Measurement of the left ventricular ejection time by digital plethysmography

Pulse wave travel distance as a novel marker of ventricular-arterial coupling

Each stroke volume ejected by the heart is distributed along the arterial system as a pressure waveform. How far the front of the pressure waveform travels within the arterial system depends both on the pulse wave velocity (PWV) and the ejection time (ET). We tested the hypothesis that ET and PWV are coupled together, in order to produce a pulse wave travel distance (PWTD = PWV × ET) which would match the distance from the heart to the most distant site in the arterial system. The study was conducted in 11 healthy volunteers. We recorded lead II of the ECG along with pulse plethysmography at ear, finger and toe. The ET at the ear and pulse arrival time to each peripheral site were extracted. We then calculated PWV followed by PWTD for each location. Taken into account the individual subject variability PWTD_Toe in the supine position was 153 cm (95% CI 146-160 cm). It was not different from arterial pathway distance from the heart to the toe (D _Toe 153 cm). The PWTD_Finger and PWTD_Ear were longer than the distance from the heart to the finger and ear irrespective of body position. ET_Ear and PWV_Toe appear to be coupled in healthy subjects to produce a PWTD that is roughly equivalent to the arterial pathway distance to the toe. We propose that PWTD should be evaluated further to test its potential as a noninvasive parameter of ventricular-arterial coupling in subjects with cardiovascular diseases.

Frequency analysis of photoplethysmogram and its derivatives

There are a limited number of studies on heat stress dynamics during exercise using the photoplethysmogram (PPG). We investigate the PPG signal and its derivatives for heat stress assessment using Welch (non-parametric) and autoregressive (parametric) spectral estimation methods. The preliminary results of this study indicate that applying the first and second derivatives to PPG waveforms is useful for determining heat stress level using 20-s recordings. Interestingly, Welch's and Yule-Walker's methods in agreement that the second derivative is an improved detector for heat stress. In fact, both spectral estimation methods showed a clear separation in the frequency domain between measurements before and after simulated heat-stress induction when the second derivative is applied. Moreover, the results demonstrate superior performance of the Welch's method over the Yule-Walker's method in separating before and after the three simulated heat-stress inductions.

Towards Investigating Global Warming Impact on Human Health Using Derivatives of Photoplethysmogram Signals

Recent clinical studies show that the contour of the photoplethysmogram (PPG) wave contains valuable information for characterizing cardiovascular activity. However, analyzing the PPG wave contour is difficult; therefore, researchers have applied first or higher order derivatives to emphasize and conveniently quantify subtle changes in the filtered PPG contour. Our hypothesis is that analyzing the whole PPG recording rather than each PPG wave contour or on a beat-by-beat basis can detect heat-stressed subjects and that, consequently, we will be able to investigate the impact of global warming on human health. Here, we explore the most suitable derivative order for heat stress assessment based on the energy and entropy of the whole PPG recording. The results of our study indicate that the use of the entropy of the seventh derivative of the filtered PPG signal shows promising results in detecting heat stress using 20-second recordings, with an overall accuracy of 71.6%. Moreover, the combination of the entropy of the seventh derivative of the filtered PPG signal with the root mean square of successive differences, or RMSSD (a traditional heart rate variability index of heat stress), improved the detection of heat stress to 88.9% accuracy.

Digital Photoplethysmography for Assessment of Arterial Stiffness: Repeatability and Comparison with Applanation Tonometry

Introduction

Arterial stiffness is an independent risk factor for cardiovascular morbidity and can be assessed by applanation tonometry by measuring pulse wave velocity (PWV) and augmentation index (AIX) by pressure pulse wave analysis (PWA). As an inexpensive and operator independent alternative, photoelectric plethysmography (PPG) has been introduced with analysis of the digital volume pulse wave (DPA) and its second derivatives of wave reflections.

Objective

The objective was to investigate the repeatability of arterial stiffness parameters measured by digital pulse wave analysis (DPA) and the associations to applanation tonometry parameters.

Methods and Results

112 pregnant and non-pregnant individuals of different ages and genders were examined with SphygmoCor arterial wall tonometry and Meridian DPA finger photoplethysmography. Coefficients of repeatability, Bland-Altman plots, intraclass correlation coefficients and correlations to heart rate (HR) and body height were calculated for DPA variables, and the DPA variables were compared to tonometry variables left ventricular ejection time (LVET), PWV and AIX. No DPA variable showed any systematic measurement error or excellent repeatability, but dicrotic index (DI), dicrotic dilatation index (DDI), cardiac ejection elasticity index (EEI), aging index (AI) and second derivatives of the crude pulse wave curve, b/a and e/a, showed good repeatability. Overall, the correlations to AIX were better than to PWV, with correlations coefficients >0.70 for EEI, AI and b/a. Considering the level of repeatability and the correlations to tonometry, the overall best DPA parameters were EEI, AI and b/a. The two pansystolic time parameters, ejection time compensated (ETc) by DPA and LVET by tonometry, showed a significant but weak correlation.

Conclusion

For estimation of the LV function, ETc, EEI and b/a are suitable, for large artery stiffness EEI, and for small arteries DI and DDI. The only global parameter, AI, showed a high repeatability and the overall best correlations with AIX and PWV.

Detection of c, d, and e waves in the acceleration photoplethysmogram

Analyzing the acceleration photoplethysmogram (APG) is becoming increasingly important for diagnosis. However, processing an APG signal is challenging, especially if the goal is to detect its small components (c, d, and e waves). Accurate detection of c, d, and e waves is an important first step for any clinical analysis of APG signals. In this paper, a novel algorithm that can detect c, d, and e waves simultaneously in APG signals of healthy subjects that have low amplitude waves, contain fast rhythm heart beats, and suffer from non-stationary effects was developed. The performance of the proposed method was tested on 27 records collected during rest, resulting in 97.39% sensitivity and 99.82% positive predictivity.

Systolic peak detection in acceleration photoplethysmograms measured from emergency responders in tropical conditions

Photoplethysmogram (PPG) monitoring is not only essential for critically ill patients in hospitals or at home, but also for those undergoing exercise testing. However, processing PPG signals measured after exercise is challenging, especially if the environment is hot and humid. In this paper, we propose a novel algorithm that can detect systolic peaks under challenging conditions, as in the case of emergency responders in tropical conditions. Accurate systolic-peak detection is an important first step for the analysis of heart rate variability. Algorithms based on local maxima-minima, first-derivative, and slope sum are evaluated, and a new algorithm is introduced to improve the detection rate. With 40 healthy subjects, the new algorithm demonstrates the highest overall detection accuracy (99.84% sensitivity, 99.89% positive predictivity). Existing algorithms, such as Billauer's, Li's and Zong's, have comparable although lower accuracy. However, the proposed algorithm presents an advantage for real-time applications by avoiding human intervention in threshold determination. For best performance, we show that a combination of two event-related moving averages with an offset threshold has an advantage in detecting systolic peaks, even in heat-stressed PPG signals.

Second derivative of photoplethysmogram in children and young people

The characteristics of the second derivative of the photoplethysmogram (SDPTG) were clarified in children and young people, and the factors affecting the SDPTG wave pattern were examined. The study group comprised 775 healthy subjects aged 3-20 years (mean, 10+/-5). The blood pressure of the left brachial artery was determined in the resting sitting position and then the fingertip PTG and the SDPTG were automatically measured using a digital photoplethysmograph, with the sensor located at the cuticle of the second digit of the right hand. The values used were the b/a, c/a, d/a, and e/a ratios, and the SDPTG aging index (SDPTG-AI). With increasing age, the systolic blood pressure and height increased (r = 0.52, 0.92). Aging decreased the b/a ratio and SDPTG-AI (r = -0.58, -0.67) and increased the c/a and e/a ratios (r = 0.42 and 0.42). There was no significant correlation between blood pressure and indices of SDPTG. As height increased, the b/a ratio and SDPTG-AI decreased (r = -0.57, -0.71), whereas the c/a and e/a ratios increased (r = 0.42 and 0.46). In males the SDPTG-AI decreased with age from 3 to 18 years and then increased, and in females it decreased with age from 3 to 15 years and then increased. Overall, the SDPTG-AI decreased with age between 3 and 18 years and then increased, forming a J curve. In the children's and young people's SDPTG, the b/a and SDPTG-AI decreased and the c/a and e/a ratios increased with age. The length of the vascular system and the inner diameter and wall thickness of vessels may modify the SDPTG wave pattern in the growth period. Thereafter, as the effects of these factors decrease, the increase in intravascular pressure and decreasing wall elasticity due to aging may affect the wave pattern.

Derivation of systolic time intervals from Doppler measurement of temporal arterial blood flow

The carotid pulse method of recording systolic time intervals is limited by significant motion-induced artifact, making it unsuitable for studying patients during exercise. As an approach to overcoming this limitation, a new method utilizing the blood velocity profile of the superficial temporal artery measured by Doppler ultrasound has been developed. When compared with the values obtained from the conventional carotid pulse method, Doppler-derived left ventricular ejection time and preejection period showed excellent correlation (r = 0.99 for both) and the Doppler-derived measurements showed little intra- or interobserver variability. Studies performed during treadmill exercise showed that in 8 of 10 subjects, suitable tracing could be recorded through stage 3 of the Bruce protocol, confirming the enhanced stability of the technique compared with the carotid pulse method.

Ejection time by ear densitogram and its derivative. Clinical and physiologic applications

Ear densitographic ejection times (EDET) and first derivative ear densitogram ejection times (dEDET) were studied to determine whether their reliability and validity justify their substitution for ejection times derived from the far less stable carotid pulse tracing. Inter- and intra-subject comparisons were made on thirty individuals under a wide variety of disease and challenge states. Statistical analysis of the data-which had been obtained through a blinded procedure-showed an overall correlation ( r ) of .98 for carotid vs EDET and .99 for carotid vs dEDET. The t -test demonstrated no significant differences among ejection times derived from the three methods. Moreover, the close tracking at rest and during challenges of ejection times derived from these curves with those from the carotid indicate that either method may be substituted for standard carotid curves without sacrificing reliability or validity of the measure. Both ear curves offer distinct advantages over carotid pulse curves because their sensor is self-retaining and they remain stable during exercise and other body and respiratory movements. The additional feature of simplicity in reading the first derivative of the ear densitogram over its undifferentiated curve makes dEDET the preferred method.