Motion artefact reduction of the photoplethysmographic signal in pulse transit time measurement

Assessing hemodynamics from the photoplethysmogram to gain insights into vascular age: A review from VascAgeNet

The photoplethysmogram (PPG) signal is widely measured by clinical and consumer devices, and it is emerging as a potential tool for assessing vascular age. The shape and timing of the PPG pulse wave are both influenced by normal vascular aging, changes in arterial stiffness and blood pressure, and atherosclerosis. This review summarizes research into assessing vascular age from the PPG. Three categories of approaches are described: 1) those which use a single PPG signal (based on pulse wave analysis), 2) those which use multiple PPG signals (such as pulse transit time measurement), and 3) those which use PPG and other signals (such as pulse arrival time measurement). Evidence is then presented on the performance, repeatability and reproducibility, and clinical utility of PPG-derived parameters of vascular age. Finally, the review outlines key directions for future research to realize the full potential of photoplethysmography for assessing vascular age.

A wearable autonomous heart rate sensor based on piezoelectric-charge-gated thin-film transistor for continuous multi-point monitoring

Autonomous wearable biomedical sensors enable continuous human body vital signs monitoring with features such as being conformable, mobile, cost-effective and self-powered. In this work, we report on an autonomous and multi-positional sensor capable of heart rate and blood pressure monitoring. The device concept is based on a piezoelectric-charge-gated thin-film transistor (PCGTFT) where a polyvinylidene fluoride (PVDF) piezoelectric sandwich structure is incorporated with an amorphous silicon (a-Si:H) dual-gate TFT (DGTFT). An analytical model and preliminary experimental results will be presented along with a demonstration of a proof-of-concept sensing system for continuous multi-point heart rate monitoring.

Increasing accuracy of pulse transit time measurements by automated elimination of distorted photoplethysmography waves

Photoplethysmography (PPG) is a widely available non-invasive optical technique to visualize pressure pulse waves (PWs). Pulse transit time (PTT) is a physiological parameter that is often derived from calculations on ECG and PPG signals and is based on tightly defined characteristics of the PW shape. PPG signals are sensitive to artefacts. Coughing or movement of the subject can affect PW shapes that much that the PWs become unsuitable for further analysis. The aim of this study was to develop an algorithm that automatically and objectively eliminates unsuitable PWs. In order to develop a proper algorithm for eliminating unsuitable PWs, a literature study was conducted. Next, a '7Step PW-Filter' algorithm was developed that applies seven criteria to determine whether a PW matches the characteristics required to allow PTT calculation. To validate whether the '7Step PW-Filter' eliminates only and all unsuitable PWs, its elimination results were compared to the outcome of manual elimination of unsuitable PWs. The '7Step PW-Filter' had a sensitivity of 96.3% and a specificity of 99.3%. The overall accuracy of the '7Step PW-Filter' for detection of unsuitable PWs was 99.3%. Compared to manual elimination, using the '7Step PW-Filter' reduces PW elimination times from hours to minutes and helps to increase the validity, reliability and reproducibility of PTT data.

Heart Rate Detection During Sleep Using a Flexible RF Resonator and Injection-Locked PLL Sensor

Novel nonintrusive technologies for wrist pulse detection have been developed and proposed as systems for sleep monitoring using three types of radio frequency (RF) sensors. The three types of RF sensors for heart rate measurement on wrist are a flexible RF single resonator, array resonators, and an injection-locked PLL resonator sensor. To verify the performance of the new RF systems, we compared heart rates between presleep time and postsleep onset time. Heart rates of ten subjects were measured using the RF systems during sleep. All three RF devices detected heart rates at 0.2 to 1 mm distance from the skin of the wrist over clothes made of cotton fabric. The wrist pulse signals of a flexible RF single resonator were consistent with the signals obtained by a portable piezoelectric transducer as a reference. Then, we confirmed that the heart rate after sleep onset time significantly decreased compared to before sleep. In conclusion, the RF system can be utilized as a noncontact nonintrusive method for measuring heart rates during sleep.

A survey on signals and systems in ambulatory blood pressure monitoring using pulse transit time

Blood pressure monitoring based on pulse transit or arrival time has been the focus of much research in order to design ambulatory blood pressure monitors. The accuracy of these monitors is limited by several challenges, such as acquisition and processing of physiological signals as well as changes in vascular tone and the pre-ejection period. In this work, a literature survey covering recent developments is presented in order to identify gaps in the literature. The findings of the literature are classified according to three aspects. These are the calibration of pulse transit/arrival times to blood pressure, acquisition and processing of physiological signals and finally, the design of fully integrated blood pressure measurement systems. Alternative technologies as well as locations for the measurement of the pulse wave signal should be investigated in order to improve the accuracy during calibration. Furthermore, the integration and validation of monitoring systems needs to be improved in current ambulatory blood pressure monitors.

Clinical applications of pulse transit time in paediatric critical care

A simple and non-invasive technique, termed pulse transit time (PTT), has shown its potential in long-term investigations such as respiratory sleep studies and cardiovascular studies. Based on these findings, the PTT technique shows relevance for continuous haemodynamic monitoring in critical care. The objective of this review is to understand the potential, applications and limitations of PTT in this clinical setting. Present non-invasive haemodynamic monitoring methods such as automated oscillometric blood pressure (BP) and auscultatory techniques have their known limitations. They tend to underestimate systolic BP while overestimating diastolic BP. Due to the periodic increase in cuff pressure cycles during data acquisition, these techniques may cause much discomfort in elderly geriatric patients, or lessen the cooperation of younger paediatric patients. Thus, there can be adverse effects on therapeutic decisions and possibly clinical outcomes. Documented evidences have indicated that changes observed in PTT are inversely correlated to the corresponding BP changes. In critical care, a simple and accommodating technique like PTT may be useful in providing better comfort for patients during extended monitoring. Being a semi-quantitative measure, blanket recommendations for its utility can then become possible. The basic instrumentations needed are often part of standard critical care monitoring system. Furthermore, PTT also has the potential to monitor the often tachypnoeic respiratory dependent BP changes seen in small infants during critical care.

Changes induced in the lower- and upper-limb pulse transit-time ratio during inspiratory resistive breathing

The ankle brachial index (ABI) has been widely used to monitor the pathogenesis of peripheral arterial diseases such as ischemia of the extremities. Owing to the occluding nature of ABI measurement, this may not be appealing to less cooperative patients when multiple prolonged screening is required. Recently, a simple non-occluding technique termed pulse transit-time ratio (PTTR) has shown potential as a surrogate ABI marker. It is also known that abrupt changes in inspiratory efforts can lead to increased blood pressure (BP) and heart rate. Since transit-time measurements can be confounded by these parameters, it is important to understand their effects on PTTR normality. We recruited 12 healthy adults (8 males, aged 27.0+/-3.1 years) to perform three inspiratory activities. Friedman and Wilcoxon statistical results both showed that significant changes in transit-time oscillations were observed for higher inspiratory loads (p<0.05). These results were verified by a corresponding air-pressure difference measurement, for which a similar significant increase was also registered (p<0.05). However, limited changes were observed in the derived PTTR parameter (p>0.05). These findings suggest that, similar to ABI, PTTR is only confounded by abnormal local changes in either of the peripheral BPs measured.

Detection of central respiratory events using pulse transit time in infants

The prevalence of sudden infant death syndrome (SIDS) has been well studied and central sleep apnea is deemed as one of the possible causes. Current gold standard for its diagnosis is nocturnal polysomnography (PSG). However, this procedure is complex and generally needs to be performed in a sleep laboratory. Pulse transit time (PTT) shows its potential to indicate abrupt blood pressure (BP) changes during the occurrences of upper airway obstruction. The main objective of this study was to assess the capability of PTT to differentiate central respiratory events from tidal breathing in infants. This study involved 5 infants (4 male) with mean age of 7.8 months. 50 valid central respiratory events were randomly selected. These events were free from motion artifacts and pre-scored in the corresponding PSG studies by two blinded observers. PTT measurements from these events were then evaluated against the PSG scorings. Using a two-tailed F-test for variance, it was observed that central events differed from tidal breathing in a significant manner (p<0.05). Furthermore, PTT has showed its sensitivity to monitor marginal BP fluctuations during tidal breathing. Hence, the results herein suggest that PTT can be a valuable non-invasive technique to monitor central apneic events in sleeping infants.

Relations between Physiologic Parameters and Pulse Transit Time during Loaded Breathing

Pulse transit time (PTT) is a non-invasive measure, defined as time taken for the pulse pressure waves to travel from the R-wave of electrocardiogram to a selected peripheral site. Baseline PTT value is known to be influenced by physiologic variables like heart rate (HR), blood pressure (BP) and arterial compliance (AC). However, few quantitative data are available describing the factors which can influence PTT measurements in a child during breathing. The aim of this study was to investigate the effects of changes in breathing efforts on PTT baseline and fluctuations. Two different inspiratory resistive loading (IRL) devices were used to simulate loaded breathing in order to induce these effects. It is known that HR can influence the normative PTT value however the effect of HR variability (HRV) is not well-studied. Two groups of 3 healthy children (&#8804;12years) were recruited; one group with insignificant (p>0.05) HR changes during all test activities. Results showed that HRV is not the sole contributor to PTT variations and suggest that changes in other physiologic parameters are also equally important. Hence, monitoring PTT measurement can be indicative of these associated changes during tidal or increased breathing efforts in healthy children.

Development of a home screening system for pediatric respiratory sleep studies

To develop a simple and portable home screening monitor for sleep-disordered breathing (SDB) in children. In such a system, identifying the respiratory events and occurrences of motional artifacts are two essential elements that can affect the accuracy of the study. Moreover, such a system needs to be easy to set up and user friendly. The proposed system includes the following: electrocardiogram, pulse oximeter, microcontroller-based computation device, and a tri-axial accelerometer. Three physiologic parameters derived with this device were used to identify central (CE) and obstructive (OE) respiratory events. The criteria used were based on documented evidence and compared against corresponding standard polysomnographic scorings. In addition, a module was constructed in conjunction with a RS232 chip to transmit the recorded data to a personal computer. The accelerometer was used as a motion artifact detector. Detectable signals were acquired from the accelerometer when artifacts were induced on the photoplethysmography by motions in three regulated test activities lasting at least 30 seconds each. In classifying respiratory events, the combined use of oxygen saturation, heart rate, and pulse transit time to produce a complex classification (logic OR) showed promise. For OE, the sensitivity and specificity were 0.828 and 0.859, respectively. For CE, these values were 0.868 and 0.762, respectively. The proposed system potentially fulfils the criterion as a home screening tool and can form an indispensable addition to the SDB investigation in the pediatric population.

Pulse transit time in paediatric respiratory sleep studies

Pulse transit time (PTT) has emerged over the recent decades as a simple and non-invasive measure to quantify inspiratory effort changes in adults with sleep disordered breathing (SDB). Hence, this shows promise to be an effective screening tool for the paediatrics. However, little is known about its utility and suitability until recent studies has been provided quantitative knowledge about its relevance in clinical investigations. In this review, the origins, normative values, current uses and technical issues in its application to paediatric monitoring, particularly during sleep are discussed. Preliminary findings from these investigations suggest favourably its potential as an important element to screen SDB in the children population.

A computational system to optimise noise rejection in photoplethysmography signals during motion or poor perfusion states

Photoplethysmography (PPG) signals can be used in clinical assessment such as heart rate (HR) estimations and extraction of arterial flow waveforms. Motion artefact and/or poor peripheral perfusion can contaminate the PPG during monitoring. A computational system is presented here to minimise these two intrinsic weaknesses of the PPG signals. Specifically, accelerometers are used to detect the presence of motion artefacts and an adaptive filter is employed to minimise induced errors. Zero-phase digital filtering is engaged to reduce inaccuracy on the PPG signals when measured from a poorly perfused periphery. In this system, a decision matrix adopts the appropriate technique to improve the PPG signal-to-noise ratio dynamically. Statistical analyses show promising results (maximum error < 7.63%) when computed HR is compared to corresponding estimates from the electrocardiogram. Hence, the results here suggest that this dual-mode approach has potential for use in relevant clinical measurements.

Estimation of breathing interval from the photoplethysmographic signals in children

Two important parameters that are generally under continual observation during clinical monitoring are heart rate (HR) variability and breathing interval (BI) of patients. Current HR monitoring during night-long childhood respiratory sleep studies is well tolerated but BI monitoring requires instrumentation, like nasal cannula, that can be less accommodating for children. In this study, BI was extracted from the photoplethysmographic (PPG) signals using a two-stage signal processing technique termed zero-phase digital filtering. Eight children (7 male) aged 8.6 +/- 2.6 years were recruited to perform two breathing activities: during tidal and with customized externally applied inspiratory resistive loading (IRL). The accuracy of BI derived from the PPG signals was compared with that estimated by a calibrated air pressure transducer in children. Statistical analysis revealed that mean BI attained from the PPG signals were significantly related during tidal breathing (r(2) = 0.76; range 0.61-0.83; p < 0.05) and with the IRL (r(2) = 0.79; range 0.68-0.85; p < 0.05) in the absence of motion artefacts. Preliminary findings herein suggest that besides having the capability to monitor HR and arterial blood oxygen saturation measurements, the PPG signals can be used to derive BI for children. This can be an attractive alternative for children who are more disturbed by intrusive techniques in prolonged clinical monitoring.

Use of Pulse Transit Time To Distinguish Respiratory Events From Tidal Breathing in Sleeping Children

STUDY OBJECTIVES

Currently, esophageal pressure monitoring is the "gold standard" measure for inspiratory efforts, but its invasive nature necessitates a better tolerated and noninvasive method to be used on children. Pulse transit time (PTT) has demonstrated its potential as a noninvasive surrogate marker for inspiratory efforts. The principle velocity determinant of PTT is the change in stiffness of the arterial wall and is inversely correlated to BP. Moreover, PTT has been shown to identify changes in inspiratory effort via the BP fluctuations induced by negative pleural pressure swings. In this study, the capability of PTT to classify respiratory events during sleep as either central or obstructive in nature was investigated.

SETTING AND PARTICIPANTS

PTT measure was used in adjunct to routine overnight polysomnographic studies performed on 33 children (26 boys and 7 girls; mean +/- SD age, 6.7 +/- 3.9 years). The accuracy of PTT measurements was then evaluated against scored corresponding respiratory events in the polysomnography recordings.

RESULTS

Three hundred thirty-four valid respiratory events occurred and were analyzed. One hundred twelve obstructive events (OEs) showed a decrease in mean PTT over a 10-sample window that had a probability of being correctly ranked below the baseline PTT during tidal breathing of 0.92 (p < 0.005); 222 central events (CEs) showed a decrease in the variance of PTT over a 10-sample window that had a probability of being ranked below the baseline PTT of 0.94 (p < 0.005). This indicates that, at a sensitivity of 0.90, OEs can be detected with a specificity of 0.82 and CEs can be detected with a specificity of 0.80.

CONCLUSIONS

PTT is able to categorize CEs and OEs accordingly in the absence of motion artifacts, including hypopneas. Hence, PTT shows promise to differentiate respiratory events accordingly and can be an important diagnostic tool in pediatric respiratory sleep studies.