Transthoracic electrical bioimpedance versus thermodilution technique for cardiac output measurement during mechanical ventilation

To study the possible influence of mechanical ventilation on the accurracy of thoracic electrical bioimpedance (TEI) in the measurement of cardiac output, we determined cardiac output concurrently by TEI using Kubicek's equation and by thermodilution in 8 acutely ill patients who were mechanically ventilated (assist/control mode) but who had no underlying respiratory failure. Cardiac outputs were lower with TEI than with thermodilution (3.97±0.80 vs 4.83±1.16 l/min p=0.004) and there was poor correlation between the values (r=0.41). Although there is a need to develop non-invasive techniques to measure cardiac output, the present study indicates that TEI is not reliable in mechanically ventilated patients.     

Noninvasive whole-body electrical bioimpedance cardiac output and invasive thermodilution cardiac output in high-risk surgical patients

OBJECTIVE: To evaluate the reliability of whole-body impedance cardiography with two electrodes on either both wrists or one wrist and one ankle for the measurement of cardiac output compared with the thermodilution method. DESIGN: Prospective, clinical investigation SETTING: Surgical intensive care unit of a university-affiliated community hospital. PATIENTS: Simultaneous cardiac output measurements by noninvasive whole-body impedance cardiography (nCO) and invasive thermodilution (thCO) in 22 high-risk surgical patients scheduled for extended surgery requiring perioperative pulmonary artery catheter monitoring. INTERVENTIONS: None. MEASUREMENTS AND MAIN RESULTS: A total of 109 sets of measurements consisting of 455 single comparison measurements between nCO and thCO were included in the analysis. The mean cardiac output difference between the two methods was 1.62 L/min with limits of agreement (2 SD) of +/- 4.64 L/min. The inter-measurement variance was slightly higher for nCO. The correlation coefficient between nCO and thCO was r2 = 0.061 (p < .001) for single measurements and r2 = 0.083 (p < .002) for sets of three to six measurements. The two most predictive factors for between-method differences were the absolute thCO value (r2 = 0.13; p < .001) and whether or not a continuous nitroglycerin infusion was used (p < .05, Student's t-test). CONCLUSIONS: Agreement between whole-body impedance cardiography and thermodilution in the measurement of cardiac output was unsatisfactory. Factors that can explain these differences are differences between the populations used for calibration of nCO and the study population, the influence of changing peripheral perfusion, and the effect of a supranormal hemodynamic state on the bioimpedance signal. Whole-body impedance cardiography cannot be recommended for assessing the hemodynamic state of high-risk surgical patients as studied in this investigation.     

Determination of cardiac output during positive end-expiratory pressure--noninvasive electrical bioimpedance compared with standard thermodilution

Many investigators have demonstrated the accuracy and reliability of thoracic electrical bioimpedance (TEB) in spontaneously breathing patients and under mechanical intermittent positive-pressure ventilation. Most of these studies showed a good correlation between TEB and invasive methods, such as thermodilution (TD) or the Fick method. But during PEEP, contrary results occur when comparing TEB and TD. In six patients undergoing neurosurgical interventions, TEB cardiac output measurements were compared during zero end-expiratory pressure (ZEEP) and during PEEP at 8 cm H2O with a low respiratory rate. The data revealed a good correlation during ZEEP (r = .93) and during PEEP (r = .91). There was no significant statistical difference when measuring cardiac output by TEB during ventilation with PEEP. During normal or decreased cardiac output, TEB overestimated cardiac output compared with TD, whereas TEB underestimated cardiac output compared with TD during increased cardiac output, especially during PEEP.     

Evaluation of a thoracic bioimpedance cardiac output monitor during cardiac catheterization

OBJECTIVE: To evaluate the accuracy and precision of an advanced thoracic bioimpedance cardiac output monitor by comparing it with conventional thermodilution. DESIGN: Prospective data collected from 47 patients undergoing routine cardiac catheterization. The new bioimpedance system differs from its predecessors in electrode system configuration, advanced signal processing, use of a modified Kubicek equation, and a reliable estimate of left ventricular ejection time from the time derivative bioimpedance signals. SETTING: A cardiac catheterization laboratory in a university affiliated teaching hospital. PATIENTS: A series of 47 relatively homogenous patients undergoing routine cardiac catheterization for suspected cardiac disease. MEASUREMENTS AND MAIN RESULTS: The data from the first 20 patients was used to determine optimal values for coefficients in the bioimpedance cardiac output equations. The coefficients found were used when the system was tested in the subsequent 27 patients. For the last 27 patients, a total of 80 simultaneous pairs of cardiac output measurements were made by conventional thermodilution and by thoracic bioimpedance. The mean difference between the two methods was -0.31 L/min and the standard deviation of the differences was (0.76 L/min). The correlation coefficient was r2 = .72 (p < .001). CONCLUSIONS: The correlation between conventional thermodilution and thoracic bioimpedance cardiac output estimates was good and the standard deviation of the differences was lower than that reported for commercially available devices. The system can be used in the cardiac catheterization lab for reliable and continuous noninvasive measurement of cardiac output.     

Systolic and diastolic changes in human coronary blood flow during Valsalva manoeuvre

Valsalva manoeuvre is reported to be sometimes successful for the relief of angina pectoris. The present study investigated how haemodynamic changes produced by Valsalva manoeuvre can interact to improve the relationship between cardiac work and coronary blood flow. Ten male subjects aged 53 ± 12 years (SD) were considered. Blood velocity in the internal mammary artery, previously anastomosed to the left descending coronary artery, was studied with Doppler technique. The subjects performed Valsalva manoeuvres by expiring into a tube connected to a mercury manometer, to develop a pressure of 40 mmHg. The arterial blood pressure curve was continuously monitored with a Finapres device from a finger of the left hand. During expiratory effort, an increase in heart rate and a decrease in arterial pulse pressure were followed by a more delayed and progressive increase in mean and diastolic pressures. Systolic blood velocity markedly decreased along with the reduction in pulse pressure and increase in heart rate. By contrast, diastolic and mean coronary blood velocities did not show any significant change. Since it is known that the Valsalva manoeuvre strongly reduces stroke volume and cardiac output, it is likely that a reduction in cardiac work also takes place. Since in diastole, i.e. when the myocardial wall is better perfused, coronary blood velocity did not show any significant reduction, it is likely that unchanged perfusion in the presence of reduced cardiac work is responsible for the relief from angina sometimes observed during Valsalva manoeuvre. It is also likely that the increase in heart rate prevents the diastolic and mean blood coronary velocity from decreasing during the expiratory strain, when an increased sympathetic discharge could cause vasoconstriction through the stimulation of the coronary α‐receptors.     

Cerebrovascular and cardiovascular responses to the Valsalva manoeuvre during hyperthermia

Background

During hyperthermia, the perturbations in mean arterial blood pressure (MAP) produced by the Valsalva manoeuvre (VM) are more severe. However, whether these more severe VM-induced changes in MAP are translated to the cerebral circulation during hyperthermia is unclear.

Methods

Healthy participants (n = 12, 1 female, mean ± SD: age 24 ± 3 years) completed a 30 mmHg (mouth pressure) VM for 15 s whilst supine during normothermia and mild hyperthermia. Hyperthermia was induced passively using a liquid conditioning garment with core temperature measured via ingested temperature sensor. Middle cerebral artery blood velocity (MCAv) and MAP were recorded continuously during and post-VM. Tieck's autoregulatory index was calculated from the VM responses, with pulsatility index, an index of pulse velocity (pulse time) and mean MCAv (MCAv_mean) also calculated.

Results

Passive heating significantly raised core temperature from baseline (37.9 ± 0.2 vs. 37.1 ± 0.1°C at rest, p < 0.01). MAP during phases I through III of the VM was lower during hyperthermia (interaction effect p < 0.01). Although an interaction effect was observed for MCAv_mean (p = 0.02), post-hoc differences indicated only phase IIa was lower during hyperthermia (55 ± 12 vs. 49.3 ± 8 cm s⁻¹ for normothermia and hyperthermia, respectively, p = 0.03). Pulsatility index was increased 1-min post-VM in both conditions (0.71 ± 0.11 vs. 0.76 ± 0.11 for pre- and post-VM during normothermia, respectively, p = 0.02, and 0.86 ± 0.11 vs. 0.99 ± 0.09 for hyperthermia p < 0.01), although for pulse time only main effects of time (p < 0.01), and condition (p < 0.01) were apparent.

Conclusion

These data indicate that the cerebrovascular response to the VM is largely unchanged by mild hyperthermia.     

Transthoracic electrical bioimpedence cardiac output: comparison with multigated equillibrium radionuclide cardiography

Introduction  

Thoracic electrical bioimpedance (TEB) for measuring cardiac output (CO) is being explored increasingly as an alternative to pulmonary artery catheter. The major advantage of this technology is that it is non-invasive and easy to perform. Several studies have compared it to thermodilution cardiac output using PA catheter, with variable correlation. Multigated radionuclide equilibrium cardiography (RNEC) method of cardiac output measurement is known to be reliable.     

Continuous noninvasive real-time monitoring of stroke volume and cardiac output by thoracic electrical bioimpedance

Thoracic electrical bioimpedance (TEB) is a noninvasive method for the estimation of left ventricular stroke volume (SV). Objections to TEB in the past have included systematic overestimation of SV and cardiac output in normals, poor correlation in absolute terms with standard cardiac output reference techniques, motion and ventilation artifacts which distorted recorded waveforms except when patients remained in apnea during measurements, and lack of a computer to yield rapid heat-to-beat quantitation of SV and cardiac output. The present study entails real-time, beat-to-beat cardiac output determinations using TEB and assessing its agreement via a digital computer against a reference standard, thermodilution. A correlation coefficient of 0.88 is reported with 85% of data points falling within 20% confidence limits on either side of the line of identity.     

Cardiac output determinations in the pig--thoracic electrical bioimpedance versus thermodilution

The accuracy of transthoracic electrical bioimpedance (TEB) for continuous, noninvasive measurement of cardiac output (Qt) in pigs was assessed in comparison with the thermodilution (TD) technique. Using the TEB technique, the different thoracic habitus of the pig had to be corrected for A good correlation with the TD technique was obtained (r = .87; p less than .001; n = 86) using thoracic length value (the measured value plus 25%) in an NCCOM3-R6 cardiodynamic computer for Qt values ranging from 2.9 to 9.8 L/min in pigs weighing from 40 to 75 kg. However, the Qt values given by the NCCOM3 were systematically 11% to 15% higher over the full range of values than the average of NCCOM-3 and TD Qt values. On the basis of the good agreement in the present study between the TEB and TD techniques over a broad range of Qt values, we conclude that TEB offers a valuable continuous, noninvasive alternative to TD for Qt determinations in experimental porcine models.     

Continuous noninvasive estimation of cardiac output by electrical bioimpedance: an experimental study in dogs

A new device has been developed to estimate continuously and noninvasively cardiac output from the thoracic electrical bioimpedance (CObi). CObi was compared to cardiac output by thermodilution (COtd) in five anesthetized dogs. Blood pressure, blood volume, and blood flow were manipulated by hemorrhage and infusions of sodium nitroprusside and phenylephrine. These data were used to determine the correlation between CObi and COtd under conditions of hypotensive normal flow and normotensive low flow, as well as during hemorrhagic shock and resuscitation. The CObi device was calibrated in vivo to COtd for each dog at the beginning of each experiment. CObi had a significant positive correlation with COtd throughout the experiments (r = 0.84, slope = 0.91, intercept = 0.55, p less than 0.01), and CObi predicted COtd with a standard error of the estimate of 0.81 L/min. Neither heart rate nor mean arterial pressure was significantly correlated with COtd or CObi. During severe hemorrhagic shock, CObi could not determine cardiac output in two of the dogs when COtd averaged 1.7 L/min. These data indicate that CObi is a blood-flow related variable that can be monitored continuously.     

Use of transthoracic bioimpedance to determine cardiac output in pediatric patients

The use of a transthoracic bioimpedance monitor to determine cardiac output was evaluated in critically ill children. The children ranged in age from 10 months to 8 yr and their height and weight ranged from the third to the 97th percentile. Each child had a thermodilution catheter in place to monitor cardiac output. The bioimpedance monitor used in this study, the NCCOM-3, required the input of a constant (L), which was obtained for each individual patient by adjusting the L setting until cardiac output measured by bioimpedance (COBI) was within 10% of cardiac output measured by thermodilution (COTD). This method of determining L was superior to using either measured thoracic length or the manufacturer's guidelines to obtain L and resulted in an excellent correlation between COTD and COBI (r = .94; p less than .05; n = 59). In children less than 125 cm in height, measured thoracic length alone was inadequate to use for L but provided a good approximation of L when multiplied by 1.25. This study suggests that the use of transthoracic bioimpedance to determine cardiac output compares favorably with thermodilution techniques and it is noninvasive.     

Autonomic responses to the Valsalva manoeuvre in healthy subjects

Summary. To assess normal autonomic haemodynamic responses to the Valsalva manoeuvre, 158 healthy unmedicated subjects, aged 25–60 years, were examined. For measurement of beat-to-beat blood pressure on a finger, the Finapres instrument was used. Phase-to-phase changes in instantaneous blood pressure and heart rate and the latency response between the end of a Valsalva manoeuvre and points on the resultant blood pressure and heart rate were calculated, and the reference limits for various indices were determined. Sex had no or only marginal effect on blood pressure or heart rate responses or latencies. Ageing was accompanied by a smaller decrease and smaller partial recovery of blood pressure during the strain, with attenuation of reflectory bradycardia, and lengthening of the latencies. It is concluded that age-related reference values should be applied in the interpretation of the Valsalva responses. The following responses should be analysed: mean blood pressure decrease and partial recovery during the strain (adrenergic vasoconstrictor function), reflectory bradycardia after the strain (parasympathetic function), and the latencies (sympathetic and parasympathetic function).     

Heart rate variability analysis using a ballistocardiogram during Valsalva manoeuvre and post exercise

We introduced a novel non-constrained technique for estimating heart rate variability (HRV) using a ballistocardiogram (BCG). To assess whether the BCG signal can be used to analyse the cardiac autonomic modulation, HRV parameters derived from the BCG signal (ballistocardiographic HRV, B-HRV) were statistically compared with the HRV parameters from the ECG signal during rest and under two different experimental conditions that induce cardiac autonomic rhythm changes: the Valsalva manoeuvre and static exercise. Time domain, frequency domain and nonlinear analyses were individually performed on 15 healthy subjects to assess whether the BCG can be used to analyse the cardiac autonomic modulation under each condition. For all subjects, the proposed method had averages of relative errors of 5.01 ± 4.72, 5.64 ± 4.83 and 5.98 ± 5.80% for resting, Valsalva and post-exercise sessions, respectively, and the correlation coefficients between the reference (ECG) and proposed (BCG) methods are 0.97, 0.98 and 0.98, for resting, Valsalva and post-exercise sessions, respectively. During cardiac autonomic changes, the B-HRV parameters changed in a pattern that is very similar to the variations in the HRV parameters based on Student's t-test results. In addition, some of the B-HRV parameters changed according to cardiac autonomic rhythms controlled by sympathetic and parasympathetic activities during the experiments. These findings indicate that BCG can provide an accurate and reliable means to evaluate autonomic system activation by HRV in its unconstrained way.     

Influence of posture on the Valsalva manoeuvre

The objective of the present study was to evaluate the influence of posture on the responses of blood pressure (BP) and heart rate (HR) to the Valsalva manoeuvre (VM). Neurohumoral activation, as well as changes in intravascular and intracardiac volumes and pressures, are well known effects of orthostatic stress. These changes are likely to have significant effects on cardiovascular reflexes, such as the response to the VM. However, the influence of posture on the VM has not been intensively evaluated, except for a few studies involving small sex- and age-selected case series. We therefore investigated the effects of posture on the VM in a larger non-selected group of healthy control subjects. In 19 healthy volunteers (ten female/nine male; age range 20-72 years, mean age 43 years), two reproducible VMs (40 mmHg; 15 s) were performed after 10 min of supine rest, 10 min of sitting and 10 min of standing. HR and BP were monitored continuously. End-diastolic volume, total peripheral resistance and cardiac output were calculated at baseline for each position. We found that assuming an upright position resulted in increases in total peripheral resistance and HR, accompanied by decreases in end-diastolic volume and cardiac output. The fall in BP during early phase II and the BP overshoot during phase IV were clearly more pronounced with increasing orthostatic stress, whereas the rise in BP during late phase II remained unchanged; pulse pressure was more compressed during phase II, but higher during phase IV. The Valsalva ratio was not significantly affected, but baroreflex gain (calculated from early phase II) was significantly decreased in the upright position. While a reduced late phase II was observed on one occasion in each of the lying and sitting positions, three abnormal responses were observed during standing. We conclude that posture has a significant influence on BP responses to the VM, probably resulting from changes in the intrathoracic blood volume. Standing results in a lower rate of 'flat-top' responses, but also seems to reduce the specificity of this test. Sympathetic activation in the upright position seems to blunt baroreflexes, leading to similar HR responses in spite of larger changes in BP.     

Noninvasive assessment of cardiac output by impedance cardiography in the newborn canine

Currently, critical care monitoring of cardiac function in the newborn human consists mainly of measuring heart rate and BP. A noninvasive technique for assessing cardiac output routinely in the critically ill neonate would facilitate clinical management. Impedance cardiography (IC) is a noninvasive technique which measures stroke volume on a beat-by-beat basis. This study compared cardiac output as measured by thermodilution (TD) to that measured by IC in seven canine pups 6 to 7 days old weighing 0.66 to 0.86 kg. Cardiac output was altered by the withdrawal and reinfusion of blood. There were no significant differences between the two methods for either the absolute value of cardiac output (r = .96) or the percent change in cardiac output (r = .97). Coefficients of variation were 3.0% for TD and 3.6% for IC. These results indicate that IC can be used to assess serially cardiac function in the newborn.     

Evaluation of changes in cardiac output from the electrical impedance waveform in the forearm

We tested the validity of regional impedance cardiography (RIC) for measuring changes in both cardiac output and stroke volume by comparing the values with a 2D ultrasound technique in response to the breath-hold manipulation. Among 13 subjects, changes in the maximum amplitude of the regional impedance waveform from the forearm conformed to those in stroke volume (r = 0.86, p < 0.001) and cardiac output (r = 0.76, p < 0.003) measured with the ultrasound technique in baseline and immediately after a 30 s breath-hold maneuver. We also found that the per cent change in cardiac output (r = 0.73, p < 0.005) and the per cent change in stroke volume (r = 0.84, p < 0.0003) by the echocardiography were both positively correlated with the per cent change in the peak impedance amplitude. In addition, both the change and the per cent change in the mean area under the impedance curve were consistent with those in the stroke volume, respectively. Accordingly, the regional electrical impedance waveform from lower limbs may be helpful in providing a non-invasive and continuous assessment of left ventricular output, especially during cardiac procedures.     

Bioimpedance cardiography measurements of cardiac output and other cardiovascular parameters

This paper describes impedance technique and device types, discusses hemodynamic data parameters that are available, discusses the differences between impedance cardiography and data that are derived from invasive pulmonary artery catheters, and explains how nurses can apply bioimpedance cardiography in a variety of patient populations.     

Comparison of cardiac output determined by bioimpedance and bioreactance methods at rest and during exercise

Bioreactance is a novel non-invasive method for cardiac output measurement that involves the analysis of blood flow-dependent changes in the phase shifts of electrical currents applied across the chest. The present study (1) compared resting and exercise cardiac outputs determined by bioreactance and bioimpedance methods and those estimated from measured oxygen consumption, (2) determined the relationship between cardiac output and oxygen consumption, and (3) assessed the agreement between the bioreactance and bioimpedance methods. Twelve healthy subjects (aged 30 ± 4 years) performed graded cardiopulmonary exercise test on a recumbent cycle ergometer on two occasions, 1 week apart. Cardiac output was monitored at rest, at 30, 50, 70, 90, 150 W and at peak exercise intensity by bioreactance and bioimpedance and expired gases collected. Resting cardiac output was not significantly different between the bioreactance and bioimpedance methods (6.2 ± 1.4 vs. 6.5 ± 1.4 l min⁻¹, P = 0.42). During exercise cardiac outputs were correlated with oxygen uptake for both bioreactance (r = 0.84, P < 0.01) and bioimpedance techniques (r = 0.82, P < 0.01). At peak exercise bioimpedance estimated significantly lower cardiac outputs than both bioreactance and theoretically calculated cardiac output (14.3 ± 2.6 vs. 17.5 ± 5.2 vs. 16.9 ± 4.9 l min⁻¹, P < 0.05). Bland–Altman analyses including data from rest and exercise demonstrated that the bioimpedance method reported ~1.5 l min⁻¹ lower cardiac output than bioreactance with lower and upper limits of agreement of −2.98 to 5.98 l min⁻¹. Bioimpedance and bioreactance methods provide different cardiac output estimates, particularly at high exercise intensity, and therefore the two methods cannot be used interchangeably. In contrast with bioimpedance, bioreactance cardiac outputs are similar to those estimated from measured oxygen consumption.