Non-invasive cardiac output evaluation during a maximal progressive exercise test, using a new impedance cardiograph device

One of the greatest challenges in exercise physiology is to develop a valid, reliable, non-invasive and affordable measurement of cardiac output (CO). The purpose of this study was to evaluate the reproducibility and accuracy of a new impedance cardiograph device, the Physio Flow, during a 1-min step incremental exercise test from rest to maximal peak effort. A group of 12 subjects was evaluated to determine the reproducibility of the method as follows: (1) each subject performed two comparable tests while their CO was measured by impedance cardiography using the new device (CO_Imp1, CO_Imp2), and (2) in a subgroup of 7 subjects CO was also determined by the direct Fick method (CO_Fick) during the second test. The mean difference between the values obtained by impedance (i.e. CO_Imp1–CO_Imp2) was –0.009 l·min^–1 (95% confidence interval: –4.2 l·min^–1, 4.2 l·min^–1), and CO ranged from 3.55 l·min^–1 to 26.75 l·min^–1 (n=146). When expressed as a percentage, the difference (CO_Imp1–CO_Imp2) did not vary with increasing CO. The correlation coefficient between the values of CO_Imp and CO_Fick obtained during the second exercise test was r=0.94 (P<0.01, n=50). The mean difference expressed as percentage was –2.78% (95% confidence interval: –27.44%, 21.78%). We conclude that CO_Imp provides a clinically acceptable evaluation of CO in healthy subjects during an incremental exercise. Electronic Publication     

Continuous cardiac output monitoring system

A continuous cardiac output monitoring system has been developed in the laboratory to allow the real-time measurement of the cardiac output. This form of continous cardiac output measurement allows the doctor to view the beat-to-beat cardiac output and can be employed to measure artery constrictions as well. The sensor comprises a laser Doppler velocimeter and an impedance measurement unit. The laser Doppler velocimeter is capable of measuring bi-directional blood flow within the vessel while the impedance measurement unit determines the cross-sectional area of the vessel. In laboratory tests, it was demonstrated on a heart-lung machine that the product of the two parameters measured is proportional to the actual flow volume of up to 6l min⁻¹ with a mean percentage error of 12.4% and a mean square error of 0.09 (using the l min⁻¹ scale) were obtained. This is significantly more accurate than the measurement made using the thermodilution cathether.     

Revised one-step method for determination of cardiac output

Cardiac output (Q) is a determinant of blood pressure and O(2) delivery and is critical in the maintenance of homeostasis, particularly during environmental stress and exercise. Cardiac output can be determined invasively in patients; however, indirect methods are required for other situations. Soluble gas techniques are widely used to determine (Q). Historically, measurements during a breathhold, prolonged expiration and rebreathing to CO(2) equilibrium have been used; however, with limitations, especially during stress. Farhi and co-workers developed a single-step CO(2) rebreathing method, which was subsequently revised by his group, and has been shown to be reliable and compared closely to direct, invasive measures. V(CO2), P(ACO2), and P(VCO2) are determined during a 12-25s rebreathing, using the appropriate tidal volume, and (Q) is calculated. This method can provide accurate data in laboratory and field experiments during exercise, increased or decreased gravity, water immersion, lower body pressure, head-down tilt, altered ambient pressure or changes in inspired gas composition.     

Reliability of the cardiac output measurement with the indirect Fick-principle for CO2 during exercise

Cardiac output measurements were performed during 50 exercise tests in 16 normal subjects employing the indirect Fick principle for CO₂. During sub-maximal steady state exercise the plateau CO₂ tension ( ) was estimated with a rebreathing procedure. The mixed venous CO₂ tension ( ) was calculated by subtracting the alveolocapillary CO₂ tension difference from the . Compared with data from the literature the most valid calculation of the cardiac output was obtained by using the . Cardiac output values, calculated via the turned out to be too low.The reproducibility was tested by repitition of 18 exercise tests at least after 5 days. The relative standard error of a single observation was 4.1% for the cardiac output, which was found to be as good as that of invasive measurements.     

Postural effect on cardiac output,oxygen uptake and lactate during cycle exercise of varying intensity

Owing to changes in cardiac output, blood volume distribution and the efficacy of the muscle pump, oxygen supply may differ during upright and supine cycle exercise. In the present study we measured, in parallel, circulatory (heart rate, stroke volume, blood pressure) and metabolic parameters (oxygen uptake, lactic acid concentration [1a]) during incremental-exercise tests and at constant power levels ranging from mild to severe exercise. In supine position, cardiac output exceeded the upright values by 1.0-1.5 1 · min^–1 during rest, light ([la] < 2 mmol · 1^–1) and moderate ([la] =2–4 mmol · 1^–1) exercise. At higher exercise intensities the cardiac output in an upright subject approached and eventually slightly exceeded the supine values. For both rest-exercise transitions and large-amplitude steps (W 140 W) the cardiac output kinetics was significantly faster in upright cycling. The metabolic parameters (VO₂ and [la]) showed no simple relationship to the circulatory data. In light to moderate exercise they were unaffected by body position. Only in severe exercise, when cardiac output differences became minimal, could significant influences be observed: with supine body posture, [la] started to rise earlier and maximal power (W=23 W) and exercise duration (64 s) were significantly reduced. However, the maximal [la] value after exercise was identical in both positions. The present findings generally show advantages of upright cycling only for severe exercise. With lower workloads the less effective muscle pump in the supine position appears to be compensated for by the improved central circulatory conditions and local vasodilatation.     

Non-invasive assessment of cardiac output and stroke volume in patients during exercise

Summary A one-step CO₂ rebreathing method for the determination of cardiac output and stroke volume (SV) has been evaluated by comparison with the direct Fick technique during recumbent exercise (10–90 W) in 13 patients. In an initial analysis, the influence of different rebreathing times and of correction for haemoglobin concentration was studied. The best correlation with the direct Fick technique was obtained with the longest analysis time, i. e. 21 s, and correction for variations in haemoglobin concentration further improved the correlation. Consequently, an analysis time of 21 s and correction for haemoglobin have been used. At low cardiac outputs, the CO₂-rebreathing method overestimated the flow compared to the Fick technique. The correlation between the methods, however, was so good that a valid estimate of cardiac output could be obtained from the CO₂ rebreathing method with appropriate corrections (Cardiac output, CO₂ method=2.7+0.77. Cardiac output, Fick; r=0.91; Residual Standard deviation (SD res) =0.77 l · min⁻¹). Stroke volumes measured with the CO₂ rebreathing method did not differ significantly from those obtained with the direct Fick technique, although there was a tendency to overestimate stroke volume with the CO₂ rebreathing method (SV, CO₂ method=12+0.89 · SV, Fick; r=0.82; SD res=11 ml).     

Elimination of breathing artefacts from impedance cardiograms at rest and during exercise

A signal processing technique was developed by which breathing artefacts can be eliminated from impedance cardiograms. The breathing component of the transthoracic impedance signal is identified by a moving-window technique using linear regression analysis, the window span being determined by the current R-R interval of the ECG. Satisfactory beat-by-beat stroke volume measurements were obtained when the method was applied to eliminate simulated breathing artefacts superimposed on distortion-free impedance signals from human subjects. In subjects performing moderate to heavy exercise the potential capability of the method to retrieve the cardiogenic impedance signal in the presence of severe interference caused by exercise hyperpnoea was demonstrated, permitting distortion-free beat-by-beat stroke volume estimations.     

Measurement of exercise cardiac output by thoracic impedance in healthy children

The purpose of this study was to track changes in stroke volume during exercise by impedance cardiography in order to validate the method, and to obtain such data in a large number of healthy children for reference purposes. One hundred and fifteen healthy children (aged 7–19 years) performed progressive exercise to voluntary exhaustion with work increments every minute on a cycle ergometer. Oxygen uptake (O₂) was measured on a breath-by-breath system. Cardiac output was measured with an ICG-M501 impedance cardiograph. Stroke volume was normalized for body surface area and expressed as stroke volume index. Cardiac output was regressed against O₂, and differences between stroke volume index at rest and exercise were assessed by repeated measures analysis of variance. Cardiac output increased linearly with O₂ in all subjects: individual slopes and intercepts averaged 5.16 (1.56) lmin^–1 per lmin^–1 O₂, and 4.25 (1.92) lmin^–1, respectively [mean (SD)]. Stroke volume index rose by an average of 29% from rest to exercise, reaching a maximum of 52 mlm^–2 in boys and girls. Most subjects demonstrated a continuous, gentle rise in stroke volume index with increasing work rate, though a minority demonstrated a falling index as work increased above the anaerobic threshold, despite rising cardiac output. Impedance cardiography accurately tracks cardiac output and can be a useful clinical and research tool in pediatric cardiology and exercise physiology.     

The importance of blood resistivity in the measurement of cardiac output by the thoracic impedance method

Twenty simultaneous pairs of cardiac output values from patients who did not have valvular abnormalities were obtained by the radioisotope method and the electrical-impedance method of Kubicek et al. (1966). If a standard value of 150Ω-cm was assumed for the resistivity of each patient's blood, the mean value for the impedance cardiac output was 14·5% high compared with the mean radioisotope value. In this study the patient's haematocrits ranged from 20 to 48%. Inserting the appropriate value of the resistivity for each patient into the stroke volume equation of Kubicek from the data of Geddes and Sadler (1973) made the mean impedance value 10·3% low compared with the mean isotope value. The use of our measured resistivity data made the mean impedance cardiac output value 21·5% lower than the mean isotope value. The correlation coefficient between the impedance and isotope techniques was 0·61 for the standard value of resistivity of 150Ω-cm. Using the resistivity data of Geddes and Sadler (1973) the correlation became 0·87, and with our data it was 0·88.     

Comparison of electrical field plethysmography with electrical impedance plethysmography

As a means for assessing cardiac function, electrical field plethysmography (EFP) has been shown to have some features quite different from electrical impedance plethysmography (EIP). Here the two techniques are compared by using the two systems simultaneously on a subject and also with independent use in different electrode configurations. The results conform with the view that EIP is related primarily to volumetric changes of the aorta, whereas EFP is affected predominantly by changes in cardiac dimensions and orientation. Because of this difference, the standard time differential formula used for EIP is not applicable for the computation of cardiac output from the EFP waveforms. An alternative method of computation based on the amplitude of the EFP waveform is suggested.     

Influence of body position and pre-exercise activity on cardiac output and oxygen uptake following step changes in exercise intensity

Summary Parallel measurements of breath-by-breath oxygen uptake, cardiac output (Doppler technique), blood pressure (Finapres technique) and heart rate were performed in nine subjects during cycle ergometer exercise in the upright and supine positions. Transients were monitored during power steps starting from and leading to either rest or lower levels of exercise intensity. Oxygen uptake ( ) and cardiac output kinetics were markedly faster than in all other conditions when exercise was started from rest. In contrast to exercise-exercise on steps, the computed arteriovenous difference in O₂ content increased almost immediately in this situation, indicating that not only the additional energy expenditure due to the acceleration of the flywheel but also an increased venous admixture from non-exercising parts of the body contributed to the early kinetics. The off kinetics generally showed a more uniform pattern and did not simply mirror the on transients. The present findings indicate that transitions from rest should be avoided when muscle kinetics are to be assessed on the basis of ₂ measurements at the mouth.     

Contributions to the impedance cardiogram waveform

We have developed a cylindrically symmetric model with which to study what physiologic variables might contribute to the impedance cardiogram signal. We find the major contributions in this model to be due to dilation of the aorta and carotid arteries, changes in conductivity of blood in these same vessels due to red cell reorientation during flow, changes in the conductivity of the lungs, and changes in heart volume. The calculations suggest that the popular equation used to determine stroke volume from thoracic impedance data is not accurate under all conditions of the circulatory system.     

Changes of cardiac output during treadmill exercise by impedance cardiography

Nine athletes and ten nonathletes were selected randomly to study the changes of cardiac function during exercise by impedance cardiography. The speed of the treadmill was maintained at 3.4 mph, and its grade was increased by 1% (Balke protocol). The exercise was continued until the target heart rate (THR), 85% of maximum oxygen uptake (VO2max). The measured parameters for pre- and post-exercise were stroke volume (SV), heart rate (HR), and cardiac output (CO). Average stroke volume of athletes at pre-exercise, 71.1 ml, was higher than that of nonathletes, 64.6 ml, and stroke volume of the former at post-exercise, 97.0 ml, was also higher than that of the latter, 85.2 ml. Therefore, despite the lower heart rate, cardiac outputs of athletes at pre- and post-exercise, 4.98 and 16.3 L/min, were higher than those of nonathletes, 4.87 and 14.2 L/min. For the second phase of the study, cardiac outputs of three subjects were measured during the continuous treadmill exercise with newly developed electrodes and shoes for minimizing motion artifact. Though there were several studies measuring cardiac output during continuous bicycle exercise, this is thought to be the first study in the world measuring cardiac output during continuous treadmill exercise without aid of ensemble averaging.     

生物电阻抗法监测心输出量在危重病中的应用

目的 探讨生物电阻抗法测量无创心输出量(CO)在危重病中应用的准确性和有效性.方法 选择2008年1月至5月间本院外科重症监护中心(SICU)收治的12例需放置SWAN-GANZ导管进行血流动力学监测的患者,同时用生物电阻抗法和SWAN-GANZ导管热稀释法测定患者CO曲线平稳后0、1、6、12、24、48 h的CO值,并应用Pearson相关对两种方法测得的CO值与时间进行分析.结果 生物电阻抗法测最的CO值在所有时间点均低于SWAN-GANZ导管热稀释法的测最值[0 h:(5.5±0.9)L/min比(7.7±1.4)L/min;1h:(5.3±0.7)L/min比(7.4±1.5)L/min;6 h:(4.6±0.6)L/min比(6.8±1.4)L/min;12 h:(5.1±0.9)L/min比(7.6±1.6)L/min;24 h:(6.1±1.1)L/min比(9.0±1.6)L/min;48 h:(6.3±1.3)L/min比(9.5±1.5)L/min;均P<0.05].两种方法测量的CO值均随着时间的延长而呈升高趋势(r=0.818,P=0.047和r=0.884,P=0.020).结论 生物电阻抗法测量的无创CO与SWAN-GANZ导管测定的患者CO同样呈时间依赖性,是一种有效的CO监测方法.     

Sources of the thoracic cardiogenic electrical impedance signal as determined by a model

A three-dimensional electrical model was developed to study the origin of ΔZ due to cardiac activity recorded from band electrodes around the neck and lower thorax. Volume changes were simulated with resistivity changes in the lungs, large arteries, large veins and atria, ventricles, small arteries and veins and the skeletal muscle for a typical 80 ml ventricular stroke volume. The results showed the contributions to ΔZ to be 61 per cent from the lungs, 23 per cent from the large arteries and 13 per cent from the skeletal muscle. The ΔZ signal was most sensitive to skeletal muscle volume change The results indicate that the ΔZ signal has many origins which could cause significant error in calculated cardiac function it all the regions do not change in the normal related pattern.     

The use of the electrical-impedance technique for the monitoring of cardiac output and limb bloodflow during anaesthesia

A need has long existed during routine anaesthesia for a convenient, noninvasive method for the monitoring of stroke volume, and hence of changes in cardiac output. If knowledge of the mean arterial blood pressure is available, it becomes possible to monitor changes occurring in the left-ventricular stroke work and the total systemic resistance. Nyboer et al. (1940) proposed the use of thoracic electrical-impedance measurements for this purpose. The system used by us, however, is based on the work of Kubicek et al. (1966). The calculation method used for the stroke volume is to some extent empirical; nonetheless the values obtained compare well with those obtained by the dye-dilution technique in patients with normal hearts and lungs.     

Relationship between cardiac output and oxygen uptake at the onset of exercise

Summary The purpose of the present study was to assess the relationship between the rapidity of increased gas exchange (i.e. oxygen uptake ) and increased cardiac output ( ) during the transient phase following the onset of exercise. Five healthy male subjects performed multiple rest-exercise or light exercise (25 W)-exercise transitions on an electrically braked ergometer at exercise intensities of 50, 75, or 100 W for 6 min, respectively. Each transition was performed at least eight times for each load in random order. The was obtained by a breath-by-breath method, and was measured by an impedance method during normal breathing, using an ensemble average. On transitions from rest to exercise, rapidly increased during phase I with time constants of 6.8–7.3 s. The also showed a similar rapid increment with time constants of 6.0–6.8 s with an apparent increase in stroke volume (SV). In this phase I, increased to about 29.7%–34.1% of the steady-state value and increased to about 58.3%–87.0%. Thereafter, some 20 s after the onset of exercise a mono-exponential increase to steady-state occurred both in and with time constants of 26.7–32.3 and 23.7–34.4 s, respectively. The insignificant difference between and time constants in phase I and the abrupt increase in both and SV at the onset of exercise from rest provided further evidence for a cardiodynamic contribution to following the onset of exercise from rest.     

Comparison of cardiac output determined by different rebreathing methods at rest and at peak exercise

Several rebreathing methods are available for cardiac output (Q _T) measurement. The aims of this study were threefold: first, to compare values for resting Q _T produced by the equilibrium-CO₂, exponential-CO₂ and inert gas-N₂O rebreathing methods and, second, to evaluate the reproducibility of these three methods at rest. The third aim was to assess the agreement between estimates of peak exercise Q _T derived from the exponential and inert gas rebreathing methods. A total of 18 healthy subjects visited the exercise laboratory on different days. Repeated measures of Q _T, measured in a seated position, were separated by a 5 min rest period. Twelve participants performed an incremental exercise test to determine peak oxygen consumption. Two more exercise tests were used to measure Q _T at peak exercise using the exponential and inert gas rebreathing methods. The exponential method produced significantly higher estimates at rest (averaging 10.9 l min⁻¹) compared with the equilibrium method (averaging 6.6 l min⁻¹) and the inert gas rebreathing method (averaging 5.1 l min⁻¹; P < 0.01). All methods were highly reproducible with the exponential method having the largest coefficient of variation (5.3%). At peak exercise, there were non-significant differences between the exponential and inert gas rebreathing methods (P = 0.14). The limits of agreement were −0.49 to 0.79 l min⁻¹. Due to the ability to evaluate the degree of gas mixing and to estimate intra-pulmonary shunt, we believe that the inert gas rebreathing method has the potential to measure Q _T more precisely than either of the CO₂ rebreathing methods used in this study. At peak exercise, the exponential and inert gas rebreathing methods both showed acceptable limits of agreement.     

An analogue circuit for the computation of stroke volume by the electrical impedance method

An analogue circuit is described for use in conjunction with thedZ/dt, e.c.g. and phonocardiogram signals from an IFM Impedance Cardiograph to measure stroke volume. This is displayed on a digital meter. The circuit can be extended to measure cardiac output.