Photoplethysmography and its application in clinical physiological measurement

Comparison of reflected green light and infrared photoplethysmography

We evaluated the accuracy of pulse rate measurements obtained by reflected green light photoplethysmography (PPG) compared to reflected infrared light photoplethysmography and ECG. The wavelengths of the green and infrared light were 525 and 880 nm, respectively, and experiments were performed at 25 degrees C and at a skin temperature below 15 degrees C. The pulse rate obtained from reflected green light PPG was compared with the ECG RR interval and the pulse rate from reflected infrared PPG. The results indicated a stronger correlation between green PPG and ECG results at both temperatures. These results suggested that reflected green light PPG had an advantage over reflected infrared PPG, especially at temperatures below 15 degrees C.

Pulse transit times to the capillary bed evaluated by laser Doppler flowmetry

The pulse transit time (PTT) of a wave over a specified distance along a blood vessel provides a simple non-invasive index that can be used for the evaluation of arterial distensibility. Current methods of measuring the PTT determine the propagation times of pulses only in the larger arteries. We have evaluated the pulse arrival time (PAT) to the capillary bed, through the microcirculation, and have investigated its relationship to the arterial PAT to a fingertip. To do so, we detected cardiac-induced pulse waves in skin microcirculation using laser Doppler flowmetry (LDF). Using the ECG as a reference, PATs to the microcirculation were measured on the four extremities of 108 healthy subjects. Simultaneously, PATs to the radial artery of the left index finger were obtained from blood pressure recordings using a piezoelectric sensor. Both PATs correlate in similar ways with heart rate and age. That to the microcirculation is shown to be sensitive to local changes in skin perfusion induced by cooling. We introduce a measure for the PTT through the microcirculation. We conclude that a combination of LDF and pressure measurements enables simultaneous characterization of the states of the macro and microvasculature. Information about the microcirculation, including an assessment of endothelial function, may be obtained from the responses to perturbations in skin perfusion, such as temperature stress or vasoactive substances.

Multimodal signal processing for the analysis of cardiovascular variability

Cardiovascular (CV) variability as a primary vital sign carrying information about CV regulation systems is reviewed by pointing out the role of the main rhythms and the various control and functional systems involved. The high complexity of the addressed phenomena fosters a multimodal approach that relies on data analysis models and deals with the ongoing interactions of many signals at a time. The importance of closed-loop identification and causal analysis is remarked upon and basic properties, application conditions and methods are recalled. The need of further integration of CV signals relevant to peripheral and systemic haemodynamics, respiratory mechanics, neural afferent and efferent pathways is also stressed.

Remote plethysmographic imaging using ambient light

Plethysmographic signals were measured remotely (> 1m) using ambient light and a simple consumer level digital camera in movie mode. Heart and respiration rates could be quantified up to several harmonics. Although the green channel featuring the strongest plethysmographic signal, corresponding to an absorption peak by (oxy-) hemoglobin, the red and blue channels also contained plethysmographic information. The results show that ambient light photo-plethysmography may be useful for medical purposes such as characterization of vascular skin lesions (e.g., port wine stains) and remote sensing of vital signs (e.g., heart and respiration rates) for triage or sports purposes.

Differential response of peripheral arterial compliance-related indices to a vasoconstrictive stimulus

Peripheral arterial elastic properties are greatly affected by cardiovascular as well as other pathologies, and their assessment can provide useful diagnostic indicators. The photoplethysmographic technique can provide finger blood volume and pressure waveforms non-invasively, which can then be processed statically or beat-to-beat to characterize parameters of the vessel wall mechanics. We employ an occlusion-deflation protocol in 48 healthy volunteers to study peripheral artery compliance-related indices over positive and negative transmural pressure values as well as under the influence of a valid vasoconstrictor (cigarette smoking). We calculate beat-to-beat indices (compliance index CI, distensibility index DI, three viscoelastic model parameters (compliance C, viscosity R and inertia L), pressure-volume loop areas A and damping factor DF as well as symmetrical (C(max)) and asymmetrical (C(A)(max)) static compliance estimates, and their distributions over transmural pressure. All distributions are bell-shaped and centred on negative transmural pressure values. Distribution heights were significantly lower in the smoking group (w.r.t. the non-smoking group) for C, CI, DI and significantly higher in R and DF. The estimated volume signal time lag was also significantly lower in the smoking group. Left and right distribution widths were significantly different in all parameters/groups but DI (both groups), C(A)(max), A (smoking group) and L (non-smoking group), and positions of maxima/minima were significantly altered in C(A)(max), R and DF. C, DF and CI are seen to be most sensitive under this protocol, while C(max) and C(A)(max) are seen to be insensitive. These quantities provide complementary, time- and transmural pressure-dependent information about arterial wall mechanics, and the choice of index should depend on the physiological conditions at hand as well as relevant time resolution and transmural pressure range.

Hemodynamic sensing using subcutaneous photoplethysmography

Pacemakers and implantable defibrillators presently operate without access to hemodynamic information. If available, such data would allow tailoring of delivered therapy according to perfusion status, optimization of device function, and enhancement of disease monitoring and management. A candidate method for hemodynamic sensing in these devices is photoplethysmography (PPG), which uses light to noninvasively detect changes in blood volume. The present study tested the hypotheses that PPG can function in a subcutaneous location, that the acute changes in blood volume it detects are directly proportional to changes in arterial pressure, and that optimum pacing intervals identified by it are concordant with those determined by arterial pressure. Aortic pressure and PPG were simultaneously recorded in 10 dogs under general anesthesia during changes in atrioventricular (AV) delay and bursts of rapid pacing to simulate tachyarrhythmias. Direct proportionality between transient changes in pressure and PPG waveforms was tested using regression analysis. Scatter plots had a linear appearance, with correlation coefficients of 0.95 (SD 0.03) and 0.72 (SD 0.24) for rapid-pacing and AV delay protocols, respectively. The data were well described by a directly proportional relationship. Optimum AV delays estimated from the induced changes in aortic pressure and PPG waveforms were concordant. This preliminary canine study demonstrates that PPG can function subcutaneously and that it may serve as a surrogate for acute changes in arterial pressure.

Discrimination of sleep-apnea-related decreases in the amplitude fluctuations of PPG signal in children by HRV analysis

In this paper, an analysis of heart rate variability (HRV) during decreases in the amplitude fluctuations of photopletysmography (PPG) [decreases in the amplitude fluctuations of photopletysmography (DAP)] events for obstructive sleep apnea syndrome (OSAS) screening is presented. Two hundred and sixty-eight selected signal segments around the DAP event were extracted and classified in five groups depending on SaO (2) and respiratory behavior. Four windows around each DAP are defined and temporal evolution of time-frequency HRV parameters was analyzed for OSAS screening. Results show a significant increase in sympathetic activity during DAP events, which is higher in cases associated with apnea. DAP events were classified as apneic or nonapneic using a linear discriminant analysis from the HRV indexes. The ratio of DAP events per hour r(DAP) and the ratio of apneic DAP events per hour r(DAP)(a) were computed. Results show an accuracy of 79% for r(DAP)(a) (12% increase with respect to r(DAP)), a sensitivity of 87.5%, and a specificity of 71.4% when classifying 1-h polysomnographic excerpts. As for clinical subject classification, an accuracy of 80% (improvement of 6.7% ), a sensitivity of 87.5%, and a specificity of 71.4% are reached. These results suggest that the combination of DAP and HRV could be an improved alternative for sleep apnea screening from PPG with the added benefit of its low cost and simplicity.

Determination of aortic valve opening time and left ventricular peak filling rate from the peripheral pulse amplitude in patients with ectopic beats

Ectopic beats are common in patients who have heart disease and are associated with reduced peripheral pulse amplitude. This study determined the start of the peripheral pulse increase and from it the opening of the aortic valve. The left ventricular peak filling rate was also estimated from the peripheral pulse. Results were compared with published invasive and cardiac imaging data. Twenty-five subjects with ectopic beat electrocardiograms (ECGs) were studied. The ECGs and the peripheral pulses, detected optically at the right index finger by a simple photoplethysmography (PPG) technique, were recorded for subsequent analysis. Peripheral pulse amplitudes for ectopic beats, post-ectopic sinus beats and normal sinus beats were determined. Ectopic beats induced a mean 68% decrease in pulse amplitude in comparison with sinus beats (p < 0.001). In contrast, the mean pulse amplitude for post-ectopic sinus beats increased by 20% (p < 0.01). Pulse amplitude changes were comparable with the published stroke volume differences for ectopic beats and post-ectopic sinus beats. The range of shortest coupling interval (CI) for ectopic beats with observable pulses was from 373 to 531 ms, with the mean value equivalent to 55% of the mean sinus RR interval, comparable with the opening of the aortic valve. Finally, as the CI increased, the pulse amplitude increased quickly from zero. The average rate of increase was equivalent to 4.8 times the normal sinus amplitude in 1 s, equal to 50% filling in 208 ms, showing diastolic rapid filling, comparable with published left ventricular peak filling rate data. In conclusion, the effect of ectopic beat CI on peripheral pulse amplitude has been determined, providing useful information for developing a technique to determine the opening of the aortic valve and the peak filling rate non-invasively and peripherally in patients with frequent ectopic beats.

Finger photoplethysmogram pulse amplitude changes induced by flow-mediated dilation

This study was conducted to investigate the utility and efficacy of finger photoplethysmogram pulse amplitude (PPG-AC) in comparison with the standard Doppler ultrasound in assessing an endothelial function via flow-mediated dilation (FMD). High-resolution B-mode scanning of the right brachial artery (BA) of 31 healthy subjects aged 39.7 +/- 11.3 (range 22-64) years and 52 risk subjects aged 47.7 +/- 10.8 (range 30-65) years were performed before and after 4 min of upper arm occlusion. Concurrent with the ultrasound measurement (where color Doppler imaging was used to enhance arterial boundary detection), PPG signals were recorded from both index fingers for cross evaluation and comparison. Our results show that the finger PPG-AC exhibits a similar response to that of the well-known BA dilation: following the release of pressure (cuff around the BA), the PPG-AC increases abruptly before slowly decreasing toward the baseline. The peak PPG-AC is reached significantly faster than the peak FMD measured by ultrasound among healthy and risk groups (P < 0.001). The proposed technique using a finger photoplethysmogram can be applied in a rapid and non-invasive assessment of peripheral vascular functions as an alternative low-cost and less operator-dependent tool compared to ultrasound.

Photoplethysmographic assessment of hemodynamic variations using pulsatile tissue blood volume

Sympathetic responses to provocative tests have shown to provide an early prognosis of abnormalities in the human autonomic nervous system. Photoplethysmographic signal characteristics have been studied to identify vascular diseases. However, knowledge about the pulse-added arterial volume of the photoplethysmographic waveforms during these clinical investigations is limited. In all, 16 normotensive adults (9 men) were recruited to perform 3 provocative test (2 postural changes and 1 resistive breathing) activities with photoplethysmographic signals being monitored on the upper and lower peripheries using customized devices. In all, 4 parameters derived from the photoplethysmographic waveforms that related to the pulsatile tissue blood volume changes were then assessed. The results obtained showed that amplitude-related parameters during these activities showed significant changes (>9.63%; P < .05). However, comparison of pulse-added arterial volume showed insignificant changes (<5.56%; P > .05) for all test settings. Hence, the findings herein suggest that there is clinical potential in using this aspect of the photoplethysmographic waveform.

Noninvasive respiratory monitoring system based on the piezoceramic transducer's pyroelectric effect

This paper presents a simple alternative method and system for noninvasive respiratory airflow monitoring. The proposed system uses a piezoceramic transducer to measure respiratory airflow. When a piezoceramic transducer is impacted by respiratory airflow, there is a piezoelectric and a pyroelectric response to pressure and thermal airflow fluctuations. In this study, the selected transducer's response output is dominated by the pyroelectricity factor. Therefore, the piezoelectric effect is not significant and can be ignored in this study. Using the transducer's pyroelectricity to measure thermal flow variations, a subject's respiratory rate and respiratory air volumetric flow rate can be monitored. The proposed system was evaluated for accuracy and response time using quiet and postphysical exertion breathing modes. Using the pneumotach system as a benchmark, the proposed system's respiratory rate measurement accuracy for the two breathing modes is approximately 98.78%. In addition, the proposed system's output voltage is highly correlated with the respiratory volumetric flow rate measured by the selected pneumotach (r2=0.9783). The average correlation coefficient between the pneumotach system's output waveform and the proposed system is approximately 0.9389. Moreover, the proposed system and the selected pneumotach have almost the same rapid response time to respiratory airflow. When compared to a temperature measurement thermistor system, the thermistor on average is approximately 25.3 ms slower than the proposed system. Furthermore, compared to the selected screen-type pneumotach system, the proposed system simplifies the respiration monitoring requirements. Instead of sensing the pressure drop across a mesh screen, like the screen-type pneumotach, it measures respiration at one point within the respiratory airflow. The proposed system benefits from simplified processing circuits and a mesh-free design. The advantages of this new respiratory airflow measurement method are fast response time, high accuracy, low cost, and ease of implementation.

Detection of obstructive sleep apnea in children using decreases in the amplitude fluctuations of PPG signal and HRV

An analysis of the HRV during decreases in the amplitude fluctuations of PPG (DAP) events, and their utility in OSAS screening is presented. The overall data set used in the study includes the polysomnographic records of 21 children. DAP events were automatically detected by an algorithm based on the envelope attenuations of the PPG. DAP events were classified as apneic or non apneic by a linear discriminant analysis. The features used by the linear discriminant come from the temporal and spectral parameters of the heart rate obtained by Smooth Pseudo Wigner Ville Distribution. Two indexes were defined: the number of DAP events per hour ratio r(DAP) and the number of apneic DAP events per hour ratio r(DAP)(alpha). Results show a 12% increase in accuracy for r(DAP)(alpha) with respect to r(DAP) in classifying 1 hour polysomnographic segments, reaching values of 72.7% and 80% for sensitivity and specificity, respectively. As for subject classification, the improvement in accuracy is 6.7% obtaining values of 87.5% and 71.4% for sensitivity and specificity respectively. These results suggest that the combination of DAP and HRV could be an alternative for sleep apnea screening with the added benefit of low cost and simplicity.

Reduction of error in spectrophotometry of scattering media using polarization techniques

Scattering can result in erroneous determination of the concentrations of constituent absorbers in spectrophotometry. This is due to the relationship between attenuation and absorption coefficient becoming nonlinear; hence, the use of the Lambert-Beer law becomes invalid. It has previously been shown that application of polarization techniques can reduce these effects, resulting in a more linear relationship. Here we quantify the impact of this improvement on measurement of the ratio of concentrations for two general absorbing species and show that measurement using polarization-maintaining light is more accurate. This is performed using a generalized version of theory previously dependent on selection of isosbestic wavelengths. For the absorbing species and geometries considered here, the mean error on the estimation of absorber concentration ratio is 18.2% for the case of detection without polarization discrimination. When polarization-maintaining light is extracted, mean errors of 1.2% and 5.1% are achieved for linear and circular polarizations, respectively. The improvement provided by the polarization techniques is observed regardless of the illuminating wavelengths but is achieved at the expense of a reduced signal-to-noise ratio. Taking this into account, for the detection scheme considered with a detector well capacity of 4 x 10(5) electrons the improvement provided by linear polarization-maintaining light is reduced to a factor of 3.6 and for circular polarizations a factor of 2.2.

Development of a temperature-controlled miniature enclosure for monitoring poor perfusion photoplethysmographic signals

A major issue in most optical-based physiological measurements is the effect of temperature fluctuations on the reliability of the acquired data. Studies have suggested that waveforms obtained from the photoplethysmography (PPG) method are temperature sensitive. The conventional approach to tackle these temperature induced variations is to post-signal process the waveforms in the hope of optimizing them to a minimum level for signal pattern and recognition. However, it is known such an approach may alter the PPG characteristics which can be used to identify the pathogenesis and/or pathophysiology of peripheral vascular diseases. Thus, appropriate temperature settings can then facilitate prolonged comparative clinical studies and minimize pathogenic survivability. In this study, a miniature enclosure with a temperature-control feature is proposed to enhance PPG acquisition in a more consistent environment. Eight healthy adults (six male; 26.4 +/- 3.0 yr) were recruited to perform three different test trials to assess the usefulness of the proposed system. The obtained results indicated that without the activation of the temperature-control feature, PPG characteristics acquired from cold fingers can be compromised. Conversely, improvements can be observed when the temperature feature was activated. Hence, the findings herein suggest that this system can be a valuable tool in acquiring PPG waveforms from poorly perfused fingers.

Multilaser photoplethysmography technique

New technique for parallel recording of reflection photoplethysmography signals in broad spectral band (violet to NIR) has been developed based on fiber-coupled laser irradiation and time-resolved spectrometric detection. Differences in photoplethysmography waveforms that were recorded simultaneously at different wavelengths confirmed the depth variety of the skin blood pulsation dynamics, thus the proposed methodology has a potential for application in skin microcirculation studies.