Investigation of oesophageal photoplethysmographic signals and blood oxygen saturation measurements in cardiothoracic surgery patients

pyPPG: a Python toolbox for comprehensive photoplethysmography signal analysis

Objective.Photoplethysmography is a non-invasive optical technique that measures changes in blood volume within tissues. It is commonly and being increasingly used for a variety of research and clinical applications to assess vascular dynamics and physiological parameters. Yet, contrary to heart rate variability measures, a field which has seen the development of stable standards and advanced toolboxes and software, no such standards and limited open tools exist for continuous photoplethysmogram (PPG) analysis. Consequently, the primary objective of this research was to identify, standardize, implement and validate key digital PPG biomarkers.Approach.This work describes the creation of a standard Python toolbox, denotedpyPPG, for long-term continuous PPG time-series analysis and demonstrates the detection and computation of a high number of fiducial points and digital biomarkers using a standard fingerbased transmission pulse oximeter.Main results.The improved PPG peak detector had an F1-score of 88.19% for the state-of-the-art benchmark when evaluated on 2054 adult polysomnography recordings totaling over 91 million reference beats. The algorithm outperformed the open-source original Matlab implementation by ∼5% when benchmarked on a subset of 100 randomly selected MESA recordings. More than 3000 fiducial points were manually annotated by two annotators in order to validate the fiducial points detector. The detector consistently demonstrated high performance, with a mean absolute error of less than 10 ms for all fiducial points.Significance.Based on these fiducial points,pyPPGengineered a set of 74 PPG biomarkers. Studying PPG time-series variability usingpyPPGcan enhance our understanding of the manifestations and etiology of diseases. This toolbox can also be used for biomarker engineering in training data-driven models.pyPPGis available onhttps://physiozoo.com/.

Feasibility and accuracy of noninvasive continuous hemoglobin monitoring using transesophageal photoplethysmography in porcine model

BACKGROUND

Continuous and noninvasive hemoglobin (Hb) monitoring during surgery is essential for anesthesiologists to make transfusions decisions. The aim of this study was to investigate the feasibility and accuracy of noninvasive and continuous Hb monitoring using transesophageal descending aortic photoplethysmography (dPPG) in porcine model.

METHODS

Nineteen landrace pigs, aged 3 to 5 months and weighing 30 to 50 kg, were enrolled in this study. A homemade oximetry sensor, including red (660 nm) and infrared (940 nm) lights, was placed in the esophagus for dPPG signal detection to pair with the corresponding reference Hb values (Hbi-STAT) measured by blood gas analysis. The decrease and increase changes in Hb concentration were achieved by hemodilution and transfusion. Metrics, including alternating current (AC), direct current (DC), and AC/DC for both red and infrared light were extracted from the dPPG signal. A receiver operating characteristic (ROC) curve was built to evaluate the performance of dPPG metrics in predicting the Hb "trigger threshold" of transfusion (Hb < 60 g/L and Hb > 100 g/L). Agreement and trending ability between Hb measured by dPPG (HbdPPG) and by blood gas analysis were analyzed by Bland-Altman method and polar plot graph. Error grid analysis was also performed to evaluate clinical significance of HbdPPG measurement.

RESULTS

The dPPG signal was successfully detected in all of the enrolled experimental pigs, without the occurrence of a continuous loss of dPPG signal for 2 min during the entire measurement. A total of 376 pairs of dPPG signal and Hbi-STAT were acquired. ACred/DCred and ACinf/DCinf had moderate correlations with Hbi-STAT, and the correlation coefficients were 0.790 and 0.782, respectively. The areas under the ROC curve for ACred/DCred and ACinf/DCinf in predicting Hbi-STAT < 60 g/L were 0.85 and 0.75, in predicting Hbi-STAT > 100 g/L were 0.90 and 0.83, respectively. Bland-Altman analysis and polar plot showed a small bias (1.69 g/L) but a wide limit of agreement (-26.02-29.40 g/L) and a poor trend ability between HbdPPG and Hbi-STAT. Clinical significance analysis showed that 82% of the data lay within the Zone A, 18% within the Zone B, and 0% within the Zone C.

CONCLUSION

It is feasible to establish a noninvasive and continuous Hb monitoring by transesophageal dPPG signal. The ACred/DCred extracted from the dPPG signal could provide a sensitive prediction of the Hb threshold for transfusion. The Hb concentration measured by dPPG signal has a moderate correlation with that measured by blood gas analysis. This animal study may provide an experimental basis for the development of bedside HbdPPG monitoring in the future.

Pressure-sensitive multivesicular liposomes as a smart drug-delivery system for high-altitude pulmonary edema

Changes in bodily fluid pressures, such as pulmonary artery pressure, play key roles in high-altitude pulmonary edema (HAPE) and other disorders. Smart delivery systems releasing a drug in response to these pressures might facilitate early medical interventions. However, pressure-responsive delivery systems are unavailable. We here constructed hydrostatic pressure-sensitive multivesicular liposomes (PSMVLs) based on the incomplete filling of the internal vesicle space with neutral lipids. These liposomes were loaded with amlodipine besylate (AB), a next-generation calcium channel inhibitor, to treat HAPE on time. AB-loaded PSMVLs (AB-PSMVLs) were destroyed, and AB was released through treatment under hydrostatic pressure of at least 25 mmHg. At 25 mmHg, which is the minimum pulmonary artery pressure value in HAPE, 38.8% of AB was released within 1 h. In a mouse HAPE model, AB-PSMVLs concentrated in the lung and released AB to diffuse into the vascular wall. Intravenously injected AB-PSMVLs before HAPE modeling resulted in a stronger protection of lung tissues and respiratory function and lower occurrence of pulmonary edema than treatment with free drug or non-pressure-sensitive AB-loaded liposomes. This study offers a new strategy for developing smart drug delivery systems that respond to changes in bodily fluid pressures.

Multimodal Finger Pulse Wave Sensing: Comparison of Forcecardiography and Photoplethysmography Sensors

Pulse waves (PWs) are mechanical waves that propagate from the ventricles through the whole vascular system as brisk enlargements of the blood vessels' lumens, caused by sudden increases in local blood pressure. Photoplethysmography (PPG) is one of the most widespread techniques employed for PW sensing due to its ability to measure blood oxygen saturation. Other sensors and techniques have been proposed to record PWs, and include applanation tonometers, piezoelectric sensors, force sensors of different kinds, and accelerometers. The performances of these sensors have been analyzed individually, and their results have been found not to be in good agreement (e.g., in terms of PW morphology and the physiological parameters extracted). Such a comparison has led to a deeper comprehension of their strengths and weaknesses, and ultimately, to the consideration that a multimodal approach accomplished via sensor fusion would lead to a more robust, reliable, and potentially more informative methodology for PW monitoring. However, apart from various multichannel and multi-site systems proposed in the literature, no true multimodal sensors for PW recording have been proposed yet that acquire PW signals simultaneously from the same measurement site. In this study, a true multimodal PW sensor is presented, which was obtained by integrating a piezoelectric forcecardiography (FCG) sensor and a PPG sensor, thus enabling simultaneous mechanical-optical measurements of PWs from the same site on the body. The novel sensor performance was assessed by measuring the finger PWs of five healthy subjects at rest. The preliminary results of this study showed, for the first time, that a delay exists between the PWs recorded simultaneously by the PPG and FCG sensors. Despite such a delay, the pulse waveforms acquired by the PPG and FCG sensors, along with their first and second derivatives, had very high normalized cross-correlation indices in excess of 0.98. Six well-established morphological parameters of the PWs were compared via linear regression, correlation, and Bland-Altman analyses, which showed that some of these parameters were not in good agreement for all subjects. The preliminary results of this proof-of-concept study must be confirmed in a much larger cohort of subjects. Further investigation is also necessary to shed light on the physical origin of the observed delay between optical and mechanical PW signals. This research paves the way for the development of true multimodal, wearable, integrated sensors and for potential sensor fusion approaches to improve the performance of PW monitoring at various body sites.

Photoplethysmogram Analysis and Applications: An Integrative Review

Beyond its use in a clinical environment, photoplethysmogram (PPG) is increasingly used for measuring the physiological state of an individual in daily life. This review aims to examine existing research on photoplethysmogram concerning its generation mechanisms, measurement principles, clinical applications, noise definition, pre-processing techniques, feature detection techniques, and post-processing techniques for photoplethysmogram processing, especially from an engineering point of view. We performed an extensive search with the PubMed, Google Scholar, Institute of Electrical and Electronics Engineers (IEEE), ScienceDirect, and Web of Science databases. Exclusion conditions did not include the year of publication, but articles not published in English were excluded. Based on 118 articles, we identified four main topics of enabling PPG: (A) PPG waveform, (B) PPG features and clinical applications including basic features based on the original PPG waveform, combined features of PPG, and derivative features of PPG, (C) PPG noise including motion artifact baseline wandering and hypoperfusion, and (D) PPG signal processing including PPG preprocessing, PPG peak detection, and signal quality index. The application field of photoplethysmogram has been extending from the clinical to the mobile environment. Although there is no standardized pre-processing pipeline for PPG signal processing, as PPG data are acquired and accumulated in various ways, the recently proposed machine learning-based method is expected to offer a promising solution.

In vivo photoacoustic assessment of the oxygen saturation changes in the human radial artery: a preliminary study associated with age

SIGNIFICANCE

We demonstrate the potential of probing the sO2 change under blood flow in vivo using photoacoustic (PA) imaging and sheds light on the complex relationship between RBC aggregation and oxygen delivery.

AIM

To conduct in vivo assessments of the sO2 in the radial artery of healthy volunteers and simultaneously probe the relation between the sO2 and hemodynamic behavior such as red blood cell (RBC) aggregation.

APPROACH

The effects of PA-based measurements of blood hemodynamics were studied as a function of the subjects' age (20s, 30s, and 40s). The pulsatile blood flow in the human radial artery of 12 healthy subjects was imaged in the 700 to 900 nm optical wavelength range using a linear array-based PA system.

RESULTS

The PA power when blood velocity is minimum (Pamax) was larger than the one attained at maximum blood velocity (Pamin), consistent with predictions based on the cyclical variation of RBC aggregation during pulsatile flow. The difference between Pamin and Pamax at 800 nm (ΔPa800) increased with age (1.7, 2.2, and 2.6 dB for age group of 20s, 30s, and 40s, respectively). The sO2 computed from Pamax was larger than the one from Pamin.

CONCLUSIONS

The ΔPa800 increased with participant age. The ΔPa800 metric could be a surrogate of noninvasively monitoring the age-induced changes in RBC aggregation. The sO2 change during a cycle of pulsatile blood flow also increased with age, demonstrating that RBC aggregation can affect the sO2 change.

The Sensing Endotracheal Tube

Current pulse oximetry sensors are not very well suited to use in anaesthetised patients as it has been shown that during episodes of reduced peripheral circulation they do not function correctly or fail all together [1], [2]. To address this problem a new design for a photoplethysmography (PPG) endotracheal (ET) sensor to monitor pulse rate and oxygen saturation (SpO₂) internally is presented. Flexible printed circuit board (PCB) technology and miniature optoelectronic components have been implemented and integrated with a custom instrumentation system [3]. The sensor adheres and conforms to the curvature of standard french-gauge 7 and 8 ET tubes at the point just above the inflatable cuff within the laryngeal positioning markings. A 3D-modelled, optically clear, soft silicon encapsulation electronically and thermally isolates the electronic components whilst providing a smooth surface to aid the insertion on the ET tube during standard intubation procedures. A pilot study with 5 patients (3 Female, 2 Male), undergoing abdominal and limb laproscopic procedures has demonstrated the operation of the sensing ET tube, showing good quality red and infra-red PPG signals. Preliminary signal analysis reveals heart rate can be measured via PPG successfully, with saturation (SpO₂) readings in close agreement with the commercial monitors of 97.9 % (STD 0.2 %) and 98.6 % (STD 0.8 %) respectively.

A Novel Photoplethysmography Sensor for Vital Signs Monitoring from the Human Trachea

Current pulse oximeter sensors can be challenged in working accurately and continuously in situations of reduced periphery perfusion, especially among anaesthetised patients. A novel tracheal photoplethysmography (PPG) sensor has been developed in an effort to address the limitations of current pulse oximeters. The sensor has been designed to estimate oxygen saturation (SpO2) and pulse rate, and has been manufactured on a flexible printed circuit board (PCB) that can adhere to a standard endotracheal (ET) tube. A pilot clinical trial was carried out as a feasibility study on 10 anaesthetised patients. Good quality PPGs from the trachea were acquired at red and infrared wavelengths in all patients. The mean SpO2 reading for the ET tube was 97.1% (SD 1.0%) vs. the clinical monitor at 98.7% (SD 0.7%). The mean pulse rate for the ET sensor was 65.4 bpm (SD 10.0 bpm) vs. the clinical monitor at 64.7 bpm (SD 9.9 bpm). This study supports the hypothesis that the human trachea could be a suitable monitoring site of SpO2 and other physiological parameters, at times where the periphery circulation might be compromised.

SPECMAR: fast heart rate estimation from PPG signal using a modified spectral subtraction scheme with composite motion artifacts reference generation

The task of heart rate estimation using photoplethysmographic (PPG) signal is challenging due to the presence of various motion artifacts in the recorded signals. In this paper, a fast algorithm for heart rate estimation based on modified SPEctral subtraction scheme utilizing Composite Motion Artifacts Reference generation (SPECMAR) is proposed using two-channel PPG and three-axis accelerometer signals. First, the preliminary noise reduction is obtained by filtering unwanted frequency components from the recorded signals. Next, a composite motion artifacts reference generation method is developed to be employed in the proposed SPECMAR algorithm for motion artifacts reduction. The heart rate is then computed from the noise and motion artifacts reduced PPG signal. Finally, a heart rate tracking algorithm is proposed considering neighboring estimates. The performance of the SPECMAR algorithm has been tested on publicly available PPG database. The average heart rate estimation error is found to be 2.09 BPM on 23 recordings. The Pearson correlation is 0.9907. Due to low computational complexity, the method is faster than the comparing methods. The low estimation error, smooth and fast heart rate tracking makes SPECMAR an ideal choice to be implemented in wearable devices. Graphical Abstract Flow chart for the heart rate estimation using modified SPEctral subtraction scheme utilizing Composite Motion Artifacts Reference generation (SPECMAR) from photoplethysmographic (PPG) signals.

Unmasking the Hypovolemic Shock Continuum: The Compensatory Reserve

Hypovolemic shock exists as a spectrum, with its early stages characterized by subtle pathophysiologic tissue insults and its late stages defined by multi-system organ dysfunction. The importance of timely detection of shock is well known, as early interventions improve mortality, while delays render these same interventions ineffective. However, detection is limited by the monitors, parameters, and vital signs that are traditionally used in the intensive care unit (ICU). Many parameters change minimally during the early stages, and when they finally become abnormal, hypovolemic shock has already occurred. The compensatory reserve (CR) is a parameter that represents a new paradigm for assessing physiologic status, as it comprises the sum total of compensatory mechanisms that maintain adequate perfusion to vital organs during hypovolemia. When these mechanisms are overwhelmed, hemodynamic instability and circulatory collapse will follow. Previous studies involving CR measurements demonstrated their utility in detecting central blood volume loss before hemodynamic parameters and vital signs changed. Measurements of the CR have also been used in clinical studies involving patients with traumatic injuries or bleeding, and the results from these studies have been promising. Moreover, these measurements can be made at the bedside, and they provide a real-time assessment of hemodynamic stability. Given the need for rapid diagnostics when treating critically ill patients, CR measurements would complement parameters that are currently being used. Consequently, the purpose of this article is to introduce a conceptual framework where the CR represents a new approach to monitoring critically ill patients. Within this framework, we present evidence to support the notion that the use of the CR could potentially improve the outcomes of ICU patients by alerting intensivists to impending hypovolemic shock before its onset.

Development of a new analgesic index using nasal photoplethysmography

Although surrogate measures to quantify pain intensity have been commercialised, there is a need to develop a new index with improved accuracy. The aim of this study was to develop a new analgesic index using nasal photoplethysmography data. The specially designed sensor was placed between the columella and the nasal septum to acquire nasal photoplethysmography in surgical patients. Nasal photoplethysmography and Surgical Pleth Index^® (GE Healthcare) data were obtained for 14 min both in the absence (pre-operatively) or presence (postoperatively) of pain in a group of surgical patients, each patient acting as their own control. Various dynamic photoplethysmography variables were extracted to quantify pain intensity; the most accurate index was selected using logistic regression as a classifier. The area under the curve of the receiver-operating characteristic curve was measured to evaluate the accuracy of final model predictions. In total, 12,012 heart beats from 89 patients were used to develop a new Nasal Photoplethysmography Index for analgesic depth quantification. The two-variable model (a combination of diastolic peak point variation and heart beat interval variation) was most accurate in discriminating between the presence and absence of pain (numerical rating scale (NRS) ≥ 3). The accuracy and area under the curve of the receiver-operating characteristic curve for the Nasal Photoplethysmography Index were 75.3% and 0.8018, respectively, and 64.8% and 0.7034, respectively, for the Surgical Pleth Index. The Nasal Photoplethysmography Index clearly distinguished pain (NRS ≥ 3) in awake surgical patients with postoperative pain. The Nasal Photoplethysmography Index performed better than the Surgical Pleth Index. Further validation studies are needed to evaluate its feasibility to quantify pain intensity during general anaesthesia.

Can the descending aortic stroke volume be estimated by transesophageal descending aortic photoplethysmography?

PURPOSE

The aim of this study was to investigate the ability of transesophageal photoplethysmography detected from the descending aorta (dPPG) for predicting low descending aortic stroke volume (dSV) level in cardiac surgical patients.

METHODS

Fifteen patients scheduled for elective cardiac surgery were enrolled in our study. A transesophageal echocardiography (TEE) probe with an attached oximetry sensor was placed into the esophagus for paired dPPG signal and descending aortic Doppler blood flow signal acquisition. Metrics, including alternating current (AC), direct current (DC), area under the curve (AUC) and width (W), were extracted from the dPPG signals. The TEE-measured dSV, which was defined as the blood flow through the descending aorta during a cardiac cycle, was chosen as the standard reference. A receiver operating characteristic (ROC) curve was built to evaluate the performance of dPPG metrics in predicting low dSV level, and dSV measuring agreement between TEE and dPPG was analyzed by the Bland-Altman method.

RESULTS

A total of 644 paired dPPG and Doppler signals of the descending aorta were acquired. Significant correlations were found between the dPPG metrics and TEE-measured dSV, and the correlation coefficients between TEE-measured dSV and AUC or AC were 0.64 and 0.66, respectively. AUC and AC values obviously decreased with the reduction of dSV level among the three groups (<20 mL, from 20-40 mL, and >40 mL). The areas under the ROC curve for AUC and AC in predicting low dSV level (<20 mL) were 0.85 and 0.88, respectively. Bland-Altman plot showed a small bias (0.02 mL) but a wide limit of agreement (-18.62 to 18.66 mL) in dSV measurement between dPPG and Doppler technology.

CONCLUSIONS

The AC and AUC extracted from the dPPG signal provided a sensitive and qualitative prediction for dSV level. The dSV value could not be accurately measured by dPPG metrics.

TRIAL REGISTRATION

Chinese Clinical Trials Register Identifier: ChiCTR-OCS-12002789.

Estimation of instantaneous venous blood saturation using the photoplethysmograph waveform

Non-invasive estimation of regional venous saturation (SxvO2) using a conventional pulse oximeter could provide a means of obtaining clinically relevant information. This study was carried out in order to investigate the hypothesis that SxvO2 could be estimated by utilising the modulations created by positive pressure ventilation in the photoplethysmograph (PPG) signals. The modulations caused by the mechanical ventilator were extracted from oesophageal PPG signals obtained from 12 patients undergoing cardiothoracic surgery. The signals analysed in this work were acquired in a previous study. For the purpose of this analysis the raw PPG signal was considered to have three major components, ac PPG signal (cardiac related component), a static component or dc PPG signal (created mostly by the absorption of light by surrounding tissue) and the ventilator modulation component. These components were then used to estimate instantaneous arterial blood oxygen saturation (SpO2) and SxvO2 by utilising time-frequency analysis technique of smoothed-pseudo Wigner-Ville distribution (SPWVD). The results showed that there was no significant difference in the traditionally-derived (time-domain) arterial saturation and the instantaneous arterial saturation. However, the instantaneous venous saturation was found to be significantly lower than the estimated time-domain and instantaneous arterial saturation (P   =  < 0.001, n = 12).

Pulse transit time as a surrogate measure of changes in systolic arterial pressure in children during sleep

Pulse transit time has been proposed as a surrogate measure of systolic arterial pressure, as it is dependent upon arterial stiffness. Past research has shown that pulse transit time has a significant inverse relationship to systolic arterial pressure in adults; however, studies in children are limited. This study aimed to explore the relationship between systolic arterial pressure and pulse transit time in children during sleep. Twenty-five children (13.1 ± 1.6 years, 48% male) underwent overnight polysomnography (PSG) with a simultaneous recording of continuous systolic arterial pressure and photoplethysmography. Pulse transit time was calculated as the time delay between the R-wave peak of the electrocardiogram (ECG) to the 50% point of the upstroke of the corresponding photoplethysmography waveform; 500 beats of simultaneous systolic arterial pressure and pulse transit time were analysed in each sleep stage for each child. Pulse transit time was normalized to each subject's mean wake pulse transit time. The ability of pulse transit time to predict systolic arterial pressure change was determined by linear mixed-effects modelling. Significant negative correlations between pulse transit time and systolic arterial pressure were found for individual children for each sleep stage [mean correlations for cohort: non-rapid eye movement (NREM) sleep 1 and 2 r = -0.57, slow wave sleep (SWS) r = -0.76, REM r = -0.65, P < 0.01 for all]. Linear mixed-model analysis demonstrated that changes in pulse transit time were a significant predictor of changes in systolic arterial pressure for each sleep stage (P < 0.001). The model of pulse transit time-predicted systolic arterial pressure closely tracked actual systolic arterial pressure changes over time. This study demonstrated that pulse transit time was accurate in tracking systolic arterial pressure changes over time. Thus, the use of pulse transit time as a surrogate measure of changes in systolic arterial pressure in children is a valid, non-invasive and inexpensive method with many potential applications.

The human ear canal: investigation of its suitability for monitoring photoplethysmographs and arterial oxygen saturation

For the last two decades, pulse oximetry has been used as a standard procedure for monitoring arterial oxygen saturation (SpO2). However, SpO2 measurements made from extremities such as the finger, ear lobe and toes become susceptible to inaccuracies when peripheral perfusion is compromised. To overcome these limitations, the external auditory canal has been proposed as an alternative monitoring site for estimating SpO2, on the hypothesis that this central site will be better perfused. Therefore, a dual wavelength optoelectronic probe along with a processing system was developed to investigate the suitability of measuring photoplethysmographic (PPG) signals and SpO2 in the human auditory canal. A pilot study was carried out in 15 healthy volunteers to validate the feasibility of measuring PPGs and SpO2 from the ear canal (EC), and comparative studies were performed by acquiring the same signals from the left index finger (LIF) and the right index finger (RIF) in conditions of induced peripheral vasoconstriction (right hand immersion in ice water). Good quality baseline PPG signals with high signal-to-noise ratio were obtained from the EC, the LIF and the RIF sensors. During the ice water immersion, significant differences in the amplitude of the red and infrared PPG signals were observed from the RIF and the LIF sensors. The average drop in amplitude of red and infrared PPG signals from the RIF was 52.7% and 58.3%. Similarly, the LIF PPG signal amplitudes have reduced by 47.52% and 46.8% respectively. In contrast, no significant changes were seen in the red and infrared EC PPG amplitude measurements, which changed by +2.5% and -1.2% respectively. The RIF and LIF pulse oximeters have failed to estimate accurate SpO2 in seven and four volunteers respectively, while the EC pulse oximeter has only failed in one volunteer. These results suggest that the EC may be a suitable site for reliable monitoring of PPGs and SpO2s even in the presence of peripheral vasoconstriction.

Laser-speckle-based detection of fluid pulsation in the presence of motion artifacts: in vitro and in vivo study

We have performed an in vitro and in vivo study, based on laser speckle contrast analysis, to detect fluid pulsation in the presence of artifacts caused by the relative motion between the sample and the illumination source. We observe that the pulsation signal is clearly detectable for a range of motion amplitudes and oscillation frequencies; however, for higher amplitudes and oscillation frequencies of motion, the signal, due to pulsation, becomes increasingly difficult to detect.

Photoplethysmographic measurements from the esophagus using a new fiber-optic reflectance sensor

A prototype fiber-optic reflectance-mode pulse oximetry sensor and measurement system is developed for the purposes of estimating arterial oxygen saturation in the esophagus. A dedicated probe containing miniature right-angled glass prisms coupled to light sources and a photodetector by means of optical fibers is designed and used to record photoplethysmographic (PPG) signals from the esophageal epithelium in anesthetized patients. The probe is inserted simply by an anesthesiologist in all cases, and signals are recorded successfully in all but one of 20 subjects, demonstrating that esophageal PPG signals can be reliably obtained. The mean value of the oxygen saturation recorded from the esophagus for all subjects is 94.0 ± 4.0%. These results demonstrate that SpO(2) may be estimated in the esophagus using a fiber-optic probe.

Measuring venous oxygenation using the photoplethysmograph waveform

OBJECTIVE

We investigate the hypothesis that the photoplethysmograph (PPG) waveform can be analyzed to infer regional venous oxygen saturation.

METHODS

Fundamental to the successful isolation of the venous saturation is the identification of PPG characteristics that are unique to the peripheral venous system. Two such characteristics have been identified. First, the peripheral venous waveform tends to reflect atrial contraction. Second, ventilation tends to move venous blood preferentially due to the low pressure and high compliance of the venous system. Red (660 nm) and IR (940 nm) PPG waveforms were collected from 10 cardiac surgery patients using an esophageal PPG probe. These waveforms were analyzed using algorithms written in Mathematica. Four time-domain saturation algorithms (ArtSat, VenSat, ArtInstSat, VenInstSat) and four frequency-domain saturation algorithms (RespDC, RespAC, Cardiac, and Harmonic) were applied to the data set.

RESULTS

Three of the algorithms for calculating venous saturation (VenSat, VenInstSat, and RespDC) demonstrate significant difference from ArtSat (the conventional time-domain algorithm for measuring arterial saturation) using the Wilcoxon signed-rank test with Bonferroni correction (p < 0.0071).

CONCLUSIONS

This work introduces new algorithms for PPG analysis. Three algorithms (VenSat, VenInstSat, and RespDC) succeed in detecting lower saturation blood. The next step is to confirm the accuracy of the measurement by comparing them to a gold standard (i.e., venous blood gas).

Evaluation of a fiber-optic esophageal pulse oximeter

A dual-wavelength fiber-optic pulse oximetry system was developed for the purposes of estimating oxygen saturation from the esophagus. A probe containing miniature right-angled glass prisms was used to record photoplethysmographic (PPG) signals from the esophageal wall. Signals were recorded successfully in 19 of 20 patients, demonstrating that PPG signals could be reliably obtained from an internal vascularized tissue site such as the esophageal epithelium. The value of the mean oxygen saturation recorded from the esophagus was 94.0 +/- 4.0%. These results demonstrate that SpO(2) may be estimated in the esophagus using a fiber-optic probe and this may be the first report of such measurements.

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.

Pulse oximetry and photoplethysmographic waveform analysis of the esophagus and bowel

PURPOSE OF REVIEW

This article reviews the development of novel reflectance pulse oximetry sensors for the esophagus and bowel, and presents some of the techniques used to analyze the waveforms acquired with such devices.

RECENT FINDINGS

There has been much research in recent years to expand the utility of pulse oximetry beyond the simple measurement of arterial oxygen saturation from the finger or earlobe. Experimental sensors based on reflectance pulse oximetry have been developed for use in internal sites such as the esophagus and bowel. Analysis of the photoplethysmographic waveforms produced by these sensors is beginning to shed light on some of the potentially useful information hidden in these signals.

SUMMARY

The use of novel reflectance pulse oximetry sensors has been successfully demonstrated. Such sensors, combined with the application of more advanced signal processing, will hopefully open new avenues of research leading to the development of new types of pulse oximetry-based monitoring techniques.

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.

Effects of poorly perfused peripheries on derived transit time parameters of the lower and upper limbs

A simple and non-intrusive approach termed the pulse transit time ratio (PTTR) has recently been shown to be a potential surrogate of the ankle-brachial index (ABI). PTTR is based on the principle of PTT, which is known to be temperature-sensitive. In this study, 23 healthy adults with normally perfused peripheries and 10 with poorly perfused peripheries were recruited. No significant change in PTTR was observed between those with cold (1.287+/-0.043) and normal (1.290+/-0.027) peripheries (p>0.05). A cold periphery may cause pulse waveform changes and indirectly affect PTT owing to poor skin microcirculation, but may have a limited effect on PTTR, which is useful as an ABI alternative.

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.

Photoplethysmography and its application in clinical physiological measurement

Photoplethysmography (PPG) is a simple and low-cost optical technique that can be used to detect blood volume changes in the microvascular bed of tissue. It is often used non-invasively to make measurements at the skin surface. The PPG waveform comprises a pulsatile ('AC') physiological waveform attributed to cardiac synchronous changes in the blood volume with each heart beat, and is superimposed on a slowly varying ('DC') baseline with various lower frequency components attributed to respiration, sympathetic nervous system activity and thermoregulation. Although the origins of the components of the PPG signal are not fully understood, it is generally accepted that they can provide valuable information about the cardiovascular system. There has been a resurgence of interest in the technique in recent years, driven by the demand for low cost, simple and portable technology for the primary care and community based clinical settings, the wide availability of low cost and small semiconductor components, and the advancement of computer-based pulse wave analysis techniques. The PPG technology has been used in a wide range of commercially available medical devices for measuring oxygen saturation, blood pressure and cardiac output, assessing autonomic function and also detecting peripheral vascular disease. The introductory sections of the topical review describe the basic principle of operation and interaction of light with tissue, early and recent history of PPG, instrumentation, measurement protocol, and pulse wave analysis. The review then focuses on the applications of PPG in clinical physiological measurements, including clinical physiological monitoring, vascular assessment and autonomic function.

Filtering techniques for the removal of ventilator artefact in oesophageal pulse oximetry

The oesophagus has been shown to be a reliable site for monitoring blood oxygen saturation (SpO(2)). However, the photoplethysmographic (PPG) signals from the lower oesophagus are frequently contaminated by a ventilator artefact making the estimation of SpO(2) impossible. A 776th order finite impulse response (FIR) filter and a 695th order interpolated finite impulse response (IFIR) filter were implemented to suppress the artefact. Both filters attenuated the ventilator artefact satisfactorily without distorting the morphology of the PPG when processing recorded data from ten cardiopulmonary bypass patients. The IFIR filter was the better since it conformed more closely to the desired filter specifications and allowed real-time processing. The average improvements in signal-to-noise ratio (SNR) achieved by the FIR and IFIR filters for the fundamental component of the red PPG signals with respect to the fundamental component of the artefact were 57.96 and 60.60 dB, respectively. The corresponding average improvements achieved by the FIR and IFIR filters for the infrared PPG signals were 54.83 and 60.96 dB, respectively. Both filters were also compared with their equivalent tenth order Butterworth filters. The average SNR improvements for the FIR and IFIR filters were significantly higher than those for the Butterworth filters.

Pulse Transit Time Based on Piezoelectric Technique at the Radial Artery

OBJECTIVES

Pulse transit time (PTT) has shown its potential in relevant cardiovascular and cardiorespiratory studies. However, the use of photoplethysmography (PPG) in PTT measurement can be limited in events of poor peripheral perfusion. Uninterrupted PTT monitoring may also not be achievable when less cooperative patients distribute the PPG probe due to its prominent light source. Hence, there is a need for an alternative method to measure PTT in such incidents.

METHODS

In this study, the piezoelectric (PIEZO) technique to detect pulsations from a human wrist above the radial artery to estimate PTT is presented. 17 healthy adults (11 male; age range of 21-33 years) were recruited to compare PTT and heart rate (HR) differences between the PPG and PIEZO methods. These time-related derivations were made with respect to an electrocardiogram (ECG).

RESULTS

The timing consistency of the PIEZO transducer shows significant correlations (p < 0.01) to those derived from the ECG and a pulse oximeter. Particularly, there is a high level of agreement of < 1 beat per minute (bpm) difference in HR estimates observed when compared to the two commercial devices in the respective Bland-Altman plots. Comparison of PTT obtained from the PIEZO transducer against the PPG signal shows constantly lower values due to the shorter path length it requires to propagate. A regression equation was formulated to relate the PTT values acquired from both these signals.

CONCLUSIONS

Preliminary findings herein suggest that the PIEZO technique can be useful as an alternative for PTT monitoring. This shows promise to be more accommodating for less cooperative patients or those with insufficient peripheral perfusion.

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.

Pulse oximetry in the oesophagus

Pulse oximetry has been one of the most significant technological advances in clinical monitoring in the last two decades. Pulse oximetry is a non-invasive photometric technique that provides information about the arterial blood oxygen saturation (SpO(2)) and heart rate, and has widespread clinical applications. When peripheral perfusion is poor, as in states of hypovolaemia, hypothermia and vasoconstriction, oxygenation readings become unreliable or cease. The problem arises because conventional pulse oximetry sensors must be attached to the most peripheral parts of the body, such as finger, ear or toe, where pulsatile flow is most easily compromised. Since central blood flow may be preferentially preserved, this review explores a new alternative site, the oesophagus, for monitoring blood oxygen saturation by pulse oximetry. This review article presents the basic physics, technology and applications of pulse oximetry including photoplethysmography. The limitations of this technique are also discussed leading to the proposed development of the oesophageal pulse oximeter. In the majority, the report will be focused on the description of a new oesophageal photoplethysmographic/SpO(2) probe, which was developed to investigate the suitability of the oesophagus as an alternative monitoring site for the continuous measurement of SpO(2) in cases of poor peripheral circulation. The article concludes with a review of reported clinical investigations of the oesophageal pulse oximeter.

Evaluation of oesophageal reflectance pulse oximetry in major burns patients

Pulse oximetry is being used in everyday clinical practice in anaesthesia utilising a peripheral probe. However, it may be unreliable in certain clinical situations such as hypothermia, hypovolemia, vasoconstriction and decreased cardiac output. Similar situations occur in burns patients and, more importantly, burns to extremities which limit the sites available for measurement of peripheral oxygen saturation (SpO(2)). To overcome these limitations, the oesophagus has been investigated as an alternative measurement site, as perfusion may be preferentially preserved centrally. A miniaturised reflectance oesophageal saturation (SpO(2)) probe has been constructed utilising infrared and red photodiodes and a photodetector. Our study was aimed at evaluating the reliability of oesophageal pulse oximetry in major burns patients. Seven adult patients (five males, two females) were studied. They were sedated and ventilated as part of their routine care. Measurable photoplethysmographic (PPG) traces and SpO(2) values were obtained in the oesophagus of all patients at a mean depth of 15.6+/-1.8 cm (measured from the lips). It was found that the oesophageal pulse oximeter results were in good agreement with oxygen saturation measurements obtained by a CO-oximeter. The mean (+/-S.D.) of the differences between the oesophageal oxygen saturation results and those from CO-oximetry was 0.50+/-0.69%. A Bland and Altman analysis showed that the bias and the limits of agreement between the oesophageal and commercial toe pulse oximeters were 0.4% and -3.6% to 4.6%, respectively. This study suggests that the oesophagus can be used as an alternative site for monitoring arterial blood oxygen saturation by pulse oximetry in burned patients.