more distal arteries, e.g., femoral instead of carotid, are used in connection with the electrocardiogram as a timing reference, then the isovolumetric period builds a smaller fraction of the arrival time and the present variations in the isovolumetric period are less significant (Geddes et al. 1981b; Sugo et al. 1999).
-
- 27.
Ultrasound is given by a mechanical vibration of particles of elastic media with a frequency above the range of human hearing ( > 20 kHz). When the particles are displaced from their equilibrium positions, internal restoration (electrostatic) forces arise between the particles, which lead to the oscillatory local motions of the particles. For instance, local oscillations in the density of tissue may be induced in the direction of ultrasound propagation, yielding the so-called compressional or longitudinal waves. It is important to note that the ultrasound waves are reflected and scattered on anatomical inhomogenities or whenever the waves encounter different acoustic impedance; specifically, anywhere there are density changes in the body.
- 28.
The Doppler effect was named after Christian Andreas Doppler (1803–1853), an Austrian mathematician and physicist, who first described the change in frequency [and wavelength, compare (2.21)] of a propagating wave as perceived by an observer moving relative to the wave’s source.
- 29.
Ultrasound imaging uses echoes from inhomogenities in the tissue, since part of the sound wave is reflected back to the ultrasound probe and detected as an echo, compare Footnote 213. The amplitude of the echo is related to the dominance of the corresponding inhomogeneity and is used to modulate the pixel brightness in the image, compare Fig. 3.16a. The time delay of the echo reflects the depth (or distance) of the inhomogeneity from the ultrasound probe and is given on the y-axis. Lastly, the location of the sound beam determines the x axis and the beam is swept over an area of interest to vary x. This type of display is often referred to as a “B-mode” display.
- 30.
The pulsed Doppler approach yields Doppler information (Footnote 214) from only a small blood sample volume, with a longitudinal size usually < 1 mm, compare Fig. 3.16a. The depth (or distance) of the sample volume from the ultrasound probe is determined by the propagation time of a short ultrasound burst (pulse), i.e., by the time from pulse emission until its reception after reflection.
- 31.
In motion mode, the so-called “M-mode,” the temporal motions of the inhomogenities within the tissue are revealed along the propagation direction of the sound beam, i.e., the motions relative to the ultrasound probe. In other words, the temporal changes of echo amplitudes are monitored along the coordinate y while x is frozen, compare Fig. 3.16a and Footnote 215. In the image of M-mode, see Fig. 3.17, the echo amplitude corresponds to the pixel brightness, with the physical depth of the inhomogenities displayed along the y-axis and the time along the horizontal t-axis.
- 32.
A single red blood cell has about 280 ⋅106 hemoglobin molecules while each molecule can bind four molecules of oxygen, see Fig. 3.18. Hemoglobin molecule includes four iron atoms in the reduced form Fe2 + , each of which combines reversibly with one oxygen molecule, forming the so-called oxyhemoglobin. Hemoglobin with released oxygen molecules is known as deoxyhemoglobin. It should also be noted that hemoglobin contributes to the transport of carbon dioxide, see Footnote 176.
- 33.
When oxygen in a gaseous state comes into contact with the blood plasma, there is a tendency for the gas to dissolve in it. At equilibrium, the resulting concentration of oxygen is equal to its partial pressure \({p}_{{\mathrm{O}}_{2}}\) in the gas times its solubility in the plasma. Likewise, the plasma is saturated with oxygen at equilibrium, whereas the amount of dissolved oxygen depends on \({p}_{{\mathrm{O}}_{2}}\) and the solubility. The level of \({p}_{{\mathrm{O}}_{2}}\) corresponds to the pressure that oxygen of a gaseous mixture would have if it alone occupied the same volume. A low amount of oxygen dissolved in the plasma is already indicated by a low solubility coefficient for oxygen in the plasma, e.g., \(10\,{\mathrm{kPa}}^{-1} \cdot \mu \mathrm{Mol/l}\) for oxygen vs. \(225\,{\mathrm{kPa}}^{-1} \cdot \mu \mathrm{Mol/l}\) for carbon dioxide (Silbernagl and Despopoulos 2007).
- 34.
Nonfunctional hemoglobins (compare Footnote 218) include, for instance, carboxyhemoglobin (combines with carbon monoxide instead of oxygen) and methemoglobin (includes iron in the Fe3 + state unable to bind oxygen), amounts of which increase in abnormal situations, e.g., in poisoning with carbon monoxide. It is important to note that these hemoglobins do not influence the results of spectrometry or oximetry, see Sect. 6, as long as their amount is relatively small (Kamat 2002; Wukitsch et al. 1988).
- 35.
To give an example, hypoxemia manifests approximately 90s later for the finger vs. the forehead in the case of peripheral vasoconstriction (Bebout et al. 2001).
- 36.
From a practical point of view, the core body temperature can be estimated by measuring axillary, oral, tympanic membrane, (superficial) temporal artery, or even rectal temperature; the latter often being considered the closest to the true core body temperature (Neuman 2010).
- 37.
From a historical perspective, it is worth noting that systematic investigations into the effects of respiration on cardiovascular function date back even to famous experiments with horses by Stephen Hales in the first half of the eighteenth century, see Footnote 204. He may have been the first to record the inspiratory fall in arterial blood pressure (Olsen et al. 1985), see section “Normal Respiration.”
- 38.
The increased return of venous blood during inspiration is actually facilitated by venous valves; thus, the blood return is increased more by inspiration than it is decreased by expiration (Footnote 117).
- 39.
This is the so-called Frank–Starling law, named after Otto Frank (1865–1944), a German physiologist, and Ernest Henry Starling (1866–1927), an English physiologist. The law states that the contraction force of the cardiac muscle is proportional to the heart volume (filling volume or end-diastolic volume), especially, to the length of the heart muscles before contraction (Footnote 104). Consequently, the end-diastolic volume of ventricle is directly proportional to the following stroke volume of this ventricle.In other words, increasing systemic venous return, e.g., during inspiration or changing fromupright to supine (gravitational force leading to an increase in the thoracic venous blood volume), increases the filling pressure of the right ventricle which leads to an increased right ventricular stroke volume and subsequently to a (delayed) increase in V S and q [compare Footnote 226 and (2.30)]. Obviously, other compensatory mechanisms such as baroreflex (Sect. 3.2.2) counteract the potential imbalance of the latter physiological parameters.
- 40.
The systemic venous return must equal q when averaged over time because the circulation system is essentially a closed loop system, see Fig. 2.39 and Footnote 225.
- 41.
The reverse thoracic pump mechanism accounts for the influence of intrapleural pressure (Sect. 2.6.2) on the arterial blood flow (Olsen et al. 1985). Factors elevating intrapleural pressure, e.g., the expiratory phase of breathing, augment intracavitary left ventricular pressure referenced to atmospheric (peripheral) pressure, i.e., augment the effective left ventricular ejection pressure ( = left ventricular pressure – atmospheric pressure). Conditions reducing intrapleural pressure, e.g., inspiratory phase, lower the left ventricular ejection pressure and thus the level of V S. It should be noted that peripheral vasculature terminates the blood flow; consequently, the atmospheric (peripheral) pressure is the appropriate reference pressure for the effective ejection pressure.Interestingly, the reverse thoracic pump or changing intrapleural pressure does not affect the right ventricular volume. During the right ventricular ejection, the right ventricle (input of the pulmonary circulation, Fig. 2.39) is in continuity with the left atrium (output), both of which are identically subjected to changes in the intrapleural pressure; i.e., to its expiratory rise and inspiratory fall.
- 42.
The thoracic pump mechanism clearly contrasts with the reverse thoracic pump (from Footnote 227). The thoracic pump refers to a blood pump to the extent that elevated intrathoracic pressure (e.g., coughing or chest compressions during cardiopulmonary resuscitation) tends to press the blood out of the pulmonary vessels into the heart and then onwards into the periphery. Therefore, the elevated intrathoracic pressure effectively increases V S. In this regard, inspiration may be referred to as a reverse thoracic pump mechanism (Olsen et al. 1985), compare Footnote 227.
- 43.
The pulsus paradoxus describes the absence of peripheral pulse despite the presence of cardiac contraction (Barach 2000; Khasnis and Lokhandwala 2002). It is due to an increased (exaggerated) amplitude of the inspiratory fall in p S by more than 10 mm Hg. Two conditions may evoke the pulsus paradoxus: large variations in intrapleural pressure (e.g., due to forced respiratory effort, severe asthma, or pulmonary embolism) or increased coupling between the right and left ventricles (e.g., acute right heart failure). Conversely, marked increases in the intrapleural pressure (e.g., coughing) may produce reversed pulsus paradoxus, i.e., the presence of a peripheral pulse in the absence of cardiac contraction.
- 44.
The sympathetic control ofR T (e.g., increase in R T) to balance the blood pressure is not likely to be involved in synchrony with the respiration cycle (e.g., inspiration), because the contraction of smooth muscles in the arterial wall needs a relatively long time to develop; compare Footnote 231.
- 45.
Here, it should be noted that multiple mechanisms of sympathetic and parasympathetic origins are involved in blood pressure control in terms of the so-called baroreflex, as will be discussed in Sect. 3.2.2.
- 46.
The Bainbridge reflex, named after Francis Arthur Bainbridge (1874–1921), an English physiologist, is related to a decrease in the efferent (parasympathetic) vagus nerve activity in response to an increased right atrial volume (or pressure), namely, in response to elongation of stretch-sensitive mechanoreceptors in the right atria. Consequently, a rise in the central venous pressure (during inspiration) or in the total blood volume (buffered mainly in the venous system, compare Sect. 2.5.1) tends to provoke an increase in f C to draw more blood out of the right atrium or to prevent the pooling of blood in the venous system, respectively.
- 47.
Interestingly, respiratory sinus arrhythmia seems to buffer respiration-synchronous fluctuations of p S in the tilted position only while these fluctuations are even reinforced by the arrhythmia in the supine position (Elstad et al. 2001).
- 48.
When different phases of cardiac cycle are corrected for varying interbeat interval, the systolic (contraction and ejection) time intervals show a stronger dependency on respiration phase than the diastolic (relaxation and filling) time intervals (Leeuwen and Kuemmell 1987). Regarding the systolic period, the left ventricular ejection time decreases during inspiration while isovolumetric contraction time (or pre-ejection period) simultaneously increases (Sect. 2.4.2). This is because V S decreases during inspiration, which shortens the ejection time and yields an earlier closure of the aortic valve but increases the pressure gradient between aortic pressure and (reduced) left ventricular pressure (increased afterload during inspiration). To overcome this relative increase in the aortic pressure before blood ejection, a longer period of pre-ejection and contraction is required. Regarding the diastolic period, a lengthening of ventricular filling phase was observed at the beginning of inspiration, which can be viewed as a compensatory effect that partly offsets the loss of V S or the loss of efficiency of the left ventricular function during inspiration (Leeuwen and Kuemmell 1987).
- 49.
Formal definitions of amplitude, frequency, and phase are given in Footnote 145.
- 50.
According to Stoohs and Guilleminault (1992), during rapid eye movement (REM) phase of sleep (Sect. 3.2.4) there is a decrease in f C during apneas while V S does not change significantly. It yields a reducedq (2.30) and a clear dissociation of f C and V S. On the other hand, nonrapid eye movement (NREM) phase shows significant V S drops during apneas with a corresponding increase in f C which actually compensates and balances q. The differing behavior of f C and V S during REM and NREM phases also demonstrates different control mechanisms of the autonomic nervous system in the course of apneas. It should be noted that REM phase normally comprises longer obstructive apneas, higher degree of desaturation, and less pronounced decreases in the intrathoracic pressure (compare reduced muscle tension in REM, Sect. 3.2.4) in comparison with NREM phase (Findley et al. 1985).
- 51.
The delay of the hemoglobin oxygendesaturation during apnea is determined by both intrinsic physiological phenomena and methodological limitations while measuring desaturation (Sect. 3.1.4). In short, the slow process of handlingoxygen (storing in hemoglobin and delivery to tissues) over an intermediate buffer (hemoglobin) facilitates this time delay. In addition, peripheral locations of sensors (e.g., on a finger, as usually chosen to measure the saturation) contribute to this time delay because of the inert blood flow from the lungs to the finger and (potentially) poor blood perfusion in the periphery.
- 52.
The chemoreceptors involved, located in the carotid sinus (Footnote 248) and aortic arch, detect levels of oxygen and carbon dioxide.
- 53.
All three, breath holding, bradycardia, and selected vasoconstriction, are usually referred to asdiving reflex. Interestingly, face immersion into cold water fortifies the diving reflex through the activation of temperature receptors in the upper airways.
- 54.
During relatively long obstructive apneas with concurrent but weak intrathoracic pressure swings in terms of inspiratory efforts, the arterial pressure may also drop during apnea (Konietzko et al. 1998). It may be due to peripheral vasodilation provoked by advanced hypoxemia.
- 55.
As shown in Bachta et al. (2009), the motion of the myocardium of the heart yields a cardiac component with f C and a superimposed respiratory component with f R. The ratio of the amplitude of the respiratory component to that of the cardiac component strongly depends on the motion direction, the ratio being the highest for motion along the interior–superior direction.
- 56.
Usually, the respiration activity derived from the electrocardiogram is referred to as EDR, i.e., electrocardiogram-derived respiration.
- 57.
From an engineering point of view, the frequency-related parameters are more robust than amplitude related. A possible impact of body movements or external noise on the electrocardiogram derived respiration is expected to be lower if frequency modulation is used instead of amplitude modulation.
- 58.
The long-term regulation of arterial blood pressure relies on specific slow hormonal and renal mechanisms which primarily affect blood volume (Silbernagl and Despopoulos 2007).
- 59.
It should be noted that end-diastolic volume (proportional to V S) is a major limiting factor of increased q when f C is elevated (2.30).
- 60.
It is interesting to note that while baroreflex controls the systemic blood pressure, i.e., increasing pressure leading to decreased f C (see text), the Bainbridge reflex (Footnote 232) controls the blood volume, i.e., increasing volume leading to increased f C.
- 61.
The baroreceptors comprise two types of nerve fibers, myelinated fibers for a rapid impulse conduction, the so-called A-delta fibres, and unmyelinated C fibres for a relatively slow impulse conduction, compare Sect. 2.1.4.1. In fact, the A-delta fibres mainly transmit rapid changes of blood pressure while the C fibres the (relatively) slowly changing mean arterial pressure. Interestingly, the efferent vagus nerve is more strongly controlled by the C fibres in comparison with the A-delta fibres (Zemaityte 1997) which seems to confirm the relevance of the mean arterial pressure control, compare section “Normal Respiration” under Sect. 3.2.1.1.
- 62.
The carotid sinus refers to the dilation of the internal carotid artery close to the bifurcation of external and internal carotids, compare Fig. 2.40.
- 63.
Sleep has always been a mystery and a source of inspiration for all artists and researchers throughout the centuries (Fig. 3.46). According to the words of Derek Walcott “All-humbling sleep, whose peace is sweet as death, whose silence has all the sea’s weight and volubility, who swings this globe by a hair’s trembling breath (Hamner 1997).”
- 64.
As reviewed in Shepard et al. (2005), epidemiologic data on more than one million subjects have shown that mortality rates were significantly increased for subjects reporting less than 4 h or more than 10 h of sleep per night. The consistency of manifold reports suggests that deviations in sleep duration from the norm, i.e., insufficient or excessive sleep, may adversely influence human longevity.
- 65.
The electroencephalogram refers to the recording of the brain’s spontaneous electrical activity by the use of multiple scalp electrodes. The potential differences are registered which arise because of equalizing currents in the extracellular space during neuronal activity (compare Sect. 2.1.4 and Sect. 4). The more neurons that are timely synchronized in their firing, the higher is the amplitude of the electroencephalogram; the contribution of a single neuron activity to the electroencephalogram is very small because of a strong signal attenuation in the tissue, scalp, and skin. Usually the course of the electroencephalogram exhibits rhythmic behavior, which justifies an establishment of predefined frequency bands being particularly relevant for sleep staging. That is, the so-called
-
Beta waves correspond to rhythmic electroencephalogram behavior in the range of 13–30 Hz reflecting an active or busy human state.
-
Alpha waves are in the range of 8–13 Hz showing relaxation or closed eyes.
-
Theta waves fill the range of 4–8 Hz and indicate drowsiness.
-
Delta waves comprise the range up to 4 Hz usually denoting sleep.
The number of electrodes ranges from two, e.g., only for discrimination in between wake and sleep, up to a few hundred for research purposes. Usually the application of the scalp electrodes is based on the international 10–20 system using letter–number designation (Malmivuo and Plonsey 1995). The 10 and 20 refer to percentage distances from reference points or neighbouring electrodes. The reference points are the nasion, the point between the forehead and the nose, and the inion, the lowest point of the skull on the midline. From these points, the skull perimeters are measured in the transverse and median planes with the electrodes being placed at the distances 10%-20%-20%-20%-20%-10% along both perimeters. The letter of the electrode position designates the anatomical area, whereas the odd numbered electrodes are placed on the left and the even numbered electrodes on the right. In total 21 electrodes are applied, including two on both ear lobes (A1 and A2). For instance, a derivation C4-A1 denotes the potential difference between the central region to the right in the frontal plane and the left ear lobe, whereas C3-A2 denotes the potential difference between the central region to the left in the frontal plane and the right ear lobe (compare Fig. 3.47).
-
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Kaniusas, E. (2012). Physiological Phenomena and Biosignals. In: Biomedical Signals and Sensors I. Biological and Medical Physics, Biomedical Engineering. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-24843-6_3
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