Noninvasive Continuous Cardiac Output Monitoring Research Articles

A retrospective analysis of liver resection performed without central venous pressure monitoring

BackgroundStudies have suggested that blood loss can be reduced during liver resection by monitoring and maintaining low central venous pressure (CVP) through fluid restriction or other means, but such a strategy carries risks to the patient including those inherent to central venous catheterization. We sought to characterize fluid management and blood loss during liver resections done without CVP monitoring. MethodsRetrospective data were extracted from electronic anesthesia records for 993 liver resections. For 135 resections, between 2011 through 2013, where a documentation template was used that recorded fluid administration prior to hepatic inflow occlusion, multivariate analysis was performed to test for an association between pre-clamp fluid volumes administered and blood loss and other adverse outcomes. ResultsThe median estimated blood loss was 300 mL and overall rate of transfusion was 8.6%. There was no statistically significant association between crystalloid volume administered prior to inflow clamping (median 900 mL) and blood loss, mortality or length of stay in the subset of patients with supplemental fluid data. ConclusionLiver resection can be performed safely without either CVP monitoring or non-invasive continuous cardiac output monitoring. Additionally, there was no disadvantage to a practical approach to fluid administration prior to inflow clamping during liver resections in the absence of CVP monitoring with regard to blood loss or short-term outcomes.

Évaluation à l’effort d’une nouvelle méthode de mesure du débit cardiaque basée sur l’analyse de l’onde de pouls (comparaison EsCCO® vs Physioflow®)

ObjectivesEsCCO is a novel non-invasive continuous cardiac output monitoring system based on pulse wave transit time already validated at rest. The aim of our study was to compare cardiac output measurements obtained simultaneously by EsCCO® (Q˙cOP) and impedance cardiography (Physioflow® ; Q˙cIMP), in healthy subjects. Patients and methodsEight healthy subjects (age: 31±9 years, weight: 76±10kg, height: 179±5cm) realized two exercise tests: an incremental ergocycle test performed until exertion (Pmax=269±48W) and a constant load exercise (P=163±27W). Comparison between measurements (Q˙cOP versus Q˙cIMP) obtained during the first test allowed to evaluate the accuracy of the device. Reliability was determined on three repeated measures during the second test, realized at ventilatory threshold. ResultsCorrelation coefficient between both methods is 0.88 (P<0.01). Mean difference is 0.04±1.49L/min (95 % limits of agreement: +2.94 to −3.00L/min) and only 3/74 measures are not included between the limits of agreement. At high intensity and for cardiac output over than 15 L/min, Q˙cOP signal is lost in almost half the time. Concerning reliability, reproducibility coefficient is 0.87 (P<0.05), only 1.8 % of this variability is due to the method. ConclusionEsCCO® measurements are accurate, reliable and allow a good estimation of cardiac output on healthy subjects. The signal lost observed for high cardiac output levels (>15L/min) can limit its utilization during very high intensity exercise.

The use of noninvasive continuous cardiac output to monitor the hemodynamic effects of propofol-fentanyl versus propofol-ketamine induction

Background Noninvasive continuous cardiac output monitoring method utilizing ECG and a pulse oximeter wave was based on hemodynamic analysis combined with pulse wave transit time. Propofol injection may induce a significant decrease in blood pressure. The use of ketamine with propofol may reduce the dose and the hemodynamic effect of propofol. Patients and methods Sixty female patients, ASA I and II, undergoing dilatation and curettage were divided into two equal groups. The first group received propofol (1 mg/kg) + fentanyl (1 μg/kg), whereas the second group received propofol (1 mg/kg) + ketamine (0.5 mg/kg) as induction agents. Continuous cardiac output monitoring was performed using pulse wave transit time technology. Hemodynamic data such as oxygen saturation through pulse oximetry, heart rate, and blood pressure were recorded every minute. Results Statistically significant differences between both groups were observed in diastolic, systolic, mean blood pressure, heart rate, and cardiac output. The propofol/fentanyl group showed a significant decrease in diastolic, systolic, and mean blood pressure, heart rate, and cardiac output compared with the propofol/ketamine group. Conclusion Noninvasive cardiac output measurement utilizing ECG, noninvasive blood pressure, and a pulse oximeter is a reliable method. Hemodynamic parameters including blood pressure, heart rate, cardiac output, and cardiac output index decreased with the induction of propofol and fentanyl combination, but were stable with the use of a propofol and ketamine combination.

Bioimpedance and bioreactance methods for monitoring cardiac output.

Noninvasive continuous cardiac output monitoring may have wide clinical applications in anaesthesiology, emergency care and cardiology. It can improve outcomes, establish diagnosis, guide therapy and help risk stratification. The present article describes the theory behind the two noninvasive continuous monitoring methods for cardiac output assessment such as bioimpedance and bioreactance. The review discusses the advantages and disadvantages of these methods and highlights the recent method comparison studies. The use of bioimpedance and bioreactance to estimate cardiac output under haemodynamic challenges is also discussed. In particular, the article focuses on performance of the two methods in the assessment of fluid responsiveness using passive leg raising test and cardiac output response to exercise stress testing.

Impact of changes in systemic vascular resistance on a novel non-invasive continuous cardiac output measurement system based on pulse wave transit time: a report of two cases

The inaccuracy of arterial waveform analysis for measuring continuos cardiac output (CCO) associated with changes in systemic vascular resistance (SVR) has been well documented. A new non-invasive continuous cardiac output monitoring system (esCCO) mainly utilizing pulse wave transit time (PWTT) in place of arterial waveform analysis has been developed. However, the trending ability of esCCO to measure cardiac output during changes in SVR remains unclear. After a previous multicenter study on esCCO measurement, we retrospectively identified two cases in which apparent changes in SVR developed in a short period during data collection. In each case, the trending ability of esCCO to measure cardiac output and time component of PWTT were analyzed. Recorded data suggest that the time component of PWTT may have a significant impact on the accuracy of estimating stroke volume during changes in SVR. However, further prospective clinical studies are required to test this hypothesis.

The Ability of Stroke Volume Variation Measured by a Noninvasive Cardiac Output Monitor to Predict Fluid Responsiveness in Mechanically Ventilated Children

Continuous noninvasive cardiac output monitoring (NICOM) is a clinically useful tool in the pediatric setting. This study compared the ability of stroke volume variation (SVV) measured by NICOM with that of respiratory variations in the velocity of aortic blood flow (△Vpeak) and central venous pressure (CVP) to predict of fluid responsiveness in mechanically ventilated children after ventricular septal defect repair. The study investigated 26 mechanically ventilated children after the completion of surgery. At 30min after their arrival in an intensive care unit, a colloid solution of 10ml/kg was administrated for volume expansion. Hemodynamic variables, including CVP, stroke volume, and △Vpeak in addition to cardiac output and SVV in NICOM were measured before and 10min after volume expansion. The patients with a stroke volume increase of more than 15% after volume expansion were defined as responders. The 26 patients in the study consisted of 13 responders and 13 nonresponders. Before volume expansion, △Vpeak and SVV were higher in the responders (both p values <0.001). The areas under the receiver operating characteristic curves of △Vpeak, SVV, and CVP were respectively 0.956 (95% CI 0.885-1.00), 0.888 (95% CI 0.764-1.00), and 0.331 (95% CI 0.123-0.540). This study showed that SVV by NICOM and △Vpeak by echocardiography, but not CVP, reliably predicted fluid responsiveness during mechanical ventilation after ventricular septal defect repair in children.

Cardiac output monitoring during Cesarean delivery in a patient with palliated tetralogy of Fallot

Tetralogy of Fallot (TOF) is one of the most common causes of cyanotic congenital heart disease. The anesthetic management of parturients with uncorrected TOF is challenging and controversial, especially for Cesarean delivery (CD). We describe the use of noninvasive cardiac output (CO) monitoring to assist the management of CD for a woman with palliated TOF under general anesthesia. A 34-yr-old woman presented for elective CD at 38weeks gestation. Having been born with TOF, she underwent a modified Blalock-Taussig shunt at six years of age, followed nine years later by creation of an aortopulmonary connection. The patient's functional status was New York Heart Association class I despite evident central cyanosis. A CD was performed under general anesthesia. Fentanyl, etomidate, and succinylcholine were utilized for induction, and intrathecal morphine was administered for postoperative pain control. The baseline CO (7.2L·min(-1)), blood pressure (156/74mmHg), heart rate (74beats·min(-1)), and total peripheral resistance (1,059dynes·sec(-1)·cm(-5)) remained stable throughout the procedure. Maintenance anesthesia consisted of rocuronium, sevoflurane, and an oxygen/nitrous oxide mixture. Upon delivery, an infusion of oxytocin combined with ergometrine was administered. Hemodynamic parameters remained stable and no vasopressor was required. Balanced general anesthesia and careful titration of uterotonic agents provided stable hemodynamic conditions during CD in a patient with a palliated TOF, as assessed by a continuous noninvasive CO monitor. Noninvasive CO monitoring may improve our understanding of the hemodynamic implications of various anesthetic techniques for CD in cardiac patients.

828 EFFICACY OF LONG ADMINISTRATION OF ANTIHYPERTENSIVE AGENTS TO THE IMPAIRED TOTAL VASCULAR TONE IN ELDERLY HYPERTENSIVE PATIENTS WITH CHRONIC KIDNEY DISEASE

Objective: We reported cardiovascular changes in chronic kidney disease (CKD) with hypertension might cause impaired circadian variation of total vascular tone (TVT). Therefore we investigated the efficacy of long term administration of antihypertensive agents to the impaired TVT in elderly hypertensive patients with CKD (HTCKD). Methods: Forty-three elderly (69.3 ± 4.2 years) HTCKD were admitted before and after administration of the following agents, and divided into two groups. Those HTCKD who were administered both ARB (telmisartan, 40 mg per day) and calcium channel blocker (amlodipine 5 mg per day) (Group 1, 22 cases), and those HTCKD who were administrated with either A or C (Group 2, 21 cases). The value of creatinine clearance of Group 1 and Group 2 were 69.4. ± 9.7 and 71.3 ± 10.6 ml/min/1.73 m2, respectively. A non-invasive ambulatory BP monitor and a non-invasive continuous cardiac output (CO) monitor were simultaneously attached to subjects for a 24 hour period. BP, CO and TVT were measured during this time. TVT was calculated as follows: [mean BP / cardiac index] x 1,332 dyne.sec/cm5. Results: 1. There was no significant differences of TVT at the untreated stage between Group 1 and Group 2. 2. The TVT after administration of the agents was significantly smaller (p < 0.05) in Group 1 than in Group 2. Conclusion: Accelerated TVT in elderly HTCKD might be ameliorated by long term administration of both ARB and C due to improving cardiovascular changes.

The ability of a new continuous cardiac output monitor to measure trends in cardiac output following implementation of a patient information calibration and an automated exclusion algorithm

A new non-invasive continuous cardiac output (esCCO) monitoring system solely utilizing a routine cardiovascular monitor was developed, even though a reference cardiac output (CO) is consistently required. Subsequently, a non-invasive patient information CO calibration together with a new automated exclusion algorithm was implemented in the esCCO system. We evaluated the accuracy and trending ability of the new esCCO system. Either operative or postoperative data of a multicenter study in Japan for evaluation of the accuracy of the original version of esCCO system were used to develop the new esCCO system. A total of 207 patients, mostly cardiac surgical patients, were enrolled in the study. Data were manually reviewed to formulate a new automated exclusion algorithm with enhanced accuracy. Then, a new esCCO system based on a patient information calibration together with the automated exclusion algorithm was developed. CO measured with a new esCCO system was compared with the corresponding intermittent bolus thermodilution CO (ICO) utilizing statistical methods including polar plots analysis. A total of 465 sets of CO data obtained using the new esCCO system were evaluated. The difference in the CO value between the new esCCO and ICO was 0.34 ± 1.50 (SD) L/min (95 % confidence limits of −2.60 to 3.28 L/min). The percentage error was 69.6 %. Polar plots analysis showed that the mean polar angle was −1.6° and radial limits of agreement were ±53.3°. This study demonstrates that the patient information calibration is clinically useful as ICO, but trending ability of the new esCCO system is not clinically acceptable as judged by percentage error and polar plots analysis, even though it’s trending ability is comparable with currently available arterial waveform analysis methods.

Left ventricular stroke volume measurement by impedance cardiography correlates with echocardiography in neonates

The aim of this study was to validate impedance cardiography (electrical velocimetry (EV)) as a continuous noninvasive cardiac output monitoring in neonates and infants. As the reference method, discontinuous transthoracic echocardiography (TTE) was used.

312 Emergency Physician Accuracy in Estimating Volume Responsive Shock Using the “CURVES” Questionnaire

Excess fluid administration and inadequate or delayed fluid resuscitation carry increased morbidity and mortality. Intensive care unit studies have shown that approximately 50% of patients given fluid boluses were truly volume responsive when compared to dynamic changes in cardiac output or stroke volume. In ED patients with undifferentiated shock, emergency physician accuracy in estimating volume responsiveness when compared to dynamic changes in cardiac output or stroke volume is unknown. To assess emergency physician accuracy in estimating volume responsiveness in patients with shock. This is a pilot study analyzing patients from a prospective randomized controlled trial of fluid optimization in the ED (“EFFORT”-ED Flow-directed Fluid Optimization Resuscitation Trial; ClinicalTrials.gov ID: NCT01128413). The study is IRB approved with informed consent. Enrollment began in August, 2010 and continues at an urban, academic tertiary care medical center with 90,000 ED visits/year. Inclusion criteria are adults (≥ 18 years) with either of the following: hypotension (SBP ≤90 or mean arterial pressure ≤65) despite 20ml/kg fluids; vasopressor use; or lactate ≥ 2.5 mmol/L. Patients are randomized to receive IV fluids via 1 of 2 arms: 1) routine care per clinician discretion 2) fluid optimization with 500ml normal saline boluses if the cardiac index or stroke volume index increases ≥ 15% with passive leg raise testing (PLRT). Patients are monitored until ED departure or a maximum of 6 hours using continuous noninvasive cardiac output monitoring (Cheetah NICOM®). During this time, passive leg raise testing is performed immediately before and after each fluid bolus or at 30-minute intervals if on a locked or maintenance fluid rate. With each passive leg raise testing, a CURVES (CardiovascUlar Response and Volume Estimation in Shock) questionnaire is administered separately to attending and resident physicians caring for the patients. Clinicians are blinded to the results of the passive leg raise testing and NICOM® monitoring. The CURVES questionnaire is a newly developed 5-item survey for this study which queries clinicians for the patient's potential shock etiology and estimations of volume responsiveness at that point in time (ie, responsive or unresponsive to further fluid administration), intravascular volume status, and cardiac index. With each questionnaire, clinicians are given the current vital signs, total volume administered, urine output, and the option for bedside re-examination, lab review, or the use of any other modality used in their routine care (ie, ultrasound, CVP, Scv02). Descriptive statistical analysis was performed using Microsoft® Excel. 11/11 per protocol patients were analyzed. The median age was 58 years (range 20-90) with 27% female. A total of 79 passive leg raise testings and 118 CURVES were acquired. 50/79 (63.3%) passive leg raise testing demonstrated volume responsiveness changes in cardiac index or stroke volume index. Attending emergency physician accuracy in estimating volume responsiveness was 68% with a sensitivity of 89% and specificity of 32%. Resident accuracy was 41% with a sensitivity of 45% and specificity of 35%. This pilot study suggests that emergency physician accuracy in estimating volume responsiveness in the setting of shock is poor when compared to passive leg raise testing changes in cardiac index or stroke volume. Routine clinical estimations of volume responsiveness not guided by dynamic changes in cardiac index or stroke volume index may lead to inadequate or excess fluid administration. The impact of ED fluid administration guided by dynamic changes in cardiac index or stroke volume index on morbidity and mortality is unknown.

Noninvasive cardiac output monitoring (NICOM): a clinical validation

To evaluate the clinical utility of a new device for continuous noninvasive cardiac output monitoring (NICOM) based on chest bio-reactance compared with cardiac output measured semi-continuously by thermodilution using a pulmonary artery catheter (PAC-CCO). Prospective, single-center study. Intensive care unit. Consecutive adult patients immediately after cardiac surgery. Cardiac output measurements obtained from NICOM and thermodilution were simultaneously recorded minute by minute and compared in 110 patients. We evaluated the accuracy, precision, responsiveness, and reliability of NICOM for detecting cardiac output changes. Tolerance for each of these parameters was specified prospectively. A total of 65,888 pairs of cardiac output measurements were collected. Mean reference values for cardiac output ranged from 2.79 to 9.27 l/min. During periods of stable PAC-CCO (slope<+/-10%, 2SD/mean<20%), the correlation between NICOM and thermodilution was R=0.82; bias was +0.16+/-0.52 l/min (+4.0+/-11.3%), and relative error was 9.1%+/-7.8%. In 85% of patients the relative error was <20%. During periods of increasing output, slopes were similar with the two methods in 96% of patients and intra-class correlation was positive in 96%. Corresponding values during periods of decreasing output were 90% and 84%, respectively. Precision was always better with NICOM than with thermodilution. During hemodynamic challenges, changes were 3.1+/-3.8 min faster with NICOM (p<0.01) and amplitude of changes did not differ significantly. Finally, sensitivity of the NICOM for detecting significant directional changes was 93% and specificity was 93%. Cardiac output measured by NICOM had most often acceptable accuracy, precision, and responsiveness in a wide range of circulatory situations.

Evaluation of a noninvasive continuous cardiac output monitoring system based on thoracic bioreactance

Noninvasive cardiac output (CO) measurement can be useful in many clinical settings where invasive monitoring is not desired. Bioimpedance (intrabeat measurement of changes in transthoracic voltage amplitude in response to an injected high-frequency current) has been explored for this purpose but is limited in some clinical settings because of inherently low signal-to-noise ratio. Since changes in fluid content also induce changes in thoracic capacitive and inductive properties, we tested whether a noninvasive CO measurement could be obtained through measurement of the relative phase shift of an injected current (i.e., bioreactance). We constructed a prototype device that applies a 75-kHz current and determines the relative phase shift (dPhi/dt) of the recorded transthoracic voltage. CO was related to the product of peak dPhi/dt, heart rate, and ventricular ejection time. The preclinical study was done in nine open-chest pigs put on right heart bypass so that CO could be varied at known values. This was followed by a feasibility study in 27 postoperative patients who had a Swan-Ganz catheter (SGC). The measurements of noninvasive CO measurement and cardiopulmonary bypass pump correlated to each other (r = 0.84) despite the large variation in CO and temperatures. Similarly, in patients, mean CO values were 5.18 and 5.17 l/min as measured by SGC and the noninvasive CO measurement system, respectively, and were highly correlated over the range of values studied (r = 0.90). Preclinical and clinical data demonstrate the feasibility of using blood flow-related phase shifts of transthoracic electric signals to perform noninvasive continuous CO monitoring.

Noninvasive cardiac output monitoring: a clinical validation

Objective To evaluate the clinical utility of a new device for continuous noninvasive cardiac output monitoring (NICOM) based on chest bio-reactance compared with cardiac output measured semi-continuously by thermodilution using a pulmonary artery catheter (PAC-CCO).

Comparison of norepinephrine and dopamine in the management of septic shock using impedance cardiography

Objective: Vasoconstrictors are one of the therapeutic modalities in the treatment of septic shock. In the present study, we have compared the effects of dopamine and norepinephrine in the treatment of septic shock with pre-defined end-points and continuous non-invasive cardiac output monitoring using impedance cardiography. Design: Randomized controlled trial. Settings: Sixteen-bedded mixed intensive care unit of a tertiary care teaching institution. Materials and Methods: The study included 50 consecutive patients presenting with septic shock and divided randomly into two groups with 25 patients in each group. Group I patients were treated with dopamine and those in Group II were treated with norepinephrine. They were optimized with fluid resuscitation upto CVP>10 cm of H 2 O, packed red cells transfusion upto hematocrit >30, oxygenation and ventilation upto PaO 2 >60 mmHg before the inotropes were started. The goal of therapy was to achieve and maintain for six hours, all of the following: (1) SBP >90 mm Hg, (2) SVRI >1100 dynes.s/cm 5 m 2 , (3) Cardiac Index >4.0L/min/m 2 , (4) IDO 2 >550 ml/min/m 2 and (5) IVO 2 >150 ml/min/m 2 . Measurements: The demographic data, baseline parameters and post-treatment parameters were statistically analyzed by using t-test. Results: The post-treatment parameters were statistically significant showing the superiority of norepinephrine over dopamine in optimization of hemodynamics and patient survival. Significant improvement in systolic blood pressure, heart rate, cardiac index, SVRI, IVO 2 and urine output were found in norepinephrine group than the dopamine group. Dopamine showed a response in 10 out of 25 patients up to a maximum dose of 25 mcg/kg/min while with norepinephrine, 19 patients responded up to a maximum dose of 2.5 mcg/kg/min The hemodynamic parameters were preserved in norepinephrine group with better preservation of organ perfusion and oxygen utilization with maintenance of splanchnic and renal blood flow as evidenced by significant increase in O2 uptake and urine flow. Conclusion: Norepinephrine was more useful in reversing the hemodynamic and metabolic abnormalities of septic shock compared to dopamine at the doses tested.

Noninvasive, Automated and Continuous Cardiac Output Monitoring by Pulmonary Capnodynamics

Cardiac output monitoring is most important where cardiovascular stability is potentially threatened, such as during major surgery and in critically ill patients. However, continuous monitoring of cardiac output is still not performed routinely during anesthesia and critical care, because of invasiveness, expense, and inaccuracy of available technologies. A technique termed the capnodynamic method was tested for breath-to-breath measurement of pulmonary blood flow from lung carbon dioxide mass balance, using measured carbon dioxide elimination and end-tidal concentration. A prototype measurement system was constructed for a feasibility study in six anesthetized sheep. Large and rapid fluctuations in cardiac output were generated by repeated dobutamine and esmolol challenge. Measurements were compared with an indwelling ultrasonic flow probe placed on the ascending aorta or pulmonary artery. Cardiac output measured by the flow probe varied between zero and 8.67 l/min, with a mean of 3.50 l/min. Overall mean bias [SD of the difference] between the methods (capnodynamic - flow probe) was -0.25 [0.94] l/min, r = 0.79 (P < 0.001). During periods of stability in cardiac output of 5 min or more, mean bias was -0.20 [0.55] l/min. The method successfully indicated two cardiac arrest events, which were induced in one of the animals. The method satisfactorily tracked wide fluctuations in cardiac output in real time. The capnodynamic method may have potential for continuous noninvasive cardiac output monitoring in patients undergoing anesthesia for major surgery, and in critical care, on a routine basis.

Reproducibility of estimated cardiovascular function by transthoracic bioimpedance cardiography in healthy volunteers

In ten healthy volunteers, stroke index (SI), heart rate (HR), thoracic fluid conductivity (TFC) and index of contractility (IC) were estimated by transthoracic bioimpedance measurements (BoMed Noninvasive Continuous Cardiac Output Monitor, model 3, Revision 7 (NCCOM3-R7)). Intra- and inter-observer variability and the influence of time of day (morning or afternoon) were determined. The intra-observer variability was significantly smaller during the morning than during the afternoon. The mean differences ± standard error between consecutive measurements for morning versus afternoon measurements were, respectively: SI, 0.8 ± 1.2 vs. 4.6 ± 1.2 ml/m 2 ( P = 0.057); HR, −1.0 ± 0.6 versus −0.9 ± 0.6 beats/min; TFC, −0.0 ± 0.3 vs. 1.0 ± 0.3 (= TFC × 1000 · Ω −1) ( P = 0.031) and IC, −0.6 ± 0.9 vs. 5.6 ± 0.9 (= IC × 1000 · s −1) ( P = 0.001). The inter-observer variability for SI, HR, TFC and IC did not reach statistical significance. These data suggest that reproducible measurements can be obtained provided that only morning or afternoon measurements are compared. Since the reproducibility of measurements was better during the morning than during the afternoon, we recommend that measurements are performed in the morning.

Application of impedance cardiography during exercise

Impedance cardiography has been used over the last 30 years to measure stroke volume on a beat-by-beat basis. Cardiac output has been successfully measured with either upper or lower body exercise during light or moderate workloads. With strenuous exercise, movement artifacts severely limit the acquisition of a quality impedance cardiogram. Advances in computer technology and signal conditioning techniques have created the next generation of impedance cardiograph systems. The purpose of this study was to evaluate such a system, the noninvasive continuous cardiac output monitor (NCCOM3-R7), at rest and during submaximal upright cycle exercise. In addition, the relationships between thoracic impedance ( Z o), first derivative of the change in thoracic impedance (d Z/d t) and posture were evaluated using the NCCOM3-R7 and the Minnesota impedance cardiograph 304B (MIC). Twenty-eight healthy men and women participated. The Z o progressively increased when moving from the supine to seated to standing position with both instruments. However, the NCCOM3-R7 yielded lower Z o values and higher d Z/d t values compared with the MIC for all postures. Z o and d Z/d t values appear to be dependent upon factors such as posture, gender, electrical current, and characteristics of the instrumentation. Exercise cardiac output values seemed reasonable for most subjects, although population subsets exist where the accuracy must be questioned. The general consensus supported by the impedance literature and reaffirmed by the present observations is that impedance cardiography provides a reasonable estimate of the directional changes in stroke volume and cardiac output during exercise and can be used to monitor changes in thoracic fluid balance. As this technology evolves and is further refined, it will undoubtedly play an increasing role in environmental medicine, exercise stress testing, cardiac rehabilitation, and sports medicine.

Acute Hemodynamic Responses to Weightlessness During Parabolic Flight

Pilots and astronauts experience fluid shifts in variable gravity. Acute effects of fluid shifts on the cardiovascular system were monitored on NASA's KC-135 aircraft during parabolic flight. The variability of R-R intervals in the electrocardiogram was measured as an indication of vagal cardiac neural activity. R-R intervals were measured during the gravity transition from 2-G to 0-G produced by parabolic flight to assess the involvement of the autonomic nervous system in regulating the acute effects of fluid shifts. In seven subjects, a BoMed noninvasive continuous cardiac output monitor (NCCOM 3) monitored thoracic fluid index (TFI, ohms), heart rate (bpm), and cardiac output (1/min). Data were stored on a lap-top computer with the subject in one of four postures: sitting, standing, supine, and semi-supine, during one of four sets of eight to ten parabolas. Five seconds of data were averaged: before parabola onset (1.3-G); parabola entry (1.9-G); 0-G; and parabola exit (1.7-G). Three to eight parabolas were averaged for subjects in each posture; the mean for each posture was calculated. In each of five additional subjects, the coefficient of variation was calculated by dividing mean value by the standard deviation of 3 to 15 R-R intervals. Eight to ten parabolas were averaged for each postural set. Compared with values collected before 0-G, standing values during 0-G showed that the thoracic fluid index decreased 2.5 ohms, heart rate decreased 22 bpm, and cardiac output increased 1 L/min. During sitting, thoracic fluid index decreased 1.25 ohms, heart rate decreased 10 bpm, whereas cardiac output increased 0.5 L/min.(ABSTRACT TRUNCATED AT 250 WORDS)