Boris Khanokh

The variability of the photoplethysmographic signal--a potential method for the evaluation of the autonomic nervous system.

The heart rate variability is composed of low- and high-frequency fluctuations, which are mediated by the sympathetic and the parasympathetic nervous systems. The baseline and the amplitude of the photoplethysmographic (PPG) signal also show fluctuations in the same frequencies. In the current study, PPG examinations were performed on the fingers of normal subjects and diabetic patients, and three parameters were derived from each PPG pulse: the baseline of the pulse, its amplitude and its period (which is equal to the heart period). The level of the variability of each PPG pulse parameter was measured by the ratio of the standard deviation of the parameter to its mean value. The level of the low-frequency fluctuations for the PPG amplitude and for the heart cycle period did not differ between males and females, but was lower for diabetic patients, indicating lower activity of the autonomic nervous system. The curves of the baseline and the amplitude of the PPG signal for the non-diabetic subjects showed high correlation between the left and the right hands. For most of the diabetic patients the right-left correlation coefficients were significantly lower than those for the non-diabetic subjects. Our initial results have shown that the variability of the PPG parameters shows promise for the assessment of the function of the autonomic nervous system.

The difference in pulse transit time to the toe and finger measured by photoplethysmography

Blood pressure pulse wave velocity (PWV) is a parameter which is related to arterial distensibility. Its direct assessment, by measuring the appearance time of a pressure pulse in two sites along an artery and the distance between the two sites, is complicated and inaccurate. In the current study, pulse transit time (PTT) to the toes and fingers of 44 normotensive male subjects was measured by photoplethysmography (PPG) and ECG. The arrival time of the pulses at the toe and finger was determined from the foot of the systolic rise of the PPG signal, i.e. at end-diastolic time. Two parameters, which are related to PWV, were tested: the time delay between the ECG R-wave and the arrival time of the pulses at the toe (E-T PTT), and the difference in the transit time of the blood pressure pulses between the toe and finger (T-F PTTD). E-T PTT and T-F PTTD decreased as functions of the subjects age and systolic blood pressure (SBP), but their dependence on the diastolic blood pressure (DBP) was not statistically significant. The decrease of the PTT parameters with age is attributed to the direct structural decrease of the arterial compliance with age and not to functional effects associated with the increase of the blood pressure with age, since the PTT parameters did not depend on DBP though the measurements were performed at end-diastole.

Measurement of oxygen saturation in venous blood by dynamic near infrared spectroscopy

A method for the measurement of oxygen saturation in the venous blood, SvO2, based on optical measurements of light absorption in the infrared region is presented. The method consists of applying relatively low external pressure of 25 mm Hg on the forearm, thereby increasing the venous blood volume in the tissue, and comparing the light absorption before and after the external pressure application. SvO2 has been determined from light absorption measurements in two wavelengths, before and after the pressure application, using a formula derived for two adjacent wavelengths. The method has been applied to the hands and fingers of 17 healthy male subjects, using wavelengths of 767 and 811 nm. SaO2, the oxygen saturation for arterial blood, was also obtained from photoplethysmographic measurements in these two wavelengths (pulse oximetry) using the same formula. The mean (+/- SD) value of SaO2 was 94.5% (+/- 3.0). The mean value of SvO2 was 86.2% (+/- 4.1) for the finger and 80.0% (+/- 8.2) for the hand. These SvO2 values are reasonable for the finger and the hand where arterio-venous anastomoses exist. The method enables the measurement of SvO2 in the limbs, a parameter which is related to tissue blood flow and oxygen consumption.

FIBER OPTIC SENSOR FOR THE MEASUREMENT OF RESPIRATORY CHEST CIRCUMFERENCE CHANGES

A fiber optic sensor for the measurement of the respiratory depth has been developed. The sensor is composed of a bent optic fiber which is connected to an elastic section of a chest belt so that its radius of curvature changes during respiration due to respiratory chest circumference changes (RCCC). The measurement of light transmission through the bent fiber provides information on its changes in curvature since a higher fraction of light escapes through the core-cladding surface of a fiber bent to a lower radius of curvature. The sensor can quantitatively measure the RCCC, although in relative terms, and it is sensitive enough to detect changes of the chest circumference due to the heart beat. Measurements of the RCCC were simultaneously performed with photoplethysmography (PPG)-the measurement by light absorption of the cardiac induced blood volume changes in the tissue-and a significant correlation was found between the RCCC and some parameters of the PPG signal. The fiber optic respiratory depth sensor enables a quantitative assessment of the respiratory induced changes in the cardiovascular parameters.

Verylow frequency variability in arterial blood pressure and blood volume pulse

Several parameters of the cardiovascular system fluctuate spontaenously owing to the activity of the autonomic nervous system. In the study, the simultaneous very low frequency (VLF) fluctuations of the arterial blood pressure, the tissue blood content and the tissue blood volume pulse are investigated. The latter two parameters are derived from the baseline BL and the amplitude AM of the photoplethysmographic (PPG) signal, measured on the fingertips of 20 healthy male subjects: the changes in the PPG parameters AM and BV, defined by BV=const.-BL, are related to the change in the tissue blood volume pulse and the total tissue blood volume, respectively. The VLF fluctuations in BV and AM are directly correlated, those of AM preceding those of BV by 4–13 heart-beats. The VLF fluctuations in the systolic (SBP) and the diastolic (DBP) blood pressure are inversely correlated to those of AM and BV, those of AM preceding those of SBP and lagging behing those of DBP by about one heart-beat. For most subjects, the period P of the PPG pulse, which is equal to the cardiac cycle period, directly correlates with AM and BV and inversely correlates with DBP and SBP. On average, the fluctuations fluctuations in tissue blood volume, systolic blood volume pulse, diastolic and systolic blood pressure, and heart period, together with their interrelationship, can provide a better understanding of the autonomic nervous control of the peripheral circulation and a potential tool for the evaluation of its function.

Right-left correlation of the sympathetically induced fluctuations of photoplethysmographic signal in diabetic and non-diabetic subjects

Photoplethysmography (PPG) records the cardiac-induced changes in tissue blood volume by light-transmission measurements. The baseline and amplitude of the PPG signal show very low-frequency (VLF) spontaneous fluctuations, which are mediated by the sympathetic nervous system, and high correlation between right and left extremities of healthy subjects. As sympathetic neuropathy is one of the diabetic complications, the right-left correlation of the PPG fluctuations was examined in diabetic patients. The PPG signal was simultaneously measured in the two index fingers and the two second toes of 35 diabetic patients and 33 non-diabetic subjects. For each PPG pulse, the baseline and amplitude were determined, and the right-left correlation coefficients of the VLF fluctuations in the baseline and amplitude were derived. The VLF fluctuations in the baseline showed high right-left correlation, both for fingers (0.93±0.05) and toes (0.93±0.06), for the non-diabetic subjects, and significantly lower correlation (0.78±0.22 and 0.84±0.17, respectively) for the diabetic patients. Similar results were obtained for the amplitude VLF fluctuations. The right-left correlation coefficients for diabetic patients decreased with the disease duration for the toe baseline and toe amplitude fluctuations and correlated with heart rate response to deep breathing for the finger baseline and toe amplitude fluctuations. The right-left correlation coefficients of the PPG fluctuations provide a simple and convenient means for assessing the adequacy of the sympathetic nervous system function.

Measurement of the variability of the skin blood volume using dynamic spectroscopy

Abstract The light incident on the skin is subject to scattering and absorption by the tissue itself and by the blood. The measurement of the transmitted or reflected light from the tissue — photoplethysmography — shows oscillation in the heart rate due to tissue blood volume increase during the systolic heart contraction period. In the current study, two parameters were derived from the photoplethysmographic signal: BV which is related to the total absorbed light in the tissue and AM, the amplitude of the pulsatile signal, which is related to the systolic blood volume increase. Both parameters fluctuate due to the activity of the autonomic nervous system, and in particular fluctuate in low frequency — below 0.05 Hz which is attributed to the sympathetic nervous system activity. The spontaneous fluctuations in BV and AM are more correlated for infrared than for red light, due to difference between their absorption constants for oxygenated and reduced hemoglobin. The dynamic spectroscopy of the skin provides information on the autonomic nervous regulation of the tissue blood flow.

Infrared radiometry of thermally insulated skin for the assessment of skin blood flow

The temperature increase of thermally insulated skin provides useful information about its blood flow and the blood temperature. The measurement of skin temperature by a contact thermometer, such as a thermistor, is not accurate, because it depends on the pressure exerted on the skin by the thermometer. In order to have reproducible measurement of the skin temperature, noncontact temperature measurement is preferable. Suitable insulation is achieved by using a cylinder of lowthermal-conductivity material, covered by polyethylene layers, which is applied on the skin. The polyethylene layers permit partial transmission of the infrared radiation through it. Preliminary results show that both the transient and the steady-state temperature increase can be obtained from measurements of radiation transmitted through the thermal insulation that was applied to the skin, and that the steady-state temperature increase is more closely related to tissue blood flow than the uncovered-skin temperature is.

Simultaneous Measurement of the Photoplethysmographic Signal Variability in the Right and Left Hands

The photoplethysmographic (PPG) signal, which is a measure of the systolic oscillations in the tissue blood volume, spontaneously fluctuates in the respiration rate and in lower frequencies similar to the heart rate fluctuations. In the current study, the low frequency fluctuations, which are attributed to the sympathetic nervous system activity, were obtained from PPG examinations simultaneously performed on the right and the left hands of healthy subjects and hemiplegic patients. For normal subjects, the fluctuations in the two hands were found to be correlated, demonstrating their central origin. For some of the hemiplegic patients, the coefficient of correlation between the right and the left hands was significantly lower than the normal range. The simultaneous measurement of the PPG signal fluctuations in both hands provides information on the adequate function of the autonomic nervous system.

Low Frequency Spontaneous Fluctuations in Tissue Blood Volume in Neonates

Low frequency spontaneous fluctuations in tissue blood volume (BV) which originate from the activity of the autonomic nervous system were studied in the toes of healthy adults and in the feet of preterm and full-term neonates. Fluctuations of cardiac-induced blood volume changes (AM) were also investigated using photoplethysmography, the measurement of changes in light absorption by tissue and its constituents. Both fluctuations showed right-left correlations, which indicates mediation of central origin. The average right-left correlation coefficient for adults was significantly higher than that for neonates, probably due to incomplete maturation of the autonomic nervous system for some of the neonates.