Assessment of changes in blood volume during lower body negative pressure-induced hypovolemia using bioelectrical impedance analysis.

BACKGROUND: Lower body negative Pressure (LBNP)-induced hypovolemia is simulating acute hemorrhage by sequestrating blood into lower extremities. Bioelectrical Impedance Analysis (BIA) is based on the electrical properties of biological tissues, as electrical current flows along highly conductive body tissues (such as blood). Changes in blood volume will lead to changes in bioimpedance. This study aims to study changes in upper (UL) and lower (LL) extremities bioimpedance during LBNP-induced hypovolemia. METHODS: This was a prospective observational study of healthy volunteers who underwent gradual LBNP protocol which consisted of 3-minute intervals: at baseline, -15, -30, -45, -60 mmHg, then recovery phases at -30 mmHg and baseline. The UL&LL extremities bioimpedance were measured and recorded at each phase of LBNP and the percentage changes of bioimpedance from baseline were calculated and compared using student's t-test. A P-value of < 0.05 was considered significant. Correlation between relative changes in UL&LL bioimpedance and estimated blood loss (EBL) from LBNP was calculated using Pearson correlation. RESULTS: 26 healthy volunteers were enrolled. As LBNP-induced hypovolemia progressed, there were a significant increase in UL bioimpedance and a significant decrease in LL bioimpedance. During recovery phases (where blood was shifted from the legs to the body), there were a significant increase in LL bioimpedance and a reduction in UL bioimpedance. There were significant correlations between estimated blood loss from LBNP model with UL (R = 0.97) and LL bioimpedance (R = - 0.97). CONCLUSION: During LBNP-induced hypovolemia, there were reciprocal changes in UL&LL bioimpedance. These changes reflected hemodynamic compensatory mechanisms to hypovolemia.

Signal quality assessment of peripheral venous pressure.

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

A Prospective Cohort Study to Evaluate Needle Passes Using a Portable Ultrasound Device versus Traditional Landmark Approach for Epidural Anesthesia in a Busy Obstetric Tertiary Care Center.

Despite its many cited benefits, ultrasound guidance for neuraxial procedures is not widespread in anesthesiology. Some cited limitations include device cost and accessibility. We test the hypothesis that a handheld and relatively inexpensive ultrasound can improve neuraxial proficiency (e.g., decreased needle manipulations and block time). This prospective study compared the number of needle passes, redirections, and procedural time between epidural placed with a handheld ultrasound versus landmarks. Needle passes and attempts were defined as the number of times the Tuhoy needle was redirected, and the times skin was punctured (re-insertion). Procedural time was defined as the time from local anesthetic infiltration until loss of resistance was obtained. The impact of level of training and accuracy of the device were also analyzed. 302 patients receiving labor epidural were included in the study. No difference in body mass index (BMI) nor distribution of level of training was noted between the groups. Regression analysis adjusted for BMI demonstrated a decrease in needle passes (-1.75 (95% CI -2.62, -0.89), p < 0.001), needle attempts (-0.51 (95% CI -0.97, -0.04), p = 0.032) and procedural time (-154.67s 95% CI -303.49s, -5.85s), p = 0.042) when a handheld ultrasound was utilized. The mean (95% Confidence interval) difference between needle depth and ultrasound depth was 0.39 cm (0.32, 0.46), p < 0.001. The use of a handheld device resulted in statistically significant decrease of needle manipulations and block time. More research is needed to evaluate the impact of and increase in accessibility of ultrasound technology.

Continuous non-invasive hemodynamic monitoring in early onset severe preeclampsia.

OBJECTIVES: Continuous hemodynamic monitoring offers the opportunity to individualize management in severe preeclampsia (PEC). We compared cardiac output (CO) and total peripheral resistance (TPR) measured by bioreactance (NICOM), Clearsite™ Fingercuff [CS), and 3D-echocardiography (3DE). STUDY DESIGN: This prospective observational study included 12 pregnant patients with early PEC. CO and TPR were measured simultaneously by NICOM, CS, and 3DE antepartum and 1-2 days postpartum. Using 3DE as the standard, CS and NICOM interchangeability, precision, accuracy, and correlation were assessed. RESULTS: Compared to 3DE-CO, CS-CO was highly correlated (R2 = 0.70, p = <0.0001) with low percentage error (PE 29%) which met criteria for interchangeablity. CS-TPR had strong correlation (R2 = 0.81, p = <0.0001) and low PE (29%). While CS tended to slightly overestimate CO (bias + 2.05 ±1.18 L/min, limit of agreement (LOA) -0.20 to 4.31) and underestimate TPR (bias -279 ±156 dyes/sec/cm5; LOA -580 to 18.4) these differences were unlikely to be clinically significant. Thus CS could be interchangeable with 3DE for CO and TPR. NICOM-CO had only moderate correlation with 3DE-CO (R2 = 0.29, p = 0.01) with high PE (52%) above threshold for interchangeability. NICOM-CO had low mean bias (-1.2 ±1.68 L/min) but wide 95% LOA (-4.41 to 2.14) suggesting adequate accuracy but low precision in relation to 3DE-CO. NICOM-TPR had poor correlation with 3DE-TPR (R2 = 0.32, p = 0.001) with high PE (67%), relatively low mean bias (238 ±256), and wide 95% LOA (-655 to 1131). NICOM did not meet the criteria for interchangeable with 3DE for CO and TPR. CONCLUSIONS: Clearsite Fingercuff, but not NICOM, has potential to be clinically useful for CO and TPR monitoring in severe preeclampsia.

Baseline rotational thromboelastometry (ROTEM) values in a healthy, diverse obstetric population and parameter changes by pregnancy-induced comorbidities.

Background: Point-of-care testing provides a representation of the patient's coagulability status during effective postpartum hemorrhage management. Baseline values of rotational thromboelastometry (ROTEM) have not yet been reported in a heterogeneous obstetric population. This study aimed to establish a baseline for a diverse population representative of the United States. The secondary aim was to evaluate the association of these hematologic parameters with comorbidities, race, and socioeconomic factors. Methods: The study was a retrospective review of collected ROTEM values of women undergoing vaginal or cesarean delivery with a history of or at risk for postpartum hemorrhage. Patients were divided into healthy and comorbid groups. Exclusion criteria for both groups included active or recent bleeding, receipt of blood products or clot-enhancing factors, and liver disease. Mean values of ROTEM by race and comorbidities were included. Median values were reported for intrinsic pathway thromboelastometry (INTEM), extrinsic pathway thromboelastometry (EXTEM), and fibrin polymerization thromboelastometry (FIBTEM) amplitude at 10 minutes (A10) and 20 minutes (A20), coagulation time, clot formation time, and maximum clot firmness. Results: A total of 681 records were reviewed; 485 met inclusion criteria, and 267 met healthy criteria. The mean (standard deviation) demographics for maternal age (years), body mass index (kg/m2), and gestational age (weeks) were 32.2 (5.7), 34 (7.3), and 35.4 (5), respectively. The median INTEM, EXTEM, and FIBTEM A10 were 63, 65, and 23 mm. The mean for INTEM, EXTEM, and FIBTEM A10 was increased for those who were Black or obese, whereas a decreased FIBTEM and EXTEM A10 was noted in those who were Asian or those who had the hemolysis, elevated liver enzymes, low platelet syndrome. Conclusions: Our heterogeneous population presents ROTEM values within the interquartile range of those previously reported in European studies. Black race, obesity, and preeclampsia were associated with hypercoagulable profiles.

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

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

Amplitude and phase measurements from harmonic analysis may lead to new physiologic insights: lower body negative pressure photoplethysmographic waveforms as an example.

The photoplethysmographic (PPG) waveform contains hemodynamic information in its oscillations. We provide a new method for quantitative study of the waveform morphology and its relationship to the hemodynamics. A data adaptive modeling of the waveform shape is used to describe the PPG waveforms recorded from ear and finger. Several indices, based on the phase and amplitude information of different harmonics, are proposed to describe the PPG morphology. The proposed approach is illustrated by analyzing PPG waveforms recorded during a lower body negative pressure (LBNP) experiment. Different phase and amplitude dynamics are observed during the LBNP experiment. Specifically, we observe that the phase difference between the high order harmonics and fundamental components change more significantly when the PPG signal is recorded from the ear than the finger at the beginning of the study. In contrast, the finger PPG amplitude changes more when compared to the ear PPG during the recovery period. A more complete harmonic analysis of the PPG appears to provide new hemodynamic information when used during a LBNP experiment. We encourage other investigators who possess modulated clinical waveform data (e.g. PPG, arterial pressure, respiratory, and autonomic) to re-examine their data, using phase information and higher harmonics as a potential source of new insights into underlying physiologic mechanisms.

Comparison between the Rotational Thromboelastometry (ROTEM) Delta device against the Cartridge-based Thromboelastography 6s and Quantra in a healthy third trimester pregnant cohort.

Rotational Thromboelastometry (ROTEM) Delta has been described in several postpartum hemorrhage algorithms, but this device requires pipetting and careful mixing of reagents to initiate the clotting reaction. In contrast, thromboelastography (TEG 6s) and the Quantra devices operate utilizing an automated pre-mixed cartridge that only requires a blood sample to start the clot strength analysis. We compared the correlation between 3 point of care viscoelastic testing (POCVT) devices to laboratory Clauss fibrinogen and platelets, their inter-device correlation, and the total running time difference between Quantra and ROTEM. A high correlation was noted between the Clauss fibrinogen and the fibrinogen parameters from ROTEM (r = 0.76-0.84, P < 0.0001), TEG6s (r = 0.71, P < 0.0001) and Quantra (r = 0.72, P = 0.0001). A moderate correlation between laboratory platelets and the ROTEM (r = 0.54;0.45, P < 0.0001; P = 0.0013) and Quantra (r = 0.66, P = 0.0001) parameters was noted. The inter-device correlation showed to be high when comparing the fibrinogen parameters of TEG6s and Quantra to that of ROTEM (r = 0.88 and 0.74, P < 0.0001, respectively). In contrast, a moderate correlation was noted between the platelet parameters of Quantra and ROTEM (r = 0.51, p = 0.0036). The Quantra device resulted 20.9 min (95% CI -0.2 to 4.7, P = 0.07) faster than the ROTEM if the warming and pipetting of reagents of the latter were considered. All the POCVT devices demonstrated a high correlation to laboratory Clauss fibrinogen, making each beneficial for the early recognition and management of hypofibrinogenemia.

Ultrasound Image Quality Comparison Between a Handheld Ultrasound Transducer and Mid-Range Ultrasound Machine.

Objectives: Not all labor and delivery floors are equipped with ultrasound machines which can serve the needs of both obstetricians and anesthesiologists. This cross-sectional, blinded, randomized observational study compares the image resolution (RES), detail (DET), and quality (IQ) acquired by a handheld ultrasound, the Butterfly iQ, and a mid-range mobile device, the Sonosite M-turbo US (SU), to evaluate their use as a shared resource. Methods: Seventy-four pairs of ultrasound images were obtained for different imaging purposes: 29 for spine (Sp), 15 for transversus abdominis plane (TAP) and 30 for diagnostic obstetrics (OB) purposes. Each location was scanned by both the handheld and mid-range machine, resulting in 148 images. The images were graded by three blinded experienced sonographers on a 10-point Likert scale. Results: The mean difference for Sp imaging favored the handheld device (RES: -0.6 [(95% CI -1.1, -0.1), p = 0.017], DET: -0.8 [(95% CI -1.2, -0.3), p = 0.001] and IQ: -0.9 [95% CI-1.3, -0.4, p = 0.001]). For the TAP images, there was no statistical difference in RES or IQ, but DET was favored in the handheld device (-0.8 [(95% CI-1.2, -0.5), p < 0.001]). For OB images, the SU was favored over the handheld device with RES, DET and IQ with mean differences of 1.7 [(95% CI 1.2, 2.1), p < 0.001], 1.6 [(95% CI 1.2, 2.0], p < 0.001] and 1.1 [(95% CI 0.7, 1.5]), p < 0.001), respectively. Conclusions: Where resources are limited, a handheld ultrasound may be considered as a potential low-cost alternative to a more expensive ultrasound machine for point of care ultrasonography, better suited to anesthetic vs. diagnostic obstetrical indications.

A new approach to complicated and noisy physiological waveforms analysis: peripheral venous pressure waveform as an example.

We introduce a recently developed nonlinear-type time-frequency analysis tool, synchrosqueezing transform (SST), to quantify complicated and noisy physiological waveform that has time-varying amplitude and frequency. We apply it to analyze a peripheral venous pressure (PVP) signal recorded during a seven hours aortic valve replacement procedure. In addition to showing the captured dynamics, we also quantify how accurately we can estimate the instantaneous heart rate from the PVP signal.

Prevalence of aberrant blood pressure readings across two automated intraoperative blood pressure monitoring systems among patients undergoing caesarean delivery.

OBJECTIVES: Aberrant automated blood pressure (BP) readings during caesarean delivery may lead to disruptions in monitoring. The present study compared the frequency of aberrant BP readings across two types of commercially available BP monitoring systems in use during caesarean delivery. METHODS: This was a retrospective observational study using two comparable patient cohorts that resulted from simultaneous introduction of two types of monitors into a single obstetric surgical center in which similar patients were treated for the same surgical procedure by the same set of clinicians during the same year. Our primary hypothesis was that aberrant readings were significantly associated with the type of monitor being used for BP measurement, controlling for a variety of relevant covariates as specified in the analytic plan. RESULTS: A total of 1418 cesarean delivery patients met inclusion criteria. Gaps of at least 6 min in machine-captured BP readings occurred in 159 (21.1%) of cases done in the operating room using a Datex-Ohmeda monitor vs. 183 (27.5%) of cases in the operating rooms using Phillips monitors (P = 0.005). In multivariable logistic regression analysis, the relative odds of the occurrence of monitoring gaps was 35% higher in rooms with the Phillips BP monitors as compared to the Datex-Ohmeda monitor while controlling for pre-specified covariates (odds ratio = 1.35, 95% confidence interval = 1.04-1.74, P = 0.02). CONCLUSION: The present analysis suggests that aberrant BP readings for parturients undergoing caesarean delivery are significantly different between the two types of automated BP monitoring systems used in the operating rooms at our institution.

Assessment of Hypertension Using Clinical Electrocardiogram Features: A First-Ever Review.

Hypertension affects an estimated 1.4 billion people and is a major cause of morbidity and mortality worldwide. Early diagnosis and intervention can potentially decrease cardiovascular events later in life. However, blood pressure (BP) measurements take time and require training for health care professionals. The measurements are also inconvenient for patients to access, numerous daily variables affect BP values, and only a few BP readings can be collected per session. This leads to an unmet need for an accurate, 24-h continuous, and portable BP measurement system. Electrocardiograms (ECGs) have been considered as an alternative way to measure BP and may meet this need. This review summarizes the literature published from January 1, 2010, to January 1, 2020, on the use of only ECG wave morphology to monitor BP or identify hypertension. From 35 articles analyzed (9 of those with no listed comorbidities and confounders), the P wave, QTc intervals and TpTe intervals may be promising for this purpose. Unfortunately, with the limited number of articles and the variety of participant populations, we are unable to make conclusions about the effectiveness of ECG-only BP monitoring. We provide 13 recommendations for future ECG-only BP monitoring studies and highlight the limited findings in pregnant and pediatric populations. With the advent of convenient and portable ECG signal recording in smart devices and wearables such as watches, understanding how to apply ECG-only findings to identify hypertension early is crucial to improving health outcomes worldwide.

PPGSynth: An Innovative Toolbox for Synthesizing Regular and Irregular Photoplethysmography Waveforms.

Photoplethysmography (PPG) is increasingly used in digital health, exceptionally in smartwatches. The PPG signal contains valuable information about heart activity, and there is lots of research interest in its means and analysis for cardiovascular diseases. Unfortunately, to our knowledge, there is no arrhythmic PPG dataset publicly available-this paper attempt to provide a toolbox that can generate synthesized arrhythmic PPG signals. The model of a single PPG pulse in this toolbox utilizes two combined Gaussian functions. This toolbox supports synthesizing PPG waveform with regular heartbeats and three irregular heartbeats: compensation, interpolation, and reset. The user can generate a large amount of PPG data with a certain irregularity, with different sampling frequency, time length, and a range of noise types (Gaussian noise and multi-frequency noise) can be added to the synthesized PPG which can all be modified from the interface, and different types of arrhythmic PPGs (as calculated by the model) generated. The generation for large PPG datasets that simulate PPG collected from real humans could be used for testing the robustness of developed algorithms that are targeting arrhythmic PPG signals. Our PPG synthesis tool is publicly available.

The Application of ROTEM in a Parturient With Antiphospholipid Syndrome in the Setting of Anticoagulation for Cesarean Delivery: A Case Report.

A 35-year-old parturient with antiphospholipid syndrome and a working diagnosis of hemolysis, elevated liver enzyme, and low platelets (HELLP) underwent a cesarean delivery 9 hours after receiving heparin. Her preoperative activated partial thromboplastin time and rotational thromboelastometry (ROTEM) intrinsic pathway (INTEM) clotting time were 120 and 1870 seconds, respectively. Fresh frozen plasma was administered for heparin neutralization. The ROTEM INTEM/heparinase assay (HEPTEM) ratio can help confirm heparin neutralization and guide intraoperative transfusion management.

Anesthesia for Obstetric Disasters.

Over the past 30 years, maternal mortality has increased in the United States to 18 deaths per 100,000 live births. Obstetric emergencies, including hemorrhage, hypertensive disorders in pregnancy, HELLP syndrome, and amniotic fluid embolism, and anesthesia complications, including high neuraxial blockade, local anesthetic systemic toxicity, and the difficult obstetric airway, contribute to maternal cardiac arrest and maternal and fetal morbidity and mortality. Expeditious intervention by the obstetric anesthesiologist is critical in these emergent scenarios, and knowledge of best practices is essential to improve maternal and fetal outcomes.

Anesthesia for Obstetric Disasters.

Over the past 30 years, maternal mortality has increased in the United States to 18 deaths per 100,000 live births. Obstetric emergencies, including hemorrhage, hypertensive disorders in pregnancy, HELLP syndrome, and amniotic fluid embolism, and anesthesia complications, including high neuraxial blockade, local anesthetic systemic toxicity, and the difficult obstetric airway, contribute to maternal cardiac arrest and maternal and fetal morbidity and mortality. Expeditious intervention by the obstetric anesthesiologist is critical in these emergent scenarios, and knowledge of best practices is essential to improve maternal and fetal outcomes.

Ventilation-Induced Modulation of Pulse Oximeter Waveforms: A Method for the Assessment of Early Changes in Intravascular Volume During Spinal Fusion Surgery in Pediatric Patients.

BACKGROUND: Scoliosis surgery is often associated with substantial blood loss, requiring fluid resuscitation and blood transfusions. In adults, dynamic preload indices have been shown to be more reliable for guiding fluid resuscitation, but these indices have not been useful in children undergoing surgery. The aim of this study was to introduce frequency-analyzed photoplethysmogram (PPG) and arterial pressure waveform variables and to study the ability of these parameters to detect early bleeding in children during surgery. METHODS: We studied 20 children undergoing spinal fusion. Electrocardiogram, arterial pressure, finger pulse oximetry (finger PPG), and airway pressure waveforms were analyzed using time domain and frequency domain methods of analysis. Frequency domain analysis consisted of calculating the amplitude density of PPG and arterial pressure waveforms at the respiratory and cardiac frequencies using Fourier analysis. This generated 2 measurements: The first is related to slow mean arterial pressure modulation induced by ventilation (also known as DC modulation when referring to the PPG), and the second corresponds to pulse pressure modulation (AC modulation or changes in the amplitude of pulse oximeter plethysmograph when referring to the PPG). Both PPG and arterial pressure measurements were divided by their respective cardiac pulse amplitude to generate DC% and AC% (normalized values). Standard hemodynamic data were also recorded. Data at baseline and after bleeding (estimated blood loss about 9% of blood volume) were presented as median and interquartile range and compared using Wilcoxon signed-rank tests; a Bonferroni-corrected P value <0.05 was considered statistically significant. RESULTS: There were significant increases in PPG DC% (median [interquartile range] = 359% [210 to 541], P = 0.002), PPG AC% (160% [87 to 251], P = 0.003), and arterial DC% (44% [19 to 84], P = 0.012) modulations, respectively, whereas arterial AC% modulations showed nonsignificant increase (41% [1 to 85], P = 0.12). The change in PPG DC% was significantly higher than that in PPG AC%, arterial DC%, arterial AC%, and systolic blood pressure with P values of 0.008, 0.002, 0.003, and 0.002, respectively. Only systolic blood pressure showed significant changes (11% [4 to 21], P = 0.003) between bleeding phase and baseline. CONCLUSIONS: Finger PPG and arterial waveform parameters (using frequency analysis) can track changes in blood volume during the bleeding phase, suggesting the potential for a noninvasive monitor for tracking changes in blood volume in pediatric patients. PPG waveform baseline modulation (PPG DC%) was more sensitive to changes in venous blood volume when compared with respiration-induced modulation seen in the arterial pressure waveform.

Photoplethysmography.

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

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

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