Measuring Venous Oxygen Saturation Using the Photoplethysmograph Waveform

The pulse oximeter measures the arterial oxygen saturation. It accomplishes this through the use of the photoplethysmograph waveform (PPG) at two or more wavelengths. It has been recognized for some time that the movement of venous blood can be detected using the PPG. We hypothesize that the PPG waveform, obtained non-invasively by modern pulse oximeters, can be analyzed via digital signal processing to infer the venous oxygen saturation. 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 (e.g., a-c-v waveform). Second, ventilation tends to move venous blood preferentially due to the low pressure and high compliance of the venous system. Red (660nm) and IR (940nm) PPG waveforms were collected from 10 cardiac surgery patients using an esophageal PPG probe. These waveforms were analyzed using algorithms written in Mathematica (Wolfram Research). The eight saturation algorithms (ArtSat, VenSat, ArtInstSat, VenInstSat, RespDC, RespAC, Cardiac, and Harmonic) were applied to the data set. Three of the methods (VenSat, VenInstSat, and RespDC) demonstrate significance difference from ArtSat using the Wilcoxon signed-rank test with Bonferroni correction (p\u3c0.0071). This thesis introduces new methods of PPG analysis. Three methods of analysis (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)

PHOTOPLETHYSMOGRAPHIC WAVEFORM ANALYSIS DURING LOWER BODY NEGATIVE PRESSURE SIMULATED HYPOVOLEMIA AS A TOOL TO DISTINGUISH REGIONAL DIFFERENCES IN MICROVASCULAR BLOOD FLOW REGULATION.

The purpose of this investigation was to explore modulation of the photoplethsymographic (PPG) waveform in the setting of simulated hypovolemia as a tool to distinguish regional differences in regulation of the microvasculature. The primary goal was to glean useful physiological and clinical information as it pertains to these regional differences in regulation of microvascular blood flow. This entailed examining the cardiovascular, autonomic nervous, and respiratory systems interplay in the functional hemodynamics of regulation of microvascular blood flow to both central (ear, forehead) and peripheral (finger) sites. We monitored ten healthy volunteers (both men and women age 24-37 ) non-invasively with central and peripheral photoplethysmographs and laser Doppler flowmeters during Lower Body Negative Pressure (LBNP). Waveform amplitude, width, and oscillatory changes were characterized using waveform analysis software (Chart, ADInstruments). Data were analyzed with the Wilcoxon Signed Ranks Test, paired t-tests, and linear regression. Finger PPG amplitude decreased by 34.6 ± 17.6% (p = 0.009) between baseline and the highest tolerated LBNP. In contrast, forehead amplitude changed by only 2.4 ± 16.0% (p=NS). Forehead and finger PPG width decreased by 48.4% and 32.7%, respectively. Linear regression analysis of the forehead and finger PPG waveform widths as functions of time generated slopes of -1.113 (R = -0.727) and -0.591 (R = -0.666), respectively. A 150% increase in amplitude density of the ear PPG waveform was noted within the range encompassing the respiratory frequency (0.19-0.3Hz) (p=0.021) attributable to changes in stroke volume. We also noted autonomic modulation of the ear PPG signal in a different frequency band (0.12 0.18 Hz). The data indicate that during a hypovolemic challenge, healthy volunteers had a relative sparing of central cutaneous blood flow when compared to a peripheral site as indicated by observable and quantifiable changes in the PPG waveform. These results are the first documentation of a local vasodilatation at the level of the terminal arterioles of the forehead that may be attributable to recently documented cholinergic mechanisms on the microvasculature

Arterial And Venous Impacts Of Transdermally Administered Vasodilators On The Local Microvasculature

Microcirculatory function is an important component of cardiovascular pharmacology as related to cardiovascular dysfunction. We used photoplethysmography (PPG) to compare the microcirculatory effects of transdermal patches of rivastigmine (Exelon, Novartis), nicotine, nitroglycerin (NTG) and topically applied EMLA (eutectic mixture of lidocaine/prilocaine). We anticipate that this initial pilot comparison of single doses of each medication will catalyze future multi-dose comparisons of the various vasodilatory features of these and other drugs. Methods: With IRB approval, 10 healthy volunteers were monitored with PPG at the time each transdermal patch was applied and every 8 minutes afterwards for a total of 40 minutes. 1x1 cm portions of patches of rivastigmine, nicotine, and NTG were placed and monitored on different sites of the forehead. Another site was isolated and pretreated 6 hours earlier with EMLA, since this drug requires many hours to induce vasodilation.1 All voltage changes were changed to ACmults, i.e., in multiples of the change in voltage associated with delivery of the stroke volume to the given site under resting conditions2. A linear mixed model was used to compare patch effects on maximum change in AC, DC, and ΔAC/ΔDC. This model accounts for the variance that can be attributed to an individual’s multiple measurements within an unstructured covariance matrix. A p-value \u3c0.05 was given to be statistically significant. Data were expressed as mean within a 95% confidence interval. Results: The max ∆AC change for each drug were significantly different from that of its control while only the max ∆DC of NTG was significantly greater than that of its control. Changes in the ΔAC/ΔDC ratio were found to be inconsistent. Rivastigmine and control had significantly lower ΔAC/ΔDC values at 8 minutes compared to that of EMLA; the differences were not significant at following time points. Discussion: These results may provide some insight into the cardiovascular effects of the study agents used. NTG, a direct NO donor, caused significant increases in AC (arteriolar) and DC (venous) values. Acting at the pre/post ganglionic junction of local parasympathetic pathways to the region of the precapillary sphincter, nicotine caused a significant increase in AC, whereas the change in DC was not found to be significant. Rivastigmine, which inhibits the metabolic degradation of acetylcholine, caused a selective increase in AC. The local anesthetic (EMLA) caused a significant increase in AC. Rivastigmine caused significantly lower ΔAC/ΔDC ratio at 8 minutes when compared to EMLA (which had been on for 6 hours previously). At \u3e16 minutes, the ΔAC/ΔDC values of rivastigmine and EMLA did not differ significantly, findings that may reflect the time needed for acetylcholine to to increase over time via the inhibition of its breakdown as opposed to an immediate introduction of additional agonist via the nicotine or NTG patch. Future studies using different and/or additional drug combinations may help give further insight into the convoluted physiology of the microvasculature

Non-invasive venous oximetry through venous blood volume modulation

For decades, the monitoring of mixed venous oxygen saturation has been done invasively using fibre-optic catheters. This procedure is not without risk as complications may arise from catheterization. This thesis describes an alternative and novel means of monitoring venous oxygen saturation. The technique outlined involves inducing regular modulations of the venous blood volume and the associated measurement of those modulations using an optical sensor. Just as pulse oximetry utilizes the natural arterial pulse to perform spectral analysis of the peripheral blood in order to estimate the arterial blood oxygen saturation, the new venous oximetry technique uses the artificially generated pulse to perform the task of measuring peripheral venous oxygen saturation. This thesis explores and investigates the feasibility of this new venous oximetry technique. A heuristic model was first developed to predict the effects of introducing an artificially generated pulsatile signal in the venous system. The effect on the underlying natural arterial pulsation was also examined. Experiments were then conducted to justify and interpret the model developed. Other experiments were also conducted to optimize the design of the artificial pulse-based venous oximeter, to explore the effects of prolonged modulation of the venous system and to establish evidence that the measurements made were indeed related to venous oxygen saturation. It is concluded that the new venous oximetry technique is indeed feasible and with further research and development would one day replace the current invasive method

Development and evaluation of venous oximetry

Photoplethysmography, a technique to measure by optical means volume changes, has been known and applied for many years. One of its most popular applications is pulse oximetry, a non-invasive method to measure oxygen content in arterial blood. It is based on the principle of arterial blood volume changes due to heart contractions, known as systoles. Systolic pulsations appear on the arterial vascular system, while blood flow in veins does not normally present pulsations, especially at remote parts of the peripheral vascular system, such as the fingers. Therefore, pulse oximetry is only applicable to arteries as their pulsations allow for separation of the pulsatile components from the rest of the absorbing components. A novel non-invasive technique permits the measurement of venous oxygen saturation by introducing a series of pulsations in the veins thus allowing the separation of venous signal components for calculation of venous oxygen saturation. This thesis presents a theoretical model describing the mechanical coupling of arteries and veins and its effects in the accuracy of oxygen saturation measurement. [Continues.

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

Pulse oximeter probes attached to the finger may fail to estimate blood oxygen saturation (SpO2) in patients with compromised peripheral perfusion (e.g. hypothermic cardiopulmonary bypass surgery). The measurement of SpO2 from a central organ such as the oesophagus is suggested as an alternative to overcome this problem. A reflectance oesophageal pulse oximeter probe and a processing system implemented in LabVIEW were developed. The system was evaluated in clinical measurements on 50 cardiothoracic surgery patients. Oesophageal photoplethysmographic (PPG) signals with large amplitudes and high signal-to-noise ratios were measured from various depths within the oesophagus from all the cardiothoracic patients. The oesophageal PPG amplitudes from these patients were in good agreement with previous oesophageal PPG amplitude measurements from healthy anaesthetized patients. The oesophageal pulse oximeter SpO2 results agreed well with the estimated arterial oxygen saturation (SaO2) values inferred from the oxygen tension obtained by blood gas analysis. The mean (+/- SD) of the differences between the oesophageal pulse oximeter SpO2 readings and those from blood gas analysis was 0.02 +/- 0.88%. Also, the oesophageal pulse oximeter was found to be reliable and accurate in five cases of poor peripheral perfusion when a commercial finger pulse oximeter probe failed to estimate oxygen saturation values for at least 10 min. These results suggest that the arterial blood circulation to the oesophagus is less subject to vasoconstriction and decreased PPG amplitudes than are the peripheral sites used for pulse oximetry such as the finger. It is concluded that oesophageal SPO2 monitoring may be of clinical value

Photoplethysmography for blood volumes and oxygenation changes during intermittent vascular occlusions

Photoplethysmography (PPG) is an optical technique that measures blood volume variations. The main application of dual-wavelength PPG is pulse oximetry, in which the arterial oxygen saturation (SpO[Formula: see text]) is calculated noninvasively. However, the PPG waveform contains other significant physiological information that can be used in conjunction to SpO[Formula: see text] for the assessment of oxygenation and blood volumes changes. This paper investigates the use of near infrared spectroscopy (NIRS) processing techniques for extracting relative concentration changes of oxygenated ([Formula: see text]HbO[Formula: see text]), reduced ([Formula: see text]HHb) and total haemoglobin ([Formula: see text]tHb) from dual-wavelength PPG signals during intermittent pressure-increasing vascular occlusions. A reflectance PPG sensor was attached on the left forearm of nineteen (n = 19) volunteers, along with a reference NIRS sensor positioned on the same forearm, above the left brachioradialis. The investigation protocol consisted of seven intermittent and pressure-increasing vascular occlusions. Relative changes in haemoglobin concentrations were obtained by applying the modified Beer-Lambert law to PPG signals, while oxygenation changes were estimated by the difference between red and infrared attenuations of DC PPGs (A[Formula: see text] = [Formula: see text]A[Formula: see text] - [Formula: see text]A[Formula: see text]) and by the conventional SpO[Formula: see text]. The [Formula: see text]HbO[Formula: see text], [Formula: see text]HHb, [Formula: see text]tHb from the PPG signals indicated significant changes in perfusion induced by either partial and complete occlusions (p < 0.05). The trends in the variables extracted from PPG showed good correlation with the same parameters measured by the reference NIRS monitor. Bland and Altman analysis of agreement between PPG and NIRS showed underestimation of the magnitude of changes by the PPG. A[Formula: see text] indicated significant changes for occlusion pressures exceeding 20 mmHg (p < 0.05) and correlation with tissue oxygenation changes measured by NIRS, while SpO[Formula: see text] had significant changes after 40 mmHg (p < 0.05). Relative changes in haemoglobin concentrations can be estimated from PPG signals and they showed a good level of accuracy in the detection of perfusion and oxygenation changes induced by different degrees of intermittent vascular occlusions. These results can open up to new applications of the PPG waveform in the detection of blood volumes and oxygenation changes

BEst (Biomarker Estimation): Health Biomarker Estimation Non-invasively and Ubiquitously

This dissertation focuses on the non-invasive assessment of blood-hemoglobin levels. The primary goal of this research is to investigate a reliable, affordable, and user-friendly point-of-care solution for hemoglobin-level determination using fingertip videos captured by a smartphone. I evaluated videos obtained from five patient groups, three from the United States and two from Bangladesh, under two sets of lighting conditions. In the last group, based on human tissue optical transmission modeling data, I used near-infrared light-emitting diode sources of three wavelengths. I developed novel image processing techniques for fingertip video analysis to estimate hemoglobin levels. I studied video images creating image histogram and subdividing each image into multiple blocks. I determined the region of interest in a video and created photoplethysmogram signals. I created features from image histograms and PPG signals. I used the Partial Least Squares Regression and Support Vector Machine Regression tools to analyze input features and to build hemoglobin prediction models. Using data from the last and largest group of patients studied, I was able to develop a model with a strong linear correlation between estimated and clinically-measured hemoglobin levels. With further data and methodological refinements, the approach I have developed may be able to define a clinically accurate public health applicable tool for hemoglobin level and other blood constituent assessment

Artefact reduction in photoplethysmography

The use of optical techniques in biomedical monitoring and diagnosis is becoming increasingly widespread, primarily because of the non-invasive nature of optically derived measurements. Physiological analysis is usually achieved by characterisation of the spectral or temporal properties of the interaction between light and the anatomy. Although some optical measurements require complex instrumentation and protocols, recent technological advances have resulted in robust and compact equipment that is now used routinely in a multitude of clinical contexts. Unfortunately, these measurements are inherently sensitive to corruption from dynamic physical conditions or external sources of light, inducing signal artefact. Artefact is the primary restriction in the applicability of many optical measurements, especially for ambulatory monitoring and tele-medicine. The most widely used optical measurement is photoplethysmography, a technique that registers dynamic changes in blood volume throughout the peripheral vasculature and can be used to screen for a number of venous disorders, as well as monitoring the cardio-vascular pulse wave. Although photoplethysmographic devices are now incorporated into many patient-monitoring systems, the prevalent application is a measurement known as pulse oximetry, which utilises spectral analysis of the peripheral blood to estimate the arterial haernoglobin oxygen saturation. Pulse oximetry is well established as an early warning for hypoxia and is now mandatory under anaesthesia in many countries. The problem of artefact is prominent in these continuous monitoring techniques, where it is often impossible to control the physical conditions during use. This thesis investigates the possibility of reducing artefact corruption of photoplethysmographic signals in real time, using an electronic processing methodology that is based upon inversion of a physical artefact model. The consequences of this non-linear artefact reduction technique for subsequent signal analysis are discussed, culminating in a modified formulation for pulse oximetry that not only has reduced sensitivity to artefact but also possesses increased generality. The design and construction of a practical electronic system is then used to explore both the implementation issues and the scope of this technique. The performance of artefact reduction obtained is then quantified under realistic experimental conditions, demonstrating that this methodology is successful in removing or reducing a large proportion of artefact encountered in clinically relevant situations. It is concluded that non-linear artefact reduction can be applied to any photoplethysmographic technology, reducing interpretation inaccuracies that would otherwise be induced by signal artefact. It is also speculated that this technology could enable the use of photoplethysmographic systems in applications that are currently precluded by the inherent severity of artefact