A review of impedance cardiography

A fresh look at sports PSM-systems

The aim of the proposed study is to reveal the correlations between the dynamics of Respiratory Rate (RR) and Heart Rate (HR) during intermittent physical work at maximum power on a cycle ergometer. The stage of investigating the General functional athlete readiness (GFAR) was conducted using the sports standard "R-Engine" and the cycle ergometer in 16 volunteers (10 men, 6 women) whose average age was 21±1.17 years. To determine the athletic potential of the volunteers in this study, we used our own Coefficient of Anaerobic Capacity (CANAC Q, beats). Continuous registration of the heart rate and respiratory rate of volunteers in the maximum power sports test was performed by the "RheoCardioMonitor" system with a module of the athlete functional readiness based on the method of Transthoracic electrical impedance rheography (TEIRG). The degree of correlation of functional indicators (M, HRM, GFAR) with CANAC Q in all experimental series of the study group as a whole (n=80) was at a very high level, which confirmed the effectiveness of using the Coefficient of Anaerobic Capacity (CANAC Q) in assessing the general functional athlete readiness of the volunteers. CANAC Q is measured in "beats" of the heart and is recorded very accurately using the method of transthoracic electrical impedance rheography (TEIRG). For this reason, as a promising sports PSM-system, CANAC Q can replace the methods for determining the functional athlete readiness by blood lactate concentration and maximum oxygen consumption.

Fluid and Hemodynamics

Several components of an Enhanced Recovery After Surgery (ERAS) pathway act to improve and simplify perioperative fluid and hemodynamic therapy. Modern perioperative fluid management has shifted away from the liberal fluid therapy and toward more individualized approaches. Clinical evidence has also emphasized the importance of maintaining adequate mean arterial pressure and avoiding intraoperative hypotension. Goal-directed hemodynamic therapy (GDHT), or the use of cardiac output monitoring to guide fluid and vasopressor use, has been shown to reduce complications, but its role within ERAS pathways is likely best-suited to high-risk patients or those undergoing high-risk procedures. This article reviews the mechanisms by which ERAS pathways aid the provider in hemodynamic management, reviews trends, and evidence regarding fluid and hemodynamic therapy approaches, and provides guidance on the practical implementation of these concepts within ERAS pathways.

Comparing square root method of measuring the cardiac output by means of aortic impedance change component to Kubicek's method

PURPOSE

The aim of this study is to explore a calculated method used to measure the cardiac output using the aortic impedance change component of reconstructed impedance cardiography.

METHODS

Routine impedance cardiography was measured using Kubicek's method with four ring electrodes. The thoracic mixed impedance changes were measured by six leads, which consisted of 15 electrodes. The aortic impedance change component was separated from six thoracic mixed impedance changes through waveform reconstruction. The square root formula used to calculate the cardiac output was deduced based on the thoracic impedance change equation and the aortic volume change hypothesis during the systole period. The cardiac outputs of 180 normal adults and 72 patients with cardiac insufficiency who could still walk freely were contrastively computed with both Kubicek's formula and the square root formula.

RESULTS

For 180 normal adults, the cardiac index (CI) computed with the square root formula was 3.60 ± 0.45 L/min/m2 , with normal values ranging from 2.7 to 4.5 L/min/m2 . A total of 163 cases (90.6%) had a CI in the standard range (2.7-4.3 L/min/m2 ) adopted in clinical applications. The CI computed with Kubicek's formula was 3.61 ± 0.86 L/min/m2 , with normal values ranging from 1.9 to 5.3 L/min/m2 , and only 115 cases (63.9%) had a CI in the above standard range. Among the 72 patients with cardiac insufficiency, 20 (27.8%) patients had a CI < 2.0 L/min/m2 with Kubicek's formula. Of these 20 cases, 9 cases had a CI < 1.5 L/min/m2 , and 4 cases had a CI < 1.1 L/min/m2 . In contrast, none of the 72 patients had a CI < 2.0 L/min/m2 with the square root formula. In addition, the influence of the chest circumference on the CI was lower for the square root formula than for Kubicek's formula.

CONCLUSIONS

The CI calculated with the square root formula had a better normal value range, was more accurate for the patients with cardiac insufficiency, and was less affected by the chest circumference than the CI calculated with Kubicek's formula.

Impedance Cardiography Derived Cardiac Output in Hemodialysis Patients: A Study of Reproducibility and Comparison with Echocardiography

Background

Hemodialysis patients experience a variety of hemodynamic abnormalities that contribute to cardiovascular disease mortality which is the leading cause of death in these patients. Impedance cardiography has been utilized in order to monitor cardiac hemodynamics with lower cost and inconvenience, but it has not been appropriately validated in the hemodialysis population.

Aim

We repeatedly used impedance cardiography to assess short- (48 hours) and long-term (15 days) reproducibility of cardiac output measurements and we compared baseline impedance cardiography measurements with echocardiographic measurements.

Patients and Methods

We studied 109 stable hemodialysis patients, aged 59.70 ± 11.97 years being on hemodialysis for 67.59 ± 40.15 months, on a non-dialysis day. Cardiac output was obtained with the BioZ impedance cardiography system (Cardiodynamics, San Diego, Ca, USA). Baseline echocardiography was performed using a Hewlett-Packard Sonos 2500 (Andover, Mass., USA).

Results

The values of impedance cardiography derived cardiac output were 5.28 ± 0.79, 5.27 ± 0.75 and 5.25 ± 0.74 l/min at baseline (107 patients), 48 hours (107 patients) and 15 days (98 patients) respectively, showing high reproducibility. Bland and Altman analysis estimated that bias at 48 hours and at 15 days were: −0.013 (95% confidence intervals = −0.045 to 0.019) and 0.028, (95% confidence intervals = −0.044 to 0.101), respectively. In addition baseline impedance cardiography derived cardiac output was significantly correlated with the echocardiographic derived cardiac output (r = 0.9, p < 0.0001).

Conclusion

Impedance cardiography is a simple non invasive technique for cardiac output estimation in hemodialysis patients which has high reproducibility when performed under controlled conditions, and is closely correlated with echocardiographic measurements of cardiac output.

Hemodynamic assessment in the contemporary intensive care unit: a review of circulatory monitoring devices

The assessment of the circulating volume and efficiency of tissue perfusion is necessary in the management of critically ill patients. The controversy surrounding pulmonary artery catheterization has led to a new wave of minimally invasive hemodynamic monitoring technologies, including echocardiographic and Doppler imaging, pulse wave analysis, and bioimpedance. This article reviews the principles, advantages, and limitations of these technologies and the clinical contexts in which they may be clinically useful.

Validation of thoracic impedance cardiography by echocardiography in healthy late pregnancy

Background

Assessment of stroke volume (SV) is often necessary in clinical and research settings. The clinically established method for SV assessment in pregnancy is echocardiography, but given its limitations, it is not always an appropriate measurement tool. Thoracic impedance cardiography (ICG) allows continuous, non-invasive SV assessment. However, SV determination relies on assumptions regarding the thoracic shape that may mean the algorithm is not valid in pregnancy. The available data regarding the validity of ICG against an established reference standard using modern SV algorithms are both limited and conflicting. We aimed to test the validity of ICG in a clinically realistic setting in late pregnancy using echocardiography.

Methods

Twenty-nine women in late pregnancy underwent standard echocardiography assessments with simultaneous ICG measurement. Agreement between devices was tested using Bland-Altman analysis.

Results

Bland-Altman analysis of the relationship between ICG and echocardiography demonstrated that the 95% limits of agreement exceeded acceptable or expected ranges. Measures of maternal and fetal anthropometry do not account for the lack of agreement.

Conclusions

Absolute values of SV as determined by ICG are not valid in pregnancy. Further work is required to examine the ability of ICG to assess relative changes in maternal haemodynamics in late pregnancy.

Hemodynamic monitoring in the critical care environment

Hemodynamic monitoring is essential to the care of the critically ill patient. In the hemodynamically unstable patient where volume status is not only difficult to determine, but excess fluid administration can lead to adverse consequences, utilizing markers that guide resuscitation can greatly affect outcomes. Several markers and devices have been developed to aid the clinician in assessing volume status with the ultimate goal of optimizing tissue oxygenation and organ perfusion. Early static measures of volume status, including pulmonary artery occlusion pressure and central venous pressure, have largely been replaced by newer dynamic measures that rely on real-time measurements of physiological parameters to calculate volume responsiveness. Technological advances have lead to the creation of invasive and noninvasive devices that guide the physician through the resuscitative process. In this manuscript, we review the physiologic rationale behind hemodynamic monitoring, define the markers of volume status and volume responsiveness, and explore the various devices and technologies available for the bedside clinician.

Relationship between cardiac output and onset of succinylcholine chloride action in electroconvulsive therapy patients

BACKGROUND

Succinylcholine chloride (Sch) is ideal for electroconvulsive therapy (ECT). However, the appropriate interval between Sch administration and electrical stimulation has not been reported. Cardiac output at the time of drug administration seems to be the major contributing factor for variability in onset time. The present study therefore investigated relationships between cardiac output before Sch administration and the onset of Sch action.

METHODS

Cardiac output and cardiac index (CI) were continuously monitored in 24 patients using a noninvasive impedance cardiac output monitor. Anesthesia was induced using intravenous propofol at 1 mg kg(-1). After loss of consciousness, dorsiflexion of the hallux was monitored as single-twitch stimulations using a peripheral nerve stimulator equipped with an acceleration sensor. A 1 mg kg(-1) dose of Sch was administered, and patients were assisted by mask ventilation with 100% oxygen. A bilateral ECT was performed after single-twitch response reached zero. We measured the intervals between Sch administration and the appearance of the first fasciculation (int-F), and between Sch administration and the loss of the single twitch response (int-S0) as time of Sch onset. To determine the effective duration of Sch action, we measured the intervals between the first fasciculation and the single-twitch response above zero (int-A) and between loss of the single-twitch response and recovery above zero (int-R).

RESULTS

The alteration in CI during ECT was biphasic. The CI before Sch administration (pre-CI) varied from 2.01 to 5.94 L min(-1) m(-2) (4.23 +/- 1.20 L min(-1) m(-2)). The int-F was 40 +/- 5 seconds (range, 31-49 seconds) and int-S0 was 90 +/- 17 seconds (range, 58-124 seconds). The correlations were significantly inverse between int-F and pre-CI (n = 10, R2 = 0.504, P = 0.0189), and between int-S0 and pre-CI (n = 17, R2 = 0.339, P = 0.0127). The int-A was 236 +/- 95 seconds (range, 119-391 seconds) and int-R was 184 +/- 106 seconds (range, 60-369 seconds). We also found correlations between int-A and pre-CI (n = 10, R2 = 0.413, P = 0.0438) and between int-R and pre-CI (n = 17, R2 = 0.405, P = 0.0466).

CONCLUSIONS

The onset of muscle relaxation varies among patients receiving ECT and is related to CI before Sch administration. In patients for whom fasciculation is difficult to determine, the effects of a muscle relaxant should be objectively confirmed before electrical stimulation of the brain.

Assessment of stroke volume index with three different bioimpedance algorithms: lack of agreement compared to thermodilution

OBJECTIVE

The accuracy of bioimpedance stroke volume index (SVI) is questionable as studies report inconsistent results. It remains unclear whether the algorithms alone are responsible for these findings. We analyzed the raw impedance data with three algorithms and compared bioimpedance SVI to transpulmonary thermodilution (SVI(TD)).

DESIGN AND SETTING

Prospective observational clinical study in a university hospital.

PATIENTS

Twenty adult patients scheduled for coronary artery bypass grafting (CABG).

INTERVENTIONS

SVI(TD) and bioimpedance parameters were simultaneously obtained before surgery (t1), after bypass (t2), after sternal closure (t3), at the intensive care unit (t4), at normothermia (t5), after extubation (t6) and before discharge (t7). Bioimpedance data were analyzed off-line using cylinder (Kubicek: SVI(K); Wang: SVI(W)) and truncated cone based algorithms (Sramek-Bernstein: SVI(SB)).

MEASUREMENTS AND RESULTS

Bias and precision between the SVI(TD) and SVI(K), SVI(SB), and SVI(W) was 1.0+/-10.8, 9.8+/-11.4, and -15.7+/-8.2 ml/m2 respectively, while the mean error was abundantly above 30%. Analysis of data per time moment resulted in a mean error above 30%, except for SVI(W) at t2 (28%).

CONCLUSIONS

Estimation of SVI by cylinder or truncated cone based algorithms is not reliable for clinical decision making in patients undergoing CABG surgery. A more robust approach for estimating bioimpedance based SVI may exclude inconsistencies in the underlying algorithms in existing thoracic bioimpedance cardiography devices.

Validation of a new arterial pulse contour-based cardiac output device

OBJECTIVE

To evaluate the accuracy and precision of an arterial pulse contour-based continuous cardiac output device (Vigileo). Vigileo cardiac output (VigileoCO) was compared with intermittent transpulmonary thermodilution cardiac output (TPCO) and an established arterial pulse contour-based cardiac output (PCCO).

DESIGN

Prospective clinical study.

SETTING

University hospital.

PATIENTS

Twenty-two patients undergoing coronary artery bypass graft surgery.

INTERVENTIONS

Defined volume load during surgery and in the postoperative period.

MEASUREMENTS AND MAIN RESULTS

We obtained 184 pairs of VigileoCO and TPCO, 140 pairs of VigileoCO and PCCO, and 140 pairs of PCCO and TPCO. Measurements were performed after induction of anesthesia (T1), after sternotomy (T2), immediately after (T3) and 20 mins after volume challenge with 10 mL.kg hydroxyethyl starch 6% (T4), 15 mins after coronary pulmonary bypass (T5), after retransfusion of autologous blood (T6), after arrival at the intensive care unit (T7), and immediately after (T8) and 20 mins after (T9) a second volume load with 10 mL.kg hydroxyethyl starch 6%. TPCO was used to calibrate PCCO. For pooled data, including uncalibrated PCCO data immediately after weaning from coronary pulmonary bypass (T5), the correlation coefficient of TPCO vs. VigileoCO, PCCO vs. VigileoCO, and TPCO vs. PCCO was 0.75, 0.60, and 0.75 respectively. Bland-Altman analysis showed a bias of 0.00, -0.01, and 0.02 L.min, the precision (=sd) was 0.87, 1.08, and 0.93 L.min, and the mean error was 33%, 40%, and 35%. When we compared calibrated PCCO values (T2-T4, T6, T7-9), the correlation coefficients of PCCO-VigileoCO and TPCO-PCCO were 0.72 and 0.85, bias was -0.16 and 0.19 L.min, and mean error was 33% and 27%, respectively. Best correlations and the least differences between TPCO and VigileoCO were observed in postbypass closed-chest conditions and in the intensive care unit.

CONCLUSIONS

Our results showed that VigileoCO enables clinically acceptable assessment of cardiac output in postbypass closed-chest conditions and during stable conditions in the intensive care unit.

Comparison of electrical velocimetry and transpulmonary thermodilution for measuring cardiac output in piglets

BACKGROUND

Monitoring of cardiovascular function is essential during major pediatric and pediatric cardiac surgery. Invasive monitoring of cardiac output (CO) and oxygen delivery is expensive and sometimes associated with adverse events. Therefore, we investigated the accuracy of a new noninvasive CO monitoring device using electrical velocimetry (EV) in comparison with the more invasive transpulmonary thermodilution (TPTD) method.

METHODS

In five fasted, anesthetized and mechanically ventilated piglets, CO was measured simultaneously using EV and TPTD under normal conditions, volume loading, inotropic support and exsanguination.

RESULTS

In five piglets, 169 measurements could be performed. The correlations between EV-CO and TPTD-CO were significant for absolute values (P < 0.0001, r = 0.82) and relative changes from baseline (P < 0.0001, r = 0.93). The receiver operating characteristic (ROC) curve analysis of the relative changes of the EV-CO values in relation to the first EV-CO measurement showed a sensitivity of 91% and specificity of 94% (AUC 0.974, 95% CI 0.96-0.99). Changes in TPTD-CO greater than 15% lead to a change of EV-CO in the same direction in 93%. Bland-Altman analysis showed a mean difference between the two methods of -0.63 l x min(-1) with an sd of 0.64 l x min(-1). The lower and upper limits of agreement were -1.88 and 0.62 l x min(-1), percentage limit of agreement was +/-82.8%.

CONCLUSIONS

The results show that EV is a safe, simple, noninvasive and cost-effective method for continuous trend monitoring of CO in piglets. The agreement of the EV-CO with TPTD-CO is not good enough to replace the standard method in our animal model. A correction factor for body habitus in piglets may be beneficial.

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 (dΦ/d t) of the recorded transthoracic voltage. CO was related to the product of peak dΦ/d t, 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.

Comparison of electrical velocimetry and transoesophageal Doppler echocardiography for measuring stroke volume and cardiac output † †Declaration of interest. This study was supported by Osypka Medical GmbH, Berlin, Germany

BACKGROUND

Impedance cardiography (ICG) has been used extensively to estimate stroke volume (SV) and cardiac output (CO) from changes of thoracic electrical bioimpedance (TEB). However, studies comparing ICG with reference methods have questioned the reliability of this approach. Electrical velocimetry (EV) provides a new algorithm to calculate CO from variations in TEB. As the transoesophageal Doppler echocardiographic quantification of CO (TOE-CO) has emerged as a reliable method, the purpose of this study was to determine the limits of agreement between CO estimations using EV (EV-CO) and TOE-CO.

METHODS

Standard ECG electrodes were used for non-invasive EV-CO measurements. These were placed on 37 patients scheduled for coronary artery surgery necessitating transoesophageal echocardiography monitoring. Simultaneous EV-CO and TOE-CO measurements were recorded after induction of anaesthesia. EV-CO was calculated using the Bernstein-Osypka equation. TOE-CO was measured across the aortic valve using continuous-wave Doppler echocardiography and a triangular orifice model.

RESULTS

A significant high correlation was found between the TOE-CO and the EV-CO measurements (r2=0.86). Data were related linearly. The slope of the line (1.10 (se 0.07)) was not significantly different from unity, and the point at which it intersected the ordinate (-0.46 (0.32) litre min(-1)) was not significantly different from zero. Bland-Altman analysis revealed a bias of 0.18 litre min(-1) with narrow limits of agreement (-0.99 to 1.36 litre min(-1)).

CONCLUSIONS

The agreement between EV-CO and TOE-CO is clinically acceptable, and these two techniques can be used interchangeably.

Hemodynamic mechanisms underlying the incomplete tolerance to caffeine's pressor effects

Blood pressure (BP) and cardiovascular hemodynamics were assessed at baseline and after caffeine administration in a 4-week, placebo-controlled, double-blind, randomized, crossover trial of caffeine tolerance formation. Half of the subjects developed tolerance to the pressor effect of caffeine, whereas the other half continued to show increases in BP after caffeine ingestion (F = 16.7, p <0.0001). In the subjects who did not develop tolerance, peripheral resistance increased incrementally as the daily dose of caffeine increased (F = 2.8, p = 0.05).

Automated Impedance Cardiography for Detecting Ischemic Left Ventricular Dysfunction during Exercise Testing

Automated impedance cardiography (ICG) is an attractive method for noninvasive hemodynamic evaluation. The objective of our study was to evaluate the feasibility and diagnostic value automated ICG in patients with suspected coronary artery disease (CAD). We measured stroke index (SI) and cardiac index (CI) in 65 patients with suspected CAD at rest and during bicycle exercise testing. All patients underwent subsequent cardiac catheterization including coronary angiography (CA). Depending on the results of CA, patients were divided into three groups, patients without significant CAD (group 0), single vessel disease (group 1) or multivessel disease (group 2-3). In a subset of 20 patients, automated ICG was compared to measurements of CI by the thermodilution (TD) method.

RESULTS

There were no significant differences in SI and CI at baseline between the three groups. At 75- and 100-watt exercise, patients in group 2-3 showed significantly lower mean values of SI and CI as compared to patients of group 0 and group 1 (all p < 0.05), indicating exercise-induced ischaemic left ventricular (LV) dysfunction. Three patients had to be excluded because of inappropriate quality of the ICG signals during exercise. Comparison of automated ICG with TD measurements of CI showed good correlations between both methods at rest (r = 0.73) and during exercise (r = 0.89-0.91).

CONCLUSIONS

We conclude that hemodynamic monitoring by automated ICG is both feasible and practical during exercise testing. Automated ICG can provide reliable and valuable additional diagnostic information on LV function during exercise which is helpful for selecting those patients for angiography who are likely to benefit from coronary interventions.

Comparison of Impedance Cardiography and Thermodilution-Derived Measurements of Stroke Volume and Cardiac Output at Rest and During Exercise Testing

BACKGROUND

Non-invasive evaluation of haemodynamic variables remains a preferable and attractive option in both pharmacologic research and clinical cardiology.

OBJECTIVES

The objective of this study was to evaluate the correlation, feasibility and diagnostic value of haemodynamic measurements by ICG with the thermodilution (TD) method at rest and during exercise testing.

METHODS

We measured stroke volume (SV) and cardiac output (CO) with both methods in 20 patients with suspected coronary artery disease (CAD). All measurements were performed simultaneously at rest and during bicycle exercise.

RESULTS

There was a highly significant correlation (p < 0.001) for measurements of SV between both methods at rest (r = 0.83) and during exercise (r = 0.85-0.87) with 50-100 watts. For measurements of CO, the respective correlations were r = 0.85 at rest and r = 0.92-0.94 during exercise. The mean difference for measurements of SV were 3.8 +/- 12.6 ml at rest and 6.5+/- 11.4 ml during exercise. For measurements of CO, the mean difference between both methods was 0.9 +/- 1.0 l/min at rest and 1.0+/- 0.8 l/min during exercise. Compared to TD measurements, ICG had a bias to overestimate SV and CO of approximately by 5-10%. One patient had to be excluded because of inappropriate quality of the ICG signals during exercise.

CONCLUSIONS

ICG is a feasible and accurate method for non-invasive measurements of SV and CO. Haemodynamic measurements by ICG were correlated highly significant to simultaneous measurements by the TD method.

Validity of the Adaptive Filter for Accurate Measurement of Cardiac Output in Impedance Cardiography

The purpose of this study was to assess the validity of an adaptive filter, the scaled Fourier linear combiner (SFLC), in the impedance cardiography (ICG). Eight healthy males underwent constant-load bicycle exercise at different intensities from unloaded to near maximal intensity. The stroke volume (SV) and cardiac output (Q) measured by ICG at each condition were compared with those by the CO2 rebreathing method. We found that the noises were greatly reduced in the impedance waveform and that the inflection points, so-called the B- and X-points, were clearly detected even during strenuous exercise using the SFLC. Although a high correlation was observed between Qs measured by the two methods, the mean values of Qs in each method differed significantly and the regression line also differed significantly from the identity line. Likewise, a significant correlation was observed between SVs obtained by the two methods, but a significant difference in the group mean values and a trend of the regression line were observed. These findings suggest that the use of SFLC in ICG improves the performance in eliminating the noises and in detecting the inflection points in the waveform, thereby contributing to the accurate and beat-to-beat measurements of SV and Q especially during exercise.

Cardiovascular responses during submaximal electrical stimulation-induced leg cycling in individuals with paraplegia

This study investigated the cardiovascular responses during electrical stimulation-induced leg cycling (ES-LCE) in people with paraplegia (PARA) compared with voluntary leg cycling (VOL) at similar levels of oxygen uptake in able-bodied (AB) individuals. Six PARA with sensory and motor complete spinal cord lesions (TS-T9) and six AB participated in this study. Oxygen uptake (VO2), stroke volume (SV), heart rate (HR) and cardiac output (Q) were measured at rest and during submaximal, steady-state leg cycling. At the highest power output achieved (9.2 +/- 2.4 W for PARA versus 42.8 +/- 1.0 W for AB), VO2 was augmented above resting levels to 0.75 +/- 0.11 min(-1) in PARA and to 0.74 +/- 0.071 min(-1) in AB. HR and SV were also increased during ES-LCE in PARA (92.1 +/- 8-6 beats min(-1) and 93.9 +/- 11.3 ml bea(-1), respectively) and during VOL in AB (83.9 +/- 9.2 beats min(-1) and 89.7 +/- 9.0 ml beat(-1), respectively). At an equivalent submaximal VO2, HR and SV were not different between the two groups, however, Q was higher in PARA (6.6 +/- 0.7 versus 4.1 +/- 0.9 1 min l(-1) deltaVO2). These data suggest that ES-LCE at relatively low power outputs elicits increases in several cardiovascular variables in PARA. Furthermore, it is possible that ES-LCE leads to a 'hyperkinetic circulation' (a greater Q for a given VO2).

A finite-element study of the effects of electrode position on the measured impedance change in impedance cardiography

Traditional impedance cardiography (ICG) technique uses band electrodes both for delivering current to and measuring impedance change in the thorax. The use of spot electrodes increases the ease of electrode placement and comfort level for patients. Research has shown that changes in thoracic impedance can have multiple causes. In this study, we used finite element modeling to investigate the sources of impedance change for both band-electrode and spot-electrode ICG, and focused on how differences in electrode location affect the contribution of different sources to changes in impedance. The ultimate purpose is to identify the optimal electrode type and placement for the sensing of stroke volume (SV). Our models were built on sets of end-diastolic and end-systolic magnetic resonance images of a healthy human subject. The results showed that the effect of ventricular contraction is opposite to that of the other changes in systole: the expansion of major vessels, decrease in blood resistivity due to increased blood flow velocity, and decrease in lung resistivity due to increased blood perfusion. Ventricular contraction, the only factor that tends to increase systolic impedance, has a larger effect than any of the other factors. When spot electrodes are placed on the anterior chest wall near the heart, ventricular contraction is so dominant that the measured impedance increases from end-diastole to end-systole, and the change represents 82% of the contribution from ventricular contraction. When using the common band-electrode configuration, the change in measured impedance is a more balanced combination of the four effects, and ventricular contraction is overcome by the other three factors so that the impedance decreases. These results suggest that the belief that ICG can be used to directly measure SV based on the change in the whole thoracic impedance may be invalid, and that spot electrodes may be more useful for understanding local physiological events such as ventricular volume change. These findings are supported by previously reported experimental observations.

Noninvasive assessment of hemodynamics: an emphasis on bioimpedance cardiography

Evaluation of the hemodynamic response in heart failure is a useful adjunct in clinical management. Invasive monitoring has been the accepted gold standard of hemodynamic assessment but carries with it significant associated risks. Noninvasive hemodynamic assessment in heart failure previously has been either unreliable or difficult to obtain. Bioimpedance relies on the proportional change in the conduction of alternating current applied across the thorax as a function of blood volume in the heart and great vessels. Stroke volume, cardiac output, thoracic fluid content, and measures of diastolic function can be determined with bioimpedance. Impedance cardiography is becoming an accepted method for safe, reliable, and reproducible assessment of hemodynamics in heart failure.

Distinct hemodynamic profiles in patients with vasovagal syncope: a heterogeneous population

OBJECTIVE

The objective was to investigate mechanisms of vasovagal syncope by identifying laboratory techniques that characterize cardiovascular profiles in patients with vasovagal syncope.

BACKGROUND

The triggering mechanisms of vasovagal syncope are complex. The patient population is likely heterogeneous. We hypothesized that distinct hemodynamic profiles are definable with provocative maneuvers.

METHODS

Three groups of subjects were matched for age and gender: 16 patients with a history of syncope and an inducible vasovagal response during passive tilt table testing (70 degrees, 45 min, group I), 16 with a history of syncope, negative passive tilt table testing but positive isoproterenol tilt table testing (0.05 microg/kg per min, 70 degrees, 10 min, group II), and 16 control subjects. Beat-to-beat hemodynamic functions were determined noninvasively by photo-plethysmography and impedance cardiography.

RESULTS

At baseline, hemodynamic functions were not different among the three groups (supine). In response to tilt before any symptoms developed, total peripheral resistance decreased 9% +/- 14% in group I from baseline supine to tilt position but increased 27% +/- 18% in group II and 28% +/- 17% in controls (p < 0.001). Responses to isoproterenol were not significantly different between group II and controls in supine position. In response to tilt during isoproterenol infusion before any symptoms developed, total peripheral resistance decreased 24% +/- 20% in group II and increased 20% +/- 48% in controls (p = 0.002).

CONCLUSIONS

Group I patients may have impaired ability to increase vascular resistance during orthostatic stress. The inability to overcome isoproterenol-induced vasodilatation during tilt is important in triggering a vasovagal response in group II patients. These data suggest that the population with vasovagal response is heterogeneous. Distinct hemodynamic profiles in response to various provocative maneuvers are definable with noninvasive, continuous monitoring techniques.

Evaluation of a new transcardiac conductance method for continuous on-line measurement of left ventricular volume

OBJECTIVE

To evaluate a new, less invasive, conductance method to measure continuous on-line left ventricular volume. End-systolic and end-diastolic volumes obtained with this transcardiac conductance method were compared with simultaneous measurements using the conventional intracardiac conductance catheter.

DESIGN

Controlled animal study.

SETTING

Research laboratory in a university hospital.

SUBJECTS

Six sheep.

INTERVENTIONS

Anesthetized sheep were instrumented and inotropic condition was varied by beta-receptor stimulation (5 microg/kg/min of dobutamine) and beta-receptor blockade (1 mg/kg of propranolol). In each condition (control, dobutamine, repeat control, propranolol), ventricular volume was varied over a wide range by gradual preload reduction using a vena caval balloon catheter.

MEASUREMENTS AND MAIN RESULTS

We compared the two methods by performing linear regression analysis on simultaneous end-systolic and end-diastolic volumes obtained during gradual caval occlusions. We statistically analyzed the intercepts, slopes, and correlation coefficients of the regression equations relating the transcardiac and conductance catheter measurements to determine the effects of interanimal variability, inotropic condition, and cardiac phase on the relationship between the two methods. The results show an excellent linear correlation between the two methods (mean intercept, -1.82+/-1.24 mL; mean slope, 0.787+/-0.024 and r2 = .94). Both slope and intercept of the relationship between the two methods show a significant interanimal and cardiac phase related variability but no significant dependence on inotropic condition.

CONCLUSIONS

The significant interanimal variability indicates that the new method requires individual calibration in each subject. However, the small variability of the regression coefficients with changes in condition indicates that after initial calibration, end-systolic and end-diastolic volume can be followed accurately even in the presence of large changes in volume and inotropic state. This new method may facilitate quantitative continuous assessment of cardiac function in clinical practice, for example, in the intensive care unit.

Application of computer modelling and lead field theory in developing multiple aimed impedance cardiography measurements

Conventional impedance cardiography (ICG) methods estimate parameters related to the function of the heart from a single waveform that reflects an integrated combination of complex sources. We have previously developed methods and tools for calculating measurement sensitivity distributions of ICG electrode configurations. In this study, the methods were applied to investigate the prospects of recording multiple aimed ICG waveforms utilizing the 12-lead electrocardiography (ECG) electrode locations. Three anatomically realistic volume conductor models were used: one based on Visible Human Man cryosection data and two on magnetic resonance (MR) images representing end diastolic and end systolic phases of the cardiac cycle. Based on the sensitivity distributions obtained, 236 electrode configurations were selected for preliminary clinical examination on 12 healthy volunteers and 9 valvular patients. The model study suggested that a variety of configurations had clearly enhanced sensitivity to the cardiovascular structures as compared to conventional ICGs. Simulation data and clinical experiments showed logical correspondence supporting the theoretically predicted differences between the configurations. Recorded 12-lead ICG signals had characteristic waveforms and landmarks not coinciding with those of conventional ICG. Furthermore, configurations showing resemblance to invasive data and morphological variations in disease are of interest. The results indicate the applicability of the modelling approach in developing ICG measurement configurations. However, the level of clinical relevance and potential of the 12-lead method remains to be explored in studies employing dynamic modelling and acquisition of invasive reference data.

A meta-analysis of three decades of validating thoracic impedance cardiography

OBJECTIVE

To provide a meta-analysis of current literature concerning the validation of thoracic impedance cardiography (TIC) and to explain the variations in the reported results from the differences in the studies.

DATA SOURCES

A computer-assisted search of English-language, German, and Dutch literature was performed for the period January 1966 to April 1997. Moreover, references from review articles were obtained.

STUDY SELECTION

A total of 154 studies comparing measurements of cardiac output or related variables obtained from TIC and a reference method were analyzed.

DATA EXTRACTION

Articles were classified by differences in TIC methodology, reference method, and subject characteristics. Fisher's Zf transformed correlation coefficients were used to compare results. Data were pooled using the random-effects method.

DATA SYNTHESIS

An overall pooled r2 value of .67 (95% confidence interval, 0.64-0.71) was found. However, the correlation was higher in repeated-measurement designs than in single-measurement designs (r2 = .53; 95% confidence interval, 0.43-0.62). Further research using analysis of variance revealed a significant influence of the reference method and the subject characteristics on the correlation coefficient. The correlation was significantly better in animals than in cardiac patients. Subgroup analysis revealed that TIC correlated significantly better to the indirect Fick method than to echocardiography in healthy subjects. No significant influence of the applied TIC methodology was found.

DISCUSSION

The overall r2 value of .67 indicates that TIC might be useful for trend analysis of different groups of patients. However, for diagnostic interpretation, a r2 value of .53 might not meet the required accuracy of the study. Great care should be taken when TIC is applied to the cardiac patient. However, because the applied reference method was of significant influence, differences between TIC and the reference method are incorrectly attributed to errors in TIC alone.

Impedance stroke volume compared with dye and electromagnetic flowmeter values during drug-induced inotropic and vascular changes in dogs

Stroke volumes measured by impedance were compared with values obtained by dye dilution and an electromagnetic flowmeter (EMF) on 14 dogs during drug-induced changes in cardiac contraction strength and peripheral resistance changes. Grouping all data for a total of 305 points showed correlations between dye and EMF, dye and impedance, and EMF and impedance of 0.89, 0.68, and 0.72, respectively. Correlations for individual dogs between dye and EMF, dye and impedance, and EMF and impedance ranged from 0.60 to 0.99, -0.39 to 0.96, and -0.26 to 0.89, respectively. These data suggest that the use of impedance cardiac output measurements to make treatment decisions about individual patients could result in serious error.

A meta-analysis of published studies concerning the validity of thoracic impedance cardiography

Our aim was to provide a meta-analysis of the literature concerning the validation of thoracic impedance cardiography (TIC) and to explain variations in reported results from differences in the studies. One hundred fifty-four studies (164 Fisher's Z-transformed correlation coefficients) comparing measurements of cardiac output or related parameters from TIC and a reference method were analyzed. Papers were classified according to differences in TIC methodology, reference method, and subject characteristics. Pooling using the random-effects method yielded an overall correlation of r = 0.82 (95% confidence interval: 0.80-0.84). ANOVA revealed a significant influence of the reference method and the subject characteristics on the correlation coefficient. In cardiac patients, the correlation was significantly decreased. No influence of the applied TIC methodology was found.

CONCLUSION

TIC might be useful for trend analysis of different groups of patients. However, since the reference method was of significant influence, differences between TIC and the reference method are incorrectly attributed to TIC alone.

Evaluation of systolic performance by automated impedance cardiography

Impedance cardiography (ICG) is a noninvasive method for evaluating cardiac function. Left ventricular stroke volume (SV) is the basic hemodynamic parameter derived from thoracic bioimpedance curves. Issues of our study were to investigate the diagnostic value of other indices of left ventricular systolic performance, such as ejection fraction (EF), index of contractility (IC), peak flow index (PFI), and acceleration index (ACI), which can also be calculated by ICG. Forty patients (PTS) with suspected coronary artery disease (CAD) were monitored by automated ICG during pharmacologic stress testing with dobutamine. All PTS underwent subsequent cardiac catheterization. In PTS with single vessel disease, the dobutamine-induced changes of SV, EF, IC, PFI, and ACI were comparable to those of PTS without CAD. In PTS with multivessel disease, the impaired systolic performance during dobutamine stimulation could be clearly demonstrated. We conclude that automated ICG is a useful method for monitoring SV and other indices of left ventricular systolic performance for detecting PTS with ischemic left ventricular dysfunction during cardiovascular stress.

Sensitivity to orthostatic stress and beta-receptor activation in patients with isoproterenol-induced vasovagal syncope: a case controlled study

Cardiomotor and vasomotor responses were assessed during isoproterenol tilt-induced vasovagal reaction in patients with a history of syncope. In a case controlled study, all patients and controls were subjected to a standard protocol: baseline supine (10 min), baseline tilt (70 degrees, 45 min), isoproterenol supine (0.05 microgram/kg per min, 10 min), and isoproterenol tilt (70 degrees, 10 min). The participants were 11 consecutive patients referred for syncope evaluation (5 men, 6 women; mean age, 34.1 +/- 10.4 years; range, 18-56 years) and 11 age and sex matched controls (5 men, 6 women; mean age, 35.5 +/- 12.2 years; range, 19-63 years). On-line, beat-to-beat measurements of cardiomotor functions (heart rate, stroke volume, and cardiac output) and vasomotor functions (systolic, mean, and diastolic blood pressures and total peripheral resistance [TPR]) were detected noninvasively by volume clamp photoplethysmography and impedance cardiography. Patients and controls had similar cardiomotor and vasomotor responses during passive tilt and during isoproterenol infusion in the supine position. Immediately after tilt during isoproterenol infusion and before the onset of symptoms, decreases in vasomotor functions were significant in study patients when compared with those in controls; whereas responses in cardiomotor functions were similar between the two groups. When compared with baseline supine findings, TPR decreased by 56.5% +/- 10.9% and 29.5% +/- 23.3% in the patient population and controls, respectively (P = 0.005). When compared with isoproterenol supine findings, TPR decreased by 27.5% +/- 22.8% in the study patients and increased by 22.6% +/- 48.1% in the controls (P = 0.005). The inability to overcome isoproterenol-induced vasodilation during orthostatic stress played an important role in the initiation of a vasovagal response. These observations hold the key to early detection of hemodynamic changes and potential therapeutic interventions before patients become symptomatic.

The influence of extravascular lung water on cardiac output measurements using thoracic impedance cardiography

The purpose of this study was to investigate the influence of pulmonary oedema as measured with the double indicator dilution technique on the accuracy of cardiac output (CO) measurement using thoracic impedance cardiography (TIC) compared with thermodilution in thirteen sepsis patients. Differences in the Kubicek and Sramek-Bernstein equation with respect to pulmonary oedema were explored theoretically and experimentally. From a parallel two cylinder model a hypothesis can be derived that CO determined with the Kubicek equation is oedema independent, whereas CO determined using the Sramek-Bernstein equation is oedema dependent. Experimentally, CO determined using Kubicek's equation correlated better with thermodilution CO (r = 0.75) than CO determined with the Sramek-Bernstein equation (r = 0.25). The effect of oedema on the accuracy of TIC was investigated by comparing the differences in the CO of impedance and thermodilution to the extravascular lung water index. For the Kubicek equation the difference was not influenced by oedema (r = 0.04, p = 0.84), whereas for the Sramek-Bernstein equation the difference was affected by oedema (r = 0.39, p = 0.05). Thus, the effects of pulmonary oedema on the accuracy of TIC measurements can better be understood with the parallel cylinder model. Moreover, the Kubicek equation still holds when pulmonary oedema is present, in contrast to the Sramek-Bernstein equation.

Thoracic geometry and its relation to electrical current distribution: consequences for electrode placement in electrical impedance cardiography

In thoracic impedance cardiography (TIC) measurements the neck electrodes are often positioned at the basis of the neck, close to the neck-thorax transition. Theoretically, this neck-thorax transition will cause inhomogeneities in the current density and potential distribution. This was simulated using a 3D finite element method, solely representing the geometrical neck-thorax transition. The specific conductivity was 7 10(-3) (omega cm)-1 and the injected current was 1 mA. As expected, the model generated inhomogeneities in the current distribution at the neck-thorax transition, which reached as far as 5 cm into the neck and 20 cm into the thorax. These results are supported by in vivo measurements performed in 10 young male subjects, in which the position of the neck electrodes was varied. A two-way ANOVA revealed that the stroke volume of the lowest neck position was significantly different from the other positions. Small shifts in the position of the neck electrode resulted in large changes in impedance and stroke volume (127 to 82 ml for the Kubicek equation). To standardise the electrode position, the authors strongly recommend placement of the neck electrodes at least 6 cm above the clavicula.

Comparison of impedance cardiography and dye dilution method for measuring cardiac output

OBJECTIVE

To assess the degree of agreement between impedance cardiography, using the NCCOM3-R7 device, and the gold standard--the dye dilution method--both under basal conditions and after stimulation of cardiac output.

PATIENTS

35 paired measurements in five healthy male volunteers.

INTERVENTIONS

To obtain higher levels of cardiac output, cardiac performance was stimulated with a dopamine infusion.

RESULTS

In 35 paired measurements, the mean of all the impedance values was higher than that of the dye dilution values, at 10.2 v 7.4 l/min (p < 0.0001). The mean discrepancy between the two methods was 3.3 l/min, and the mean bias -2.9 l/min, with limits of agreement of -9.0 and 3.2 l/min. A change in cardiac output could not adequately be predicted by the NCCOM3-R7. In 20 of 25 measurements obtained during continuous intravenous dopamine infusions there was a rise in dye dilution cardiac output (range 0.2 to 5.9 l/min). Neither the magnitude nor the direction of the change in dye dilution values corresponded with the change measured by impedance cardiography. The mean discrepancy here between the two methods was 1.8 l/min, and the mean bias -0.8 l/min, with limits of agreement of -4.9 and 3.3 l/min.

CONCLUSIONS

In healthy volunteers, impedance cardiography with NCCOM3-R7 is inadequate for assessing cardiac output when compared with the dye dilution method.

Hemodynamic analysis during isoproterenol-induced vasovagal syncope

An isoproterenol-mediated increase in cardiomotor tone and a decrease in afterload contribute to the induction of vasovagal syncope. Contrary to conventional belief, a significant decrease in preload is not observed immediately before isoproterenol-induced syncope.

Whole-body impedance cardiography in the measurement of cardiac output

OBJECTIVE

To evaluate the reliability of whole-body impedance cardiography with electrodes on wrists and ankles in the measurement of cardiac output compared with the thermodilution method.

DESIGN

Prospective, clinical investigation.

SETTING

Surgical intensive care unit and operating room at a university hospital.

PATIENTS

Simultaneous cardiac output measurements by thermodilution and whole-body impedance cardiography were performed in 74 patients undergoing a coronary artery bypass grafting operation.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

A total of 97 triplicate, simultaneous cardiac output measurements were carried out with thermodilution and whole-body impedance cardiography: 74 measurements were conducted in patients who were awake and 23 measurements were conducted during anesthesia but before the commencement of surgery. The mean cardiac output difference (bias) between the two methods was 0.25 +/- 0.81 (SD) L/min; the limits of agreement (2 SD) were-1.37 and 1.87 L/min, respectively. The repeatability value (rv = 2.83 x SD) for whole-body impedance cardiography (rv = 0.46 L/min) was considerably better than for the thermodilution method (rv = 1.05 L/min). Whole-body impedance cardiography reliably detected cardiac output changes induced by head-up tilt before anesthesia, by anesthesia induction, and by intubation. Two factors predicted the between-methods stroke volume difference: hematocrit (correlation coefficient r = -.36, r2 = .13; p < .001); and body mass index (r = .29, r2 = .08; p < .01). Using the multiple linear regression equation for correcting the stroke volume by hematocrit and body mass index, the limits of agreement (2 SD) between the methods studied were reduced to +/-1.28 L/min for cardiac output and +/-0.72 L/min/m2 for cardiac index.

CONCLUSIONS

There was close agreement between whole-body impedance cardiography and thermodilution in the measurement of cardiac output in patients with coronary artery disease without cardiac shunts and valvular lesions. The repeatability of the impedance method was significantly better than the repeatability of thermodilution. Whole-body impedance cardiography can be recommended for the assessment of cardiac output and its changes in the resting state. Whole-body impedance cardiography is a feasible and handy method for noninvasive and continuous measurement of cardiac output.