Signal quality assessment of peripheral venous pressure.

Develop a signal quality index (SQI) for the widely available peripheral venous pressure waveform (PVP). We focus on the quality of the cardiac component in PVP. We model PVP by the adaptive non-harmonic model. When the cardiac component in PVP is stronger, the PVP is defined to have a higher quality. This signal quality is quantified by applying the synchrosqueezing transform to decompose the cardiac component out of PVP, and the SQI is defined as a value between 0 and 1. A database collected during the lower body negative pressure experiment is utilized to validate the developed SQI. All signals are labeled into categories of low and high qualities by experts. A support vector machine (SVM) learning model is trained for practical purpose. The developed signal quality index coincide with human experts' labels with the area under the curve 0.95. In a leave-one-subject-out cross validation (LOSOCV), the SQI achieves accuracy 0.89 and F1 0.88, which is consistently higher than other commonly used signal qualities, including entropy, power and mean venous pressure. The trained SVM model trained with SQI, entropy, power and mean venous pressure could achieve an accuracy 0.92 and F1 0.91 under LOSOCV. An exterior validation of SQI achieves accuracy 0.87 and F1 0.92; an exterior validation of the SVM model achieves accuracy 0.95 and F1 0.96. The developed SQI has a convincing potential to help identify high quality PVP segments for further hemodynamic study. This is the first work aiming to quantify the signal quality of the widely applied PVP waveform.

Improving pulse oximetry accuracy in dark-skinned patients: technical aspects and current regulations.

Recent concerns regarding the clinical accuracy of pulse oximetry in dark-skinned patients, specifically in detecting occult hypoxaemia, have motivated research on this topic and recently reported in this journal. We provide an overview of the technical aspects of the issue, the sources of inaccuracy, and the current regulations and limitations. These insights offer perspectives on how pulse oximetry can be improved to address these potential limitations.

Using the ear photoplethysmographic waveform as an early indicator of central hypovolemia in healthy volunteers utilizing LBNP induced hypovolemia model.

Objective. To study the photoplethysmographic (PPG) waveforms of different locations (ear and finger) during lower body negative pressure (LBNP) induced hypovolemia. Then, to determine whether the PPG waveform can be used to detect hypovolemia during the early stage of LBNP.Approach. 36 healthy volunteers were recruited for progressive LBNP induced hypovolemia, with an endpoint of -60 mmHg or development of hypoperfusion symptoms, whichever comes first. Subjects tolerating the entire protocol without symptoms were designated as high tolerance (HT), while symptomatic subjects were designated as low tolerance (LT). Subjects were monitored with an electrocardiogram, continuous noninvasive blood pressure monitor, and two pulse oximetry probes, one on the ear (Xhale) and one the finger (Nellcor). Stroke volume was measured non-invasively utilizing Non-Invasive Cardiac Output Monitor (NICOM, Cheetah Medical). The waveform morphology was analyzed using novel PPG waveforms indices, including phase hemodynamic index (PHI) and amplitude hemodyamaic index and were evaluated from the ear PPG and finger PPG at different LBNP stages.Main results. The PHI, particularly the phase relationship between the second harmonic and the fundamental component of the ear PPG denoted as∇φ2,during the early stage of LBNP (-15 mmHg) in the HT and LT groups is statistically significantly different (pvalue = 0.0033) with the area under curve 0.81 (CI: 0.616-0.926). The other indices are not significantly different. The 5 fold cross validation shows that∇φ2during the early stage of LBNP (-15 mmHg) as the single index could predict the tolerance of the subject with the sensitivity, specificity, accuracy andF1 as 0.771 ± 0.192, 0.71 ± 0.107, 0.7 ± 0.1 and 0.771 ± 0.192 respectively.Significance. The ear's PPG PHI which compares the phases of the fundamental and second harmonic has the potential to be used as an early predictor of central hypovolemia.

Inaccuracy of pulse oximetry with dark skin pigmentation: clinical implications and need for improvement.

Recent reports highlight potential inaccuracies of pulse oximetry in patients with various degrees of skin pigmentation. We summarise the literature, provide an overview of potential clinical implications, and provide insights into how pulse oximetry could be improved to mitigate against such potential shortcomings.

Photoplethysmography.

The photoplethysmographic (PPG) waveform, also known as the pulse oximeter waveform, is one of the most commonly displayed clinical waveforms. First described in the 1930s, the technology behind the waveform is simple. The waveform, as displayed on the modern pulse oximeter, is an amplified and highly filtered measurement of light absorption by the local tissue over time. It is optimized by medical device manufacturers to accentuate its pulsatile components. Physiologically, it is the result of a complex, and not well understood, interaction between the cardiovascular, respiratory, and autonomic systems. All modern pulse oximeters extract and display the heart rate and oxygen saturation derived from the PPG measurements at multiple wavelengths. "As is," the PPG is an excellent monitor for cardiac arrhythmia, particularly when used in conjunction with the electrocardiogram (ECG). With slight modifications in the display of the PPG (either to a strip chart recorder or slowed down on the monitor screen), the PPG can be used to measure the ventilator-induced modulations which have been associated with hypovolemia. Research efforts are under way to analyze the PPG using improved digital signal processing methods to develop new physiologic parameters. It is hoped that when these new physiologic parameters are combined with a more modern understanding of cardiovascular physiology (functional hemodynamics) the potential utility of the PPG will be expanded. The clinical researcher's objective is the use of the PPG to guide early goal-directed therapeutic interventions (fluid, vasopressors, and inotropes), in effect to extract from the simple PPG the information and therapeutic guidance that was previously only obtainable from an arterial pressure line and the pulmonary artery catheter.

Impact of lower body negative pressure induced hypovolemia on peripheral venous pressure waveform parameters in healthy volunteers.

Lower body negative pressure (LBNP) creates a reversible hypovolemia by sequestrating blood volume in the lower extremities. This study sought to examine the impact of central hypovolemia on peripheral venous pressure (PVP) waveforms in spontaneously breathing subjects. With IRB approval, 11 healthy subjects underwent progressive LBNP (baseline, -30, -75, and -90 mmHg or until the subject became symptomatic). Each was monitored for heart rate (HR), finger arterial blood pressure (BP), a chest respiratory band and PVP waveforms which are generated from a transduced upper extremity intravenous site. The first subject was excluded from PVP analysis because of technical errors in collecting the venous pressure waveform. PVP waveforms were analyzed to determine venous pulse pressure, mean venous pressure, pulse width, maximum and minimum slope (time domain analysis) together with cardiac and respiratory modulations (frequency domain analysis). No changes of significance were found in the arterial BP values at -30 mmHg LBNP, while there were significant reductions in the PVP waveforms time domain parameters (except for 50% width of the respiration induced modulations) together with modulation of the PVP waveform at the cardiac frequency but not at the respiratory frequency. As the LBNP progressed, arterial systolic BP, mean BP and pulse pressure, PVP parameters and PVP cardiac modulation decreased significantly, while diastolic BP and HR increased significantly. Changes in hemodynamic and PVP waveform parameters reached a maximum during the symptomatic phase. During the recovery phase, there was a significant reduction in HR together with a significant increase in HR variability, mean PVP and PVP cardiac modulation. Thus, in response to mild hypovolemia induced by LBNP, changes in cardiac modulation and other PVP waveform parameters identified hypovolemia before detectable hemodynamic changes.

Analysis of plethysmographic waveform changes induced by beach chair positioning under general anesthesia.

During shoulder surgery, patients typically are placed in the beach chair position. In rare cases, this positioning has resulted in devastating outcomes of postoperative cerebral ischemia (Cullen and Kirby in APSF Newsl 22(2):25-27, 2007; Munis in APSF Newsl 22(4):82-83, 2008). This study presents a method to noninvasively and continuously hemodynamically monitor patients during beach chair positioning by using the photoplethysmograph signal recorded from a commercial pulse oximeter. Twenty-nine adults undergoing shoulder surgery were monitored before and after beach chair positioning with electrocardiogram, intermittent blood pressure, end tidal carbon dioxide, and photoplethysmograph via Nellcor finger pulse oximeter. Fast Fourier transform (FFT) was used to perform frequency-domain analysis on the photoplethysmograph (PPG) signal for data segments taken 80-120 s before and after beach chair positioning. The amplitude density of respiration-associated PPG oscillations was quantified measuring the height of the FFT peak at respiratory frequency. Results were reported as (median, interquartile range) and statistical analysis was performed using Wilcoxon sign rank test. Data were also collected when vasoactive drugs phenylephrine and ephedrine were used to maintain acceptable mean arterial pressure during a case. With beach chair positioning, all subjects who did not receive vasoactive drugs showed an increase in the FFT amplitude density of respiration-associated PPG oscillations (p < 0.0001) without change in pulse-associated PPG oscillations. The PPG was more accurate at monitoring the change to beach chair position than blood pressure or heart rate. With vasoactive drugs, pulse-associated PPG oscillations decreased only with phenylephrine while respiration-associated oscillations did not change. Frequency domain analysis of the PPG signal may be a better tool than traditional noninvasive hemodynamic parameters at monitoring patients during beach chair position surgery.

New utility for an old tool: can a simple gait speed test predict ambulatory surgical discharge outcomes?

OBJECTIVE: The primary aims of this study were to design prediction models based on a functional marker (preoperative gait speed) to predict readiness for home discharge time of 90 mins or less and to identify those at risk for unplanned admissions after elective ambulatory surgery. DESIGN: This prospective observational cohort study evaluated all patients scheduled for elective ambulatory surgery. Home discharge readiness and unplanned admissions were the primary outcomes. Independent variables included preoperative gait speed, heart rate, and total anesthesia time. The relationship between all predictors and each primary outcome was determined in separate multivariable logistic regression models. RESULTS: After adjustment for covariates, gait speed with adjusted odds ratio of 3.71 (95% confidence interval, 1.21-11.26), P = 0.02, was independently associated with early home discharge readiness of 90 mins or less. Importantly, gait speed dichotomized as greater or less than 1 m/sec predicted unplanned admissions, with odds ratio of 0.35 (95% confidence interval, 0.16-0.76, P = 0.008) for those with speeds 1 m/sec or greater in comparison with those with speeds less than 1 m/sec. In a separate model, history of cardiac surgery with adjusted odds ratio of 7.5 (95% confidence interval, 2.34-24.41; P = 0.001) was independently associated with unplanned admissions after elective ambulatory surgery, when other covariates were held constant. CONCLUSIONS: This study demonstrates the use of novel prediction models based on gait speed testing to predict early home discharge and to identify those patients at risk for unplanned admissions after elective ambulatory surgery.

Is pulse oximetry an essential tool or just another distraction? The role of the pulse oximeter in modern anesthesia care.

Since the discovery of anesthetic agents, patient monitoring has been considered one of the core responsibilities of the anesthesiologist. As depicted in Robert Hinckley's famous painting, The First Operation with Ether, one observes William Thomas Green Morton carefully watching over his patient. Since its founding in 1905, 'Vigilance' has been the motto of the American Society of Anesthesiologists (ASA). Over a hundred years have passed, and one would think we would be clear regarding what we are watching for and how we should be watching. On the contrary, the introduction of new technology and outcome research is requiring us to re-examine our fundamental assumptions regarding what is and what is not important in the care of the patient. A vast majority of anesthesiologists would refuse to proceed with an anesthetic without the presence of a pulse oximeter. On the other hand, outcome studies have failed to demonstrate an improvement in patient care with their use. For that matter, it can be argued that outcome studies have yet to demonstrate an unambiguous role for any monitor of any type (i.e. blood pressure cuff or ECG), as outcome studies may fail to capture rare events. Because of the increased safety that has been attributed to pulse oximetry, it is unlikely that further studies can or will be conducted. As we enter a new era of clinical monitoring, with an emphasis on noninvasive cardiovascular monitoring, it might be of benefit to examine the role of the pulse oximeter in clinical care. This article reviews the available evidence for pulse oximetry. Further, it discusses contemporary issues, events, and perceptions that may help to explain how and why pulse oximetry may have been adopted as a standard of care despite the lack of supportive. Lastly, it discusses less obvious benefits of pulse oximetry that may have further implications on the future of anesthesia care and perhaps even automated anesthesia.

Using time-frequency analysis of the photoplethysmographic waveform to detect the withdrawal of 900 mL of blood.

BACKGROUND: We designed this study to determine if 900 mL of blood withdrawal during spontaneous breathing in healthy volunteers could be detected by examining the time-varying spectral amplitude of the photoplethysmographic (PPG) waveform in the heart rate frequency band and/or in the breathing rate frequency band before significant changes occurred in heart rate or arterial blood pressure. We also identified the best PPG probe site for early detection of blood volume loss by testing ear, finger, and forehead sites. METHODS: Eight subjects had 900 mL of blood withdrawn followed by reinfusion of 900 mL of blood. Physiological monitoring included PPG waveforms from ear, finger, and forehead probe sites, standard electrocardiogram, and standard blood pressure cuff measurements. The time-varying amplitude sequences in the heart rate frequency band and breathing rate frequency band present in the PPG waveform were extracted from high-resolution time-frequency spectra. These amplitudes were used as a parameter for blood loss detection. RESULTS: Heart rate and arterial blood pressure did not significantly change during the protocol. Using time-frequency analysis of the PPG waveform from ear, finger, and forehead probe sites, the amplitude signal extracted at the frequency corresponding to the heart rate significantly decreased when 900 mL of blood was withdrawn, relative to baseline (all P < 0.05); for the ear, the corresponding signal decreased when only 300 mL of blood was withdrawn. The mean percent decrease in the amplitude of the heart rate component at 900 mL blood loss relative to baseline was 45.2% (38.2%), 42.0% (29.2%), and 42.3% (30.5%) for ear, finger, and forehead probe sites, respectively, with the lower 95% confidence limit shown in parentheses. After 900 mL blood reinfusion, the amplitude signal at the heart rate frequency showed a recovery towards baseline. There was a clear separation of amplitude values at the heart rate frequency between baseline and 900 mL blood withdrawal. Specificity and sensitivity were both found to be 87.5% with 95% confidence intervals (47.4%, 99.7%) for ear PPG signals for a chosen threshold value that was optimized to separate the 2 clusters of amplitude values (baseline and blood loss) at the heart rate frequency. Meanwhile, no significant changes in the spectral amplitude in the frequency band corresponding to respiration were found. CONCLUSION: A time-frequency spectral method detected blood loss in spontaneously breathing subjects before the onset of significant changes in heart rate or blood pressure. Spectral amplitudes at the heart rate frequency band were found to significantly decrease during blood loss in spontaneously breathing subjects, whereas those at the breathing rate frequency band did not significantly change. This technique may serve as a valuable tool in intraoperative and trauma settings to detect and monitor hemorrhage.

Respiratory physiology and the impact of different modes of ventilation on the photoplethysmographic waveform.

The photoplethysmographic waveform sits at the core of the most used, and arguably the most important, clinical monitor, the pulse oximeter. Interestingly, the pulse oximeter was discovered while examining an artifact during the development of a noninvasive cardiac output monitor. This article will explore the response of the pulse oximeter waveform to various modes of ventilation. Modern digital signal processing is allowing for a re-examination of this ubiquitous signal. The effect of ventilation on the photoplethysmographic waveform has long been thought of as a source of artifact. The primary goal of this article is to improve the understanding of the underlying physiology responsible for the observed phenomena, thereby encouraging the utilization of this understanding to develop new methods of patient monitoring. The reader will be presented with a review of respiratory physiology followed by numerous examples of the impact of ventilation on the photoplethysmographic waveform.

Autonomic control mechanism of maximal lower body negative pressure application.

Autonomic control mechanisms during progressive hemorrhage in humans remain complex and unclear. The present study investigates the autonomic reflexes during maximal application of lower body negative pressure (LBNP) that mimics severe hemorrhage in conscious human subjects (n=10) using analyses of heart rate variability (HRV) and systolic blood pressure variability (BPV) and baroreflex sensitivity. Spectral analysis of HRV included linear power spectral density (PSD), and nonlinear principal dynamic modes (PDM) methods. The maximal LBNP application decreased (P<0.01) the systolic and pulse pressures (PP), root mean square successive differences, normalized high frequency (HF) power of HRV, and transfer function gains at low frequency (LF) and HF bands. Meanwhile, increases (P<0.05) in heart rate, diastolic blood pressure (DBP), LFHRV, LF/HFHRV, and sympathetic activity of HRV using PDM were observed during maximal LBNP tolerance. After the termination of LBNP, no significant changes (P>0.05) were found in all the parameters except DBP and PP between recovery and baseline conditions. Rapid application of maximal LBNP that simulated severe hemorrhage was found to be associated with unloading of baroreflex mediated increased sympathetic reflex.

Modulation of finger photoplethysmographic traces during forced respiration: venous blood in motion?

Photoplethysmographic (PPG) signals were recorded from the fingers of 10 healthy volunteers during forced respiratory inspiration. The aim of this pilot study was to assess the effect of negative airway pressure on the blood volumes within the tissue bed of the finger, and the resultant modulation of PPG signals. The acquired signals were analysed and oxygen saturations estimated from the frequency spectra in the cardiac and respiratory frequency ranges. Assuming that respiratory modulation affects blood volumes in veins to a greater extent than in arteries, the local venous oxygen saturation was estimated. Estimated venous oxygen saturation was found to be 3.1% (±4.2%) lower than the estimated arterial saturation.

Impact of central hypovolemia on photoplethysmographic waveform parameters in healthy volunteers. Part 1: time domain analysis.

INTRODUCTION: Our study sought to explore changes in photoplethysmographic (PPG) waveform param- eters, during lower body negative pressure (LBNP) which simulated hypovolemia, in spontaneously breathing volunteers. We hypothesize that during progressive LBNP; there will be a preservation of ear PPG parameters and a decrease in finger PPG parameters. METHODS: With IRB approval, 11 volunteers underwent a LBNP protocol at baseline, 30, 75, and 90 mm Hg (or until the subject became symptomatic). Subjects were monitored with finger and ear pulse oximeter probes, an ECG, and a finger arterial blood pressure monitor. The square root of the mean of the squared differences between adjacent NN intervals (RMSSD) which is the time domain analysis of the heart rate variability (HRV) was measured. PPG waveforms were analyzed for height, area, width 50, maximum and minimum slope. Data are presented as median and inter-quartile range. Friedman ANOVA and Wilcoxon tests were used to identify changes in hemo- dynamic and PPG parameters, P < 0.017 was considered statistically significant. RESULTS: There were no significant changes in the blood pressure variables at LBNP(30), but at and beyond LBNP(75), the decreases in systolic, mean and pulse pressure were significant as was the increase in diastolic pressure. Heart rate increased significantly at LBNP(30), reaching a maximum of 75.4% above baseline at the symptomatic phase while RMSSD showed significant reduction at LBNP(75). Finger PPG height, area, width 50, and maximum slope decreased significantly at LBNP(30) and during symptomatic phase they showed a reduction of 59.4, 76.9, 27.4 and 51.6%, respectively. Ear PPG height, area, width 50 and maximum slope did not change significantly until the LBNP(75), reached. During symptomatic phase, the respective declines reached 39.3, 61.0, 21.4 and 34.9%. CONCLUSION: PPG waveform parameters may prove to be sensitive and specific as early indicators of blood loss. These PPG changes were observed before profound decreases in arterial blood pressure. The relative sparing of central cutaneous blood flow is consistent with the increased parasympathetic innervation of central structures.

Impact of central hypovolemia on photoplethysmographic waveform parameters in healthy volunteers part 2: frequency domain analysis.

OBJECTIVE: The photoplethysmographic (PPG) waveforms are modulated by the respiratory, cardiac and autonomic nervous system. Lower body negative pressure (LBNP) has been used as an experimental tool to simulate loss of central blood volume in humans. The aim of our research is to understanding PPG waveform changes during progressive hypovolemia. METHODS: With IRB approval, 11 volunteers underwent a LBNP protocol at baseline, 30, 75, and 90 mmHg (or until the subject became symptomatic). Subjects were monitored with finger and ear pulse oximeter probes, ECG, and finger arterial blood pressure monitor (FABP). Heart rate variability (HRV) was analyzed to high frequency (HRV-HF) (0.12-0.4 Hz) and low frequency (HRV-LF) (0.04-0.12 Hz). Frequency analysis of PPG waveforms were computed to low (0.04-0.11 Hz) frequency (PPG-LF), intermediate (0.12-0.18 Hz) frequency (PPG-IF), respiratory (0.19-0.3 Hz) frequency (PPG-Resp.) and cardiac (0.75-2.5 Hz) frequency (PPG-Cardiac)during different phases of LBNP protocol RESULTS: Heart rate increased significantly while systolic, mean and pulse pressure of the FABP declined slowly together with significant reductions in HRV-HF (0.12-0.4 Hz) and HRV-LF (0.04-0.12 Hz) power at LBNP(75). There was significant reduction in finger PPG-Cardiac modulation which is consistent with the reduction in the pulse pressure of the FABP. As the LBNP progress there was shift in the amplitude density of the ear PPG-Cardiac to PPG-Resp. Oscillation as an evidence of progressive hypovolemia with reduction in pulse pressure and increase in the respiratory induced variations. At LBNP(75), there were significant increased (>140% increase from the baseline) in ear PPG-IF (0.12-0.18 Hz) in the meantime HRV-HF showed significant reduction (>89%) from the baseline. At the symptomatic phase; there was a shift in ear PPG-IF to PPG-Resp. With an increase in the ear PPG-Resp. Modulation to ≥175% from the baseline CONCLUSION: The pulse oximeter waveform contains a complex mixture of the effect of cardiac, venous, autonomic, and respiratory systems on the central and peripheral circulation. The occurrence of autonomic modulation needs to be taken into account when studying signals that have their origins from central sites (e.g. ear and forehead).

A novel approach using time-frequency analysis of pulse-oximeter data to detect progressive hypovolemia in spontaneously breathing healthy subjects.

Accurate and early detection of blood volume loss would greatly improve intraoperative and trauma care. This study has attempted to determine early diagnostic and quantitative markers for blood volume loss by analyzing photoplethysmogram (PPG) data from ear, finger and forehead sites with our high-resolution time-frequency spectral (TFS) technique in spontaneously breathing healthy subjects (n = 11) subjected to lower body negative pressure (LBNP). The instantaneous amplitude modulations present in heart rate (AM HR) and breathing rate (AMBR) band frequencies of PPG signals were calculated from the high-resolution TFS. Results suggested that the changes (P < 0.05) in AMBR and especially in AMHR values can be used to detect the blood volume loss at an early stage of 20% LBNP tolerance when compared to the baseline values. The mean percent decrease in AMHR values at 100% LBNP tolerance was 78.3%, 72.5%, and 33.9% for ear, finger, and forehead PPG signals, respectively. The mean percent increase in AMBR values at 100% LBNP tolerance was 99.4% and 19.6% for ear and finger sites, respectively; AMBR values were not attainable for forehead PPG signal. Even without baseline AMHR values, our results suggest that hypovolemia detection is possible with specificity and sensitivity greater than 90% for the ear and forehead locations when LBNP tolerance is 100%. Therefore, the TFS analysis of noninvasive PPG waveforms is promising for early diagnosis and quantification of hypovolemia at levels not identified by vital signs in spontaneously breathing subjects.

Statistical approach for the detection of motion/noise artifacts in Photoplethysmogram.

Motion and noise artifacts (MNA) have been a serious obstacle in realizing the potential of Photoplethysmogram (PPG) signals for real-time monitoring of vital signs. We present a statistical approach based on the computation of kurtosis and Shannon Entropy (SE) for the accurate detection of MNA in PPG data. The MNA detection algorithm was verified on multi-site PPG data collected from both laboratory and clinical settings. The accuracy of the fusion of kurtosis and SE metrics for the artifact detection was 99.0%, 94.8% and 93.3% in simultaneously recorded ear, finger and forehead PPGs obtained in a clinical setting, respectively. For laboratory PPG data recorded from a finger with contrived artifacts, the accuracy was 88.8%. It was identified that the measurements from the forehead PPG sensor contained the most artifacts followed by finger and ear. The proposed MNA algorithm can be implemented in real-time as the computation time was 0.14 seconds using Matlab®.