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.     

Detection of a spontaneous pulse in photoplethysmograms during automated cardiopulmonary resuscitation in a porcine model

Introduction

Reliable, non-invasive detection of return of spontaneous circulation (ROSC) with minimal interruptions to chest compressions would be valuable for high-quality cardiopulmonary resuscitation (CPR). We investigated the potential of photoplethysmography (PPG) to detect the presence of a spontaneous pulse during automated CPR in an animal study.

Methods

Twelve anesthetized pigs were instrumented to monitor circulatory and respiratory parameters. Here we present the simultaneously recorded PPG and arterial blood pressure (ABP) signals. Ventricular fibrillation was induced, followed by 20 min of automated CPR and subsequent defibrillation. After defibrillation, pediatric-guidelines-style life support was given in cycles of 2 min. PPG and ABP waveforms were recorded during all stages of the protocol. Raw PPG waveforms were acquired with a custom-built photoplethysmograph controlling a commercial reflectance pulse oximetry probe attached to the nose. ABP was measured in the aorta.

Results

In nine animals ROSC was achieved. Throughout the protocol, PPG and ABP frequency content showed strong resemblance. We demonstrate that (1) the PPG waveform allows for the detection of a spontaneous pulse during ventilation pauses, and that (2) frequency analysis of the PPG waveform allows for the detection of a spontaneous pulse and the determination of the pulse rate, even during ongoing chest compressions, if the pulse and compression rates are sufficiently distinct.

Conclusions

These results demonstrate the potential of PPG as a non-invasive means to detect pulse presence or absence, as well as pulse rate during CPR.     

Advantages of imaging photoplethysmography for migraine modeling: new optical markers of trigemino‐vascular activation in rats

BackgroundExistent animal models of migraine are not without drawbacks and limitations. The aim of our study was to evaluate imaging photoplethysmography (PPG) as a method of assessing intracranial blood flow in rats and its changes in response to electrical stimulation of dural trigeminal afferents.MethodsExperiments were carried out with 32 anesthetized adult male Wistar rats. Trigeminovascular system (TVS) was activated by means of electrical stimulation of dural afferents through a closed cranial window (CCW). Parameters of meningeal blood flow were monitored using a PPG imaging system under green illumination with synchronous recording of an electrocardiogram (ECG) and systemic arterial blood pressure (ABP). Two indicators related to blood-flow parameters were assessed: intrinsic optical signals (OIS) and the amplitude of pulsatile component (APC) of the PPG waveform. Moreover, we carried out pharmacological validation of these indicators by determining their sensitivity to anti-migraine drugs: valproic acid and sumatriptan. For statistical analysis the non-parametric tests with post-hoc Bonferroni correction was used.ResultsSignificant increase of both APC and OIS was observed due to CCW electrical stimulation. Compared to saline (n = 11), intravenous administration of both the sumatriptan (n = 11) and valproate (n = 10) by using a cumulative infusion regimen (three steps performed 30 min apart) lead to significant inhibitory effect on the APC response to the stimulation. In contrast, intravenous infusion of any substance or saline did not affect the OIS response to the stimulation. It was found that infusion of either sumatriptan or valproate did not affect the response of ABP or heart rate to the stimulation.ConclusionsImaging PPG can be used in an animal migraine model as a method for contactless assessment of intracranial blood flow. We have identified two new markers of TVS activation, one of which (APC) was pharmacologically confirmed to be associated with migraine. Monitoring of changes in APC caused by CCW electrical stimulation (controlling efficiency of stimulation by OIS) can be considered as a new way to assess the peripheral mechanism of action of anti-migraine interventions.     

Ambiguity of mapping the relative phase of blood pulsations

Blood pulsation imaging (BPI) is a non-invasive optical method based on photoplethysmography (PPG). It is used for the visualization of changes in the spatial distribution of blood in the microvascular bed. BPI specifically allows measurements of the relative phase of blood pulsations and using it we detected a novel type of PPG fast waveforms, which were observable in limited areas with asynchronous regional blood supply. In all subjects studied, these fast waveforms coexisted with traditional slow waveforms of PPG. We are therefore presenting a novel lock-in image processing technique of blood pulsation imaging, which can be used for detailed temporal characterization of peripheral microcirculation.OCIS codes: (170.3880) Medical and biological imaging, (280.1415) Biological sensing and sensors     

Comparison of non-invasive peripheral venous saturations with venous blood co-oximetry

The estimation of venous oxygen saturations using photoplethysmography (PPG) may be useful as a noninvasive continuous method of detecting changes in regional oxygen supply and demand (e.g. in the splanchnic circulation). The aim of this research was to compare PPG-derived peripheral venous oxygen saturations directly with venous saturation measured from co-oximetry blood samples, to assess the feasibility of non-invasive local venous oxygen saturation. This paper comprises two similar studies: one in healthy spontaneously-breathing volunteers and one in mechanically ventilated anaesthetised patients. In both studies, PPG-derived estimates of peripheral venous oxygen saturations (SxvO₂) were compared with co-oximetry samples (ScovO₂) of venous blood from the dorsum of the hand. The results were analysed and correlation between the PPG-derived results and co-oximetry was tested for. In the volunteer subjects,moderate correlation (r = 0.81) was seen between SxvO₂ values and co-oximetry derived venous saturations (ScovO₂), with a mean (±SD) difference of +5.65 ± 14.3% observed between the two methods. In the anaesthetised patients SxvO₂ values were only 3.81% lower than SpO₂ and tended to underestimate venous saturation (mean difference = –2.67 ± 5.89%) while correlating weakly with ScovO₂ (r = 0.10). The results suggest that significant refinement of the technique is needed to sufficiently improve accuracy to produce clinically meaningful measurement of peripheral venous oxygen saturation. In anaesthetised patients the use of the technique may be severely limited by cutaneous arteriovenous shunting.     

The relationship between the area of peripherally-derived pressure volume loops and systemic vascular resistance

Arterial and photoplethysmographic (PPG) waveforms have been utilized to non-invasively estimate stroke volume from the pulse contour. The ability of these pulse contour devices to accurately predict stroke volume is degraded when afterload changes significantly. There is a need for a non-invasive device capable of identifying when vascular tone has changed. Shelley et al. previously described a qualitative relationship between peripheral pressure volume (PV) loops (in which pressure waveforms from an intra-arterial catheter are combined with volume waveforms from the PPG waveform) and changes in vascular tone. The purpose of this study was to quantitatively compare changes in the area of peripheral PV loops with changes in systemic vascular resistance (SVR) in a patient population undergoing major surgery. Physiologic data from ten patients undergoing liver transplantation was extracted from a hemodynamic database. A peak detection algorithm was applied to both the arterial and PPG waveforms, which were manually aligned so that the troughs occurred at identical time points. PV loop area (PVA) for each heartbeat was calculated and median PVA was recorded for each minute. PVA for each patient was indexed to the average value for the first 5 min (because PPG amplitude has no standard and is not comparable between patients) and compared to indexed SVR at all points for which SVR was available. SVR and PVA were plotted as a function of time and outliers (3.1 %) removed. The Pearson correlation coefficient describing the relationship between PVA_i and SVR_i was 0.67 (1,728 min of data, p = 0.0020, sign test over 10 patients) and between MAP and SVR was 0.71. There was no meaningful correlation between ΔSVR and either ΔPVA or ΔMAP (based on minute-to-minute changes). Indexed values of PVA are correlated with indexed values of SVR and may serve as a useful monitor for changes in afterload but in their present form do not offer added value above the measurement of MAP. Incorporation of different (e.g. finger, forehead) and redundant (e.g. bilateral) sites may significantly improve the accuracy of this technique.     

Identifying Airway Obstructions Using Photoplethysmography (PPG)

Objective. Central and obstructive apneas are sources of morbidity and mortality associated with primary patient conditions as well as secondary to medical care such as sedation/analgesia in post-operative patients. This research investigates the predictive value of the respirophasic variation in the noninvasive photoplethysmography (PPG) waveform signal in detecting airway obstruction. Methods. PPG data from 20 consenting healthy adults (12 male, 8 female) undergoing anesthesia were collected directly after surgery and before transfer to the Post Anesthesia Care Unit (PACU). Features of the PPG waveform were calculated and used in a neural network to classify normal and obstructive events. Results. During the postoperative period studied, the neural network classifier yielded an average (±standard deviation) 75.4 (±3.7)% sensitivity, 91.6 (±2.3)% specificity, 84.7 (±3.5)% positive predictive value, 85.9 (±1.8)% negative predictive value, and an overall accuracy of 85.4 (±2.0)%. Conclusions. The accuracy of this method shows promise for use in real-time monitoring situations. Knorr-Chung BR, McGrath SP, Blike GT. Identifying airway obstructions using Photoplethysmography (PPG).     

Alar Photoplethysmography: A New Methodology for Monitoring Fluid Removal and Carotid Circulation during Hemodialysis

Objective Hemodialysis (HD) hemoconcen tration measurements may not predict hypotension, and are confounded by impaired compensatory responses to ultrafiltration (UF). We devised noninvasive photople thysmograph (PPG) technology to monitor carotid blood flow at the nasal alar, quantify cardiac and respiratory components, and study the effect of UF and resistance breathing in HD patients and blood donors. Methods The PPG was recorded using a novel alar probe in 40 subjects (20 each group), before and after their procedure, with 3 airway resistances. Raw data were separated into a low frequency component (LFC, based on the effect of respiration on thoracic pressure and blood capacitance) and a rapid pulsatile cardiac component (PCC), yielding 3 measures of carotid flow and vascular tone. Results The device produced stable signals amenable to automated processing. The LFC (respiration-induced variation in carotid flow) increased with UF (P as low as 0.03, depending on airway resistance), not changing in blood donors. Two PCC variables (measuring blood vessel distention with each heart beat) decreased (P ≤ 0.03) with blood donation, but not UF. Conclusions This new noninvasive PPG method detects altered respiration-associated carotid circulation during UF. With blood donation there is dampening of pulsatile vessel distention, consistent with increased vascular tone. That compensatory mechanism was impaired in HD patients and helps explain their instability with fluid removal. Fuehrlein B, Melker R, Ross EA. Alar photoplethysmography: a new methodology for monitoring fluid removal and carotid circulation during hemodialysis.     

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.     

The Use of Joint Time Frequency Analysis to Quantify the Effect of Ventilation on the Pulse Oximeter Waveform

Objective. In the process of determining oxygen saturation, the pulse oximeter functions as a photoelectric plethysmograph. By analyzing how the frequency spectrum of the pulse oximeter waveform changes over time, new clinically relevant features can be extracted. Methods. Thirty patients undergoing general anesthesia for abdominal surgery had their pulse oximeter, airway pressure and CO₂ waveforms collected (50 Hz). The pulse oximeter waveform was analyzed with a short-time Fourier transform using a moving 4096 point Hann window of 82 seconds duration. The frequency signal created by positive pressure ventilation was extracted using a peak detection algorithm in the frequency range of ventilation (0.08–0.4 Hz = 5–24 breaths/minute). The respiratory rate derived in this manner was compared to the respiratory rate as determined by CO₂ detection. Results. In total, 52 hours of telemetry data were analyzed. The respiratory rate measured from the pulse oximeter waveform was found to have a 0.89 linear correlation when compared to CO₂ detection and airway pressure change. the bias was 0.03 breath/min, SD was 0.557 breath/min and the upper and lower limits of agreement were 1.145 and −1.083 breath/min respectively. The presence of motion artifact proved to be the primary cause of failure of this technique. Conclusion. Joint time frequency analysis of the pulse oximeter waveform can be used to determine the respiratory rate of ventilated patients and to quantify the impact of ventilation on the waveform. In addition, when applied to the pulse oximeter waveform new clinically relevant features were observed.     

Monitoring of reactive hyperemia using photoplethysmographic pulse amplitude and transit time

Background: Peripheral arterial tonometry and Ultrasound measurement of flow mediated dilation have been the widely reported noninvasive techniques to assess vasodilation during reactive hyperemia (RH). Objective: Simultaneous monitoring of dilatation and tone of the vasculature during RH induced by venous occlusion (VO) and arterial occlusion (AO) has been presently attempted using simple noninvasive measures of photoplethysmography (PPG). Methods: Finger-PPG characteristics that include pulse timings, amplitude, upstroke-slope and pulse transit time (PTT) were studied before (1 min), post-VO (5 min) and post-AO (5 min) in 11 healthy volunteers. Results: PPG amplitude was significantly increased to maximum at 2nd min of post-AO (1.28±0.11 vs. 1.0 nu, P<0.05) as compared to the baseline; meanwhile, no significant changes (P>0.05) in PPG amplitude was observed during post-VO. Tremendous increase in PTT was evident at 1st min of post-AO (196.6±3.3 vs. 185.3±3.6 ms, P<0.0001) and was maintained significantly longer through 1–5 min of post-AO. Relatively small but significant increase in PTT was noticed only at 1st min of post-VO (193.9±6.8 vs. 189.6±6.2 ms, P<0.0001), followed by an immediate recovery to baseline by 2nd min of post-VO. The increase in PTT (i.e. ΔPTT) was higher at 1st min of post-AO (11.4±1.3 vs. 4.3±1.1 ms) as compared to post-VO. Conclusion: Results suggests that PTT response reflects the myogenic components in the early part of RH and PPG amplitude response reflects the metabolic component reinforcing the later course of RH. PPG amplitude and PTT can be used to quantify the changes in diameter and tone of the vessel wall, respectively during RH. The collective responses of PPG amplitude and PTT can be more appropriate to facilitate PPG technique for monitoring of vasodilation caused by RH.     

Detection of Lung Injury with Conventional and Neural Network-Based Analysis of Continuous Data

Objective. To test if analysis of pressure and flow waveform patterns with an artificial intelligence neural network could distinguish between normal and injured lungs. Methods. Acute lung injury was induced in ten healthy anesthetized, mechanically ventilated dogs with repeated injections of oleic acid, until arterial blood oxyhemoglobin saturation reached 85% breathing room air. Airway pressure, esophageal pressure, airway flow, and arterial and mixed venous saturation signals were stored at 2 min intervals. Hemodynamic and blood gas data were collected every 10 min. Back-propagation neural networks were trained with normalized airway pressure and flow waveforms from normal and fully injured lungs. Results. The networks scored lung injury on a continuous scale from +1 (normal) to –1 (injured). Network scores unequivocally distinguished between normal and fully injured lungs and suggested a gradual transition from normal to injury pattern. However, the response of the network was slow compared to compliance, resistance and venous admixture. Conclusions. Normal and fully injured lungs display distinct flow and pressure waveform patterns which are independent of changes in calculated pulmonary mechanics variables. These patterns can be recognized by a neural network. Further research is needed to determine the full potential of automated pattern recognition for lung monitoring.     

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\(_2\)) is calculated noninvasively. However, the PPG waveform contains other significant physiological information that can be used in conjunction to SpO\(_2\) 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 (\(\Delta\)HbO\(_2\)), reduced (\(\Delta\)HHb) and total haemoglobin (\(\Delta\)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\(_{Ox}\) = \(\Delta\)A\(_{IR}\) ? \(\Delta\)A\(_R\)) and by the conventional SpO\(_2\). The \(\Delta\)HbO\(_2\), \(\Delta\)HHb, \(\Delta\)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\(_{Ox}\) indicated significant changes for occlusion pressures exceeding 20 mmHg (p < 0.05) and correlation with tissue oxygenation changes measured by NIRS, while SpO\(_2\) 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.     

Measurement of the liver tissue oxygenation by near-infrared spectroscopy

Objective To study the relation between the liver tissue oxygenation index (TOI), transcutaneously measured with spatially resolved spectroscopy (a new method of near-infrared spectroscopy or NIRS), the mixed venous oxygen saturation and the blood flow in the different parts of the splanchnic circulation in newborn piglets.Design Tissue oxygenation index of the liver was measured in six newborn piglets at 33°C, 35°C, 37°C and after a decrease in arterial carbon dioxide pressure (PaCO₂).Measurements Mixed venous oxygen saturation, blood gas analysis and peripheral oxygen saturation were measured at each step. Gastric, proximal jejunal, midgut, distal ileal, splenic and hepatic arterial blood flow were measured by injection of coloured microspheres into the left atrium. NIRS optodes were attached to the skin over the liver and TOI was calculated.Results No significant changes of TOI of the liver were seen during the increase in temperature or change in PaCO₂. TOI correlated well with mixed venous oxygen saturation (r=0.85), the mid-ileal blood flow (r=0.57) and the distal ileal blood flow (r=0.72).Conclusions Measurement of the TOI of the liver might be a non-invasive way to measure the distal ileal blood flow.     

Monitoring of respiratory rate in postoperative care using a new photoplethysmographic technique

Objective.Photoplethysmography (PPG) is a non-invasive optical technique that measures variations in skin blood volume and perfusion. The PPG signal contains components that are synchronous with respiratory and cardiacrhythms. We undertook this study to evaluate PPG for monitoring patients' respiratory rate in the postoperative care unit, using a new prototype device. We compared it with the established technique, transthoracic impedance (TTI). Methods.PPG signals from 16 patients(ASA classes 1–2, mean age 43 years) who were recovering from general anaesthesia after routine operations were recorded continuously for 60minutes/patient. The respiratory synchronous part of the PPG signal was extracted by using a band pass filter. Detection of breaths in the filtered PPG signals was done both visually and by using an automated algorithm. In both procedures, the detected breaths were compared with the breaths detected in the TTI reference. Results.A total of 10.661 breaths were recorded, and the mean ± SD respiratory rate was 12.3 ± 3.5breaths/minute. When compared with TTI, the rates of false positive and false negative breaths detected by PPG (visual procedure) were 4.6 ±4.5% and 5.8 ± 6.5%, respectively. When using the algorithm for breath detection from PPG, the rates of false positive andfalse negative breaths were 11.1 ± 9.7% and 3.7 ±3.8%, respectively, when compared to TTI. Lower respiratory rates increased the occurrence of false-positive breaths that were detected by the PPG using visual identification (p< 0.05). The same tendency was seen with the automated PPG procedure (p< 0.10). Conclusions.Our results indicate that PPG has the potential to be useful for monitoring respiratory rate in the postoperative period. This revised version was published online in July 2006 with corrections to the Cover Date.     

Analyzing the value of monitoring duodenal mucosal perfusion using photoplethysmography

Photoplethysmography (PPG) is a technique that permits noninvasive measurement of changes in the volume of tissues. A novel device uses PPG to assess changes in duodenal mucosal perfusion. When tested in septic piglets, data obtained using this device correlate with the blood lactate concentration and duodenal serosal microvascular blood flow as measured with a laser Doppler flowmeter. This new PPG-based approach for continuously monitoring gut mucosal perfusion warrants further development, leading to prospective clinical trials in patients.In the previous issue of Critical Care, Jacquet-Lagrèze and colleagues from several institutions in Lyon, France, report results from a preclinical study of a novel perfusion monitoring device [1]. The study was carried out using anesthetized and mechanically ventilated piglets. Some of the animals were infused with a suspension of viable Pseudomonas aeruginosa to induce septic shock; the remaining (control) animals were not challenged with the Gram-negative bacteria preparation.The novel monitoring device was developed by Advanced Perfusion Diagnostics [2], a biotechnology start-up company in Lyon. The device makes use of photoplethysmography (PPG) to assess changes in duodenal mucosal blood flow.PPG and its applications in medicine have been well described in an excellent review article by Allen [3]. PPG uses light in the visible red and near-infrared regions of the spectrum to non-invasively assess changes in the volume of a specific region of tissue. PPG can be carried out using light that is transmitted through tissue or using light that is reflected by the tissue. Light-emitting diodes provide the light in present-day commercially available devices that employ PPG for medical applications, such as beat-to-beat monitoring of blood pressure.The characteristic signal from a PPG device is a waveform. The dominant (peak and valley) aspect of the tracing is synchronized with the beating of the heart, and is usually called the alternating current (AC) component. The origins of the other components of the PPG signal remain to be completely elucidated, although important factors are recognized to be changes in circulating blood volume, respiration and vasomotor tone. The quasi-static component of the PPG signal is usually called the direct current (DC) component.The device, which was evaluated by Jacquet-Lagrèze and colleagues, features a reflectance type of PPG device fitted onto the surface of a balloon located near the distal end of a small bore feeding tube. When the end of the feeding tube is advanced through the pylorus into the duodenum and the balloon is inflated, the PPG element presses against the mucosal surface of this portion of the intestine, allowing continuous monitoring of the AC and DC components of the waveform. In their studies using pigs, the research group from Lyon showed that the variations from baseline in both the AC and DC components of the duodenal mucosal PPG signal were significantly correlated with the variations from baseline in duodenal serosal microvascular blood flow measured using laser Doppler flowmetry. Importantly, the AC and DC components of the PPG signal also were significantly correlated with the blood lactate concentration. The findings from this preclinical validation study thus support the view that the PPG-based duodenal mucosal blood flow monitoring device provides meaningful data.While the report by Jacquet-Lagrèze and colleagues should provide encouragement for the company that is developing the new monitoring device, a look back at the history of this field should temper their enthusiasm and ours. As pointed out by the authors, use of another gastrointestinal perfusion monitoring device – the gastric tonometer – was shown to significantly improve survival in a subset of critically ill patients [4]. But the widespread adoption of this device or even an improved version of it never happened, possibly because some clinical studies showed that the method failed to provide reliable information about splanchnic blood flow [5] or, more importantly, failed to provide added value compared with that obtained from routine measurements of arterial blood gases [6]. It seems most likely, however, that gastric tonometry fell out of favor because clinicians, when confronted with evidence for inadequate gut mucosal perfusion, were perplexed about the proper intervention(s), especially when other commonly used indices of perfusion – such as arterial blood pressure, mixed venous oxygen saturation, and cardiac output – were sending a different message.Duodenal mucosal perfusion monitoring, using a cleverly designed PPG-based device, warrants further development, ultimately leading to prospective clinical trials. When these trials are designed, it will be important to pay careful attention to the intervention(s) that are triggered when the device indicates that intestinal mucosal perfusion is low.     

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 < 1beat 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.     

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.     

Frequency domain analysis of cerebral near infrared spectroscopy signals during application of an impedance threshold device in spontaneously ventilating volunteers

Currently available near infrared spectroscopy (NIRS) devices are unable to discriminate between arterial and venous blood, a potential source of artifact. The purpose of this study was to test the hypothesis that oscillations in NIR signals at the respiratory and cardiac frequency could be attributed to venous and arterial blood, respectively, and thereby isolated. After written informed consent was obtained, a two-wavelength NIRS device was placed over the left frontal cortex in 20 volunteers. After 5 min of unimpeded spontaneous ventilation, an impedance threshold device (ITD, average resistance—7 cm H₂O) was applied and an additional two minutes of data recorded. Tissue saturation (StO₂) calculated at the ventilatory and cardiac frequencies was compared to non-pulsatile StO₂, before and after application of the ITD using spectral peak and power algorithms. The ITD increased non-pulsatile cerebral saturation by 3.6 %. The ITD had no discernable effect on pulsatile estimates of StO₂ at either the ventilatory or cardiac frequencies. StO₂ estimated at the NIRS spectral peak from 0.75 to 1.75 Hz was 24 % higher than non-pulsatile StO₂ (p = 0.0013). There were no other significant differences between pulsatile and non-pulsatile algorithms in the estimation of StO₂. In 64 % of cases, both the low (ventilator) and high (cardiac) frequency estimates of StO₂ were either both larger or both smaller than non-pulsatile StO₂, suggesting that they were interrogating the same vascular bed. Frequency domain analysis cannot reliably separate NIRS waveforms into arterial and venous components.