Cardiac Output in Healthy Subjects Determined with a CO2Rebreathing 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.

Noninvasive monitoring of cardiac output during exercise by inductance cardiography

INTRODUCTION/PURPOSE

Inductance cardiography is a noninvasive technique that monitors changes in cardiac output from an inductance plethysmographic transducer encircling the chest at the level of the heart. The method has been previously validated in supine patients at rest by comparisons to thermodilution. Our purpose was to investigate whether the technique can be employed during bicycle exercise.

METHODS

We simultaneously measured cardiac output by inductance cardiography and by two gas exchange methods based on the Fick principle during upright cycle ergometry in healthy volunteers.

RESULTS

In 11 subjects, comparisons of changes in cardiac output by inductance cardiography agreed well with values measured by carbon dioxide rebreathing during a steady-state exercise protocol. In 12 subjects, cardiac output changes measured by inductance cardiography and an oxygen uptake method agreed closely during a progressive ramp exercise protocol to exhaustion. The bias (mean difference to reference methods) and limits of agreement (+/-2 SD of bias) for estimation of relative changes in cardiac output by inductance cardiography were 1% +/- 21% in 67 comparisons to the carbon dioxide rebreathing technique, and 0% +/- 22% in 98 comparisons to the oxygen uptake method.

CONCLUSION

In healthy subjects, inductance cardiography accurately and unobtrusively estimates changes in cardiac output during bicycle exercise in comparison to gas exchange methods.

The effect of submaximal exercise on recovery hemodynamics and thermoregulation in men and women

Five women and 5 men were studied to examine the effects of submaximal exercise on thermoregulatory and hemodynamic variables during recovery in two environments: (a) control (C), 22 degrees C, 33% rh; and (b) hot humid (H), 32 degrees C. The participants exercised on a cycle ergometer at 60% of peak oxygen consumption for 35 min prior to 90 min of seated recovery. Sessions were identical, except for environment. Variables evaluated (p < .05) were: core temperature (TR), mean skin temperature (Ts), sweat rate (SR), heart rate (HR), stroke index (SI), cardiac index (CI), forearm blood flow (FBF), systolic blood pressure (SBP), and diastolic blood pressure (DBP). Men and women exhibited similar patterns of TR, Ts, and SR in both environments. Ts and SR (collapsed means for gender) were higher in the H than in the C. DBP was higher in men than in women throughout recovery in both environments. With combined means for gender, HR was higher in the H than in the C. CI, SI, FBF, and SBP were similar in both environments and returned to baseline within 15 min into recovery. These data suggest that heat dissipation during extended recovery was accomplished with similar contributions of cutaneous vasodilation and sweating in M and F. Furthermore, the moderate exercise level did not influence hemodynamics beyond 15 min of recovery in either environment.

Comparison of cardiac output measurement techniques: thermodilution, Doppler, CO2-rebreathing and the direct Fick method

Simultaneously measured cardiac output obtained by thermodilution (TD), transcutaneous suprasternal ultrasonic Doppler (DOP), CO2-rebreathing (CR) and the direct Fick method (FI) were compared in eleven healthy subjects in a supine position (SU), a sitting position (SI), and during sitting exercise at a workload of 50 W (EX). The agreements between the techniques, two by two, were expressed as the bias calculated as the averaged differences between the techniques. Precision was expressed as the standard deviation of the bias. The overall agreement (bias +/- precision) between TD, DOP and CR respectively and FI were 2.3 +/- 1.6, -0.1 +/- 1.4, and -0.2 +/- 1.1 l/min. TD overestimated cardiac output consistently in SU, SI and EX. DOP was in-accurate during EX and agreed well with FI in SU and SI. CR agreed closely with FI in SI and EX, but values were underestimated in SU. The overall agreement between DOP and CR, respectively, and TD were 2.5 +/- 2.2 and 2.6 +/- 1.6 l/min. The overall agreement between DOP and CR was 0.1 +/- 1.6 l/min. In conclusion, TD overestimated cardiac output compared to the other techniques and the poor agreement has to be taken into consideration especially in measures of low values. The precision of DOP and CR against FI seems to be within clinically acceptable limits, and these methods may provide interchangeable alternatives to the invasive Fick method.

On-line determination of pulmonary blood flow using respiratory inert gas analysis

An inert gas analysis method has been developed to perform on-line real time determination of pulmonary blood flow using a nonrebreathing approach. This technique is based on a mathematical model describing mass balance of two inert gases which are breathed using an open gas circuit. The measurements using this method are noninvasive, easy to peform, and do not disturb normal physiological processes. As well, since data are collected on a breath-by-breath basis, it is possible to estimate other respiratory, cardiopulmonary, and metabolic parameters simultaneously in a breath-by-breath manner. Special consideration was given to developing effective data processing algorithms to minimize the influence of measurement noise and respiratory variations. Experimental studies to compare this method with other accepted techniques were conducted to validate the present technique.

Evaluation of cardiac output by thoracic electrical bioimpedance during exercise in normal subjects

We compared cardiac output determined simultaneously by two methods, the CO2 rebreathing technique and the thoracic electrical bioimpedance method (Bomed NCCOM-3 equipment). The studies were performed in duplicate in 11 healthy male subjects at rest and during three levels of steady-state exercise on a cycle ergometer at 60, 120, and 180 W. Cardiac output at 60 and 120 W was slightly lower (p less than 0.01) by the thoracic impedance method (12.2 +/- SE 2.2 and 15.7 +/- SE 3.5 L/min, respectively) than by the CO2 rebreathing method (14.0 +/- SE 2.1 and 17.9 +/- SE 3.0 L/min, respectively), suggesting a systematic bias between the two methods of measurement. However, if allowance is made for that bias, there would be acceptable agreement between the two methods at 60 and 120 W. Although the results were not significantly different between the two methods at rest and at 180 W, there was no acceptable agreement between the two methods probably because the CO2 rebreathing method at rest was more liable to show error due to the small arteriovenous CO2 difference, while the impedance method was less reliable at 180 W. Cardiac output by both methods correlated with O2 consumption, with the correlation being higher for cardiac output by the rebreathing method (r = 0.94) than for thoracic impedance (r = 0.88). The results suggest that the thoracic electrical bioimpedance method can be used for determination of cardiac output during mild or moderate levels of exercise in normal subjects.

Dynamics of changes in the cardiovascular response to submaximal exercise during low-intensity endurance training with particular reference to the systolic time intervals

Eighteen male volunteers (aged 20-23 years), not involved in any sporting activities, were submitted to 13 weeks of training consisting of 30 min exercise [at 50%-75% maximal oxygen intake (VO2max)] on a cycle ergometer, performed 3 times a week. Every 4 weeks cardiac function was evaluated by measuring the systolic time intervals at rest and during submaximal cycle exercise. Stroke volume (SV), heart rate (HR) and blood pressure (BP) responses to submaximal exercise, VO2max and anaerobic threshold (AT) were also determined. Significant increases in VO2max, increases in AT and SV at the submaximal exercise intensities, as well as decreases in HR and BP were found after 4 weeks of training. Resting systolic time intervals were not affected by training, but during the submaximal cycle exercise the values of the pre-ejection period (PEP) and isovolumic contraction time (ICT) corresponding to HR of 100 beats.min-1 were significantly lowered after 13 weeks of training, whereas PEP, ICT and total electromechanical systole corresponding to HR of 130 beats.min-1 were significantly shortened by the 4th week. The ratios of PEP:LVET (left ventricular ejection time) and ICT:LVET during submaximal exercise were significantly lowered by training starting from the 8th week. These changes might be interpreted as evidence of the training-induced enhancement of the "contractility reserve", i.e. the ability to increase heart muscle contractility with increasing exercise intensity.

Cardiovascular changes associated with decreased aerobic capacity and aging in long-distance runners

Fifty-five male runners aged between 30 to 80 years were examined to determine the relative roles of various cardiovascular parameters which may account for the decrease in maximal oxygen uptake (VO2max) with aging. All subjects had similar body fat composition and trained for a similar mileage each week. The parameters tested were VO2max, maximal heart rate (HRmax), cardiac output (Q), and arteriovenous difference in oxygen concentration (Ca-Cv)O2 during graded, maximal treadmill running. Average body fat and training mileage were roughly 12% and 50 km.week-1, respectively. The average 10-km run-time slowed significantly by 6.0%.decade-1 [( 10-km run-time (min) = 0.323 x age (years) + 24.4] (n = 49, r = 0.692, p less than 0.001]. A strong correlation was found between age and VO2max [( VO2max (ml.kg-1.min-1) = -0.439 x age + 76.5] (n = 55, r = -0.768, p less than 0.001]. Thus, VO2max decreased by 6.9%.decade-1 along with reductions of HRmax (3.2%.decade-1, p less than 0.001) and Q (5.8%.decade-1, p less than 0.001), while no significant change with age was observed in estimated (Ca-Cv)O2. It was concluded that the decline of VO2max with aging in runners was mainly explained by the central factors (represented by the decline of HR and Q in this study), rather than by the peripheral factor (represented by (Ca-Cv)O2).

Measurement of cardiac output by CO2 rebreathing in unsteady state exercise

The ability to determine cardiac output (Q) noninvasively during a nonsteady state (NSS) incremental exercise test was assessed. Seven healthy subjects performed two maximal incremental cycle ergometer exercise tests (100 kpm/min increments every minute), and also steady state exercise (SS) at 25, 50, and 75 percent of their maximum power output. The Q was determined by the indirect CO2 Fick method; mixed venous PCO2 was calculated using the exponential CO2 rebreathing method. No significant differences were observed for the cardiac output/oxygen uptake relationship (Q/VO2) obtained between the two incremental exercise tests. During NSS, the Q/VO2 was linear (r = .89; intercept = 5.69 L/min; slope = 5.39). During the SS, Q/VO2 was linear (r = .90; intercept = 5.47 L/min; slope = 4.87). No significant difference was observed between the SS and NSS Q/VO2 relationships (p greater than 0.05), and the NSS relationship was similar to Q/VO2 values previously reported in the literature. Accurate and reproducible measurements of Q can be obtained noninvasively in healthy subjects using the exponential CO2 rebreathing method during incremental progressive exercise tests, with similar values at comparable VO2 to those obtained in the steady state.

Effects of physical deconditioning after intense endurance training on left ventricular dimensions and stroke volume

To determine the role of preload in maintaining the enhanced stroke volume of upright exercise-trained endurance athletes after deconditioning, six highly trained subjects undergoing upright and supine bicycle ergometry were characterized before and after 3, 8 and 12 weeks of inactivity that reduced oxygen uptake by 20%. During exercise, oxygen uptake, cardiac output by carbon dioxide rebreathing, cardiac dimensions by M-mode echocardiography, indirect arterial blood pressure and heart rate were studied simultaneously. Two months of inactivity resulted in a reduction in stroke volume, calculated as cardiac output/heart rate, during upright exercise (p less than 0.005) without a significant change during supine exercise. A concomitant decrease in the left ventricular end-diastolic dimension from the trained to the deconditioned state was observed in the upright posture (5.1 +/- 0.3 versus 4.6 +/- 0.3 cm; p = 0.02) but not with recumbency (5.4 +/- 0.2 versus 5.1 +/- 0.3 cm; p = NS). There was a strong correlation between left ventricular end-diastolic dimension and stroke volume (r greater than 0.80) in all subjects. No significant changes in percent fractional shortening or left ventricular end-systolic dimension occurred in either position after cessation of training. Estimated left ventricular mass was 20% lower after 3 and 8 weeks of inactivity than when the subjects were conditioned (p less than 0.05 for both). Thus, the endurance-trained state for upright exercise is associated with a greater stroke volume during upright exercise because of augmented preload. Despite many years of intense training, inactivity for only a few weeks results in loss of this adaptation in conjunction with regression of left ventricular hypertrophy.

Measurement of mixed venous carbon dioxide pressure by rebreathing during exercise

This study compared rebreathing methods currently used for the measurement of mixed venous PCO2 (PVCO2) during exercise. Four mathematical procedures were used to derive the asymptote of the exponential rise in end-tidal PCO2 during 15 sec rebreathing of low (2-6%) CO2 mixtures ('exponential' methods); the derived PVCO2 was compared to that obtained with an 'equilibrium' method, in which high CO2 mixtures were rebreathed to obtain an equilibrium of CO2 between the lungs and rebreathing bag. Precision was established by analysing duplicate rebreathings at each power output. The most precise of the four exponential procedures (coefficient of determination 0.98) used iterative statistical analysis to obtain an equation for the best fit curve of end-tidal PCO2 with time (t), solved for t = 20 sec. The PVCO2 derived by this procedure was similar to that obtained by the equilibrium method (r = 0.96), and both yielded estimates of cardiac output that were within the previously published expected range. Variations in the initial rebreathing bag volume and CO2 concentration, and in the breathing frequency during rebreathing had little effect on the derived PVCO2.

Effect of 12 months of intense exercise training on stroke volume in patients with coronary artery disease

The purpose of this study was to determine whether 12 months of intense endurance exercise training can induce an increase in left ventricular stroke volume and in stroke work during exercise in patients with coronary artery disease. Eleven male patients were studied. With training, mean maximal oxygen uptake capacity (Vo2max) increased 39%, from 1.85 +/- 0.36 to 2.57 +/- 0.43 l/min. Stroke volume during upright exercise that required 35-65% of Vo2max was 18% higher after training. At the same percentage of Vo2max, mean blood pressure was the same before and after training; as a result, left ventricular stroke work (mean blood pressure X stroke volume) increased 18% (p less than 0.01). These findings suggest that in patients with coronary artery disease, prolonged, intense training induces an increase in stroke volume, and this is a result of cardiac rather than peripheral adaptations.

A multivariate analysis of the role of certain anthropometric and physiological attributes in distance running

Previous studies have indicated the relative importance of not only cardiovascular-réspiratory function but physical dimensions and body composition as factors involved in success in distance running. A total of 114 Japanese young, middle- and long-distance runners (age = 19 X 0 +/- 1 X 7 yr) served as subjects. Anthropometric characteristics were assessed before the measurement of VO2max (open circuit method) and Qmax (CO2 rebreathing method). Among many anthropometric variables, chest girth, upper leg length and thigh girth were best related to performances over 800, 1500 and 5000 metres, while upper arm girth, the Rohrer Index, and the Ponderal Index represented the 10000m performance. As a result of factor analysis and the multiple regression analysis, three factors (i.e., linearity of physique, girth of physique, and subcutaneous fat) were extracted, and the first two factors were nearly equally related to the 800m, and 1500m and 5000m performances. The 10000m, however, was best accounted for by the second factor. Approximately 20-40% of the variance in the performances was explained by three anthropometric variables. VO2max and Qmax, combined in a linear model, also accounted for approximately 20-40% of the variance in the performances. We conclude that anthropometric attributes would predict the distance running performance to about the same degree as physiological attributes. This may be partly attributed to the homogeneity of the present subjects in terms of their running abilities.

CO2 dissociation curves of oxygenated whole blood obtained at rest and in exercise

In vitro CO2 dissociation curves for oxygenated whole blood were determined in 19 healthy male subjects at rest and during submaximal and maximal bicycle work. Hemoglobin concentration and blood lactate increased with increasing work load and accordingly buffer value of the whole blood increased while bicarbonate and Base Excess (BE) decreased, resulting in a downward shift of the CO2 dissociation curve during exercise. Despite the marked increase in buffer values of the blood, the slopes of the CO2 dissociation curves during exercise were found to be about the same as those obtained at rest. It was inferred that the increasing effect of increased buffer value, on the dissociation slope, was essentially compensated by the decreasing effect of diminished bicarbonate content. The advantages of this relatively constant CO2 dissociation slope for the indirect measurement of cardiac output by the Fick principle are discussed.

Differences in cardio-respiratory responses to exhaustive exercise between athletes and non-athletes

To study the factors limiting the O2 supply in heavy exercise, O2 uptake at exhaustion was determined by progressive loading method with a bicycle ergometer in 33 well-trained male runners and 34 male sedentary adults. Pulmonary ventilation, oxygen removal, respiratory rate, tidal volume, pulmonary diffusing capacity, alveolar-capillary oxygen difference, cardiac output, arterial-venous oxygen difference, stroke volume and heart rate were measured. It was found that pulmonary diffusing capacity, cardiac output and stroke volume were correlated with the difference in O2 uptake at exhaustion between athletes and non-athletes.

Evaluation of the CO2 rebreathing cardiac output method in seriously ill patients

The CO2 rebreathing cardiac output method is a totally noninvasive Fick procedure needing validation in various disease states to become clinically applicable. Simultaneous measurements of cardiac output by CO2 rebreathing and dye-dilution or direct Fick techniques were performed in 53 patients. In nine patients with pulmonary disease rebreathing cardiac output averaged 4.85 L/min compared to 5.18 L/min by dye-dilution or Fick (r = 0.16). In 14 instances of acute myocardial infarction cardiac output was 5.53 L/min by rebreathing and 5.87 L/min by dye-dilution (r = 0.95), while in nine shock cases it averaged 3.98 L/min by dye-dilution or Fick and 3.75 L/min by CO2 rebreathing (r = 0.94). In five heart failure cases with mitral insufficiency, which may distort dye durves, correlation between standard and rebreathing methods was r = 0.09, but in 16 cases without mitral regurgitation, r = 0.89. Acute interventions in ten patients increased dye-dilution cardiac output by 0.92 L/min and rebreathing outputs by 0.60 L/min (r = 0.87). The data suggest that the CO2 rebreathing cardiac output method may be useful in the CCU-MICU setting.

Oxygen transport system during exhaustive exercise in Japanese boys

The study was designed to investigate the age-related increase in maximal oxygen uptake with special reference to the other physiological parameters (ventilation, pulmonary diffusing capacity, cardiac output etc.). 77 normal boys aged 10-18 years were tested by the progressive loading method on a bicycle ergometer. VO2 max increased by approximately 55% with the age from 10-18 years. The increase in VO2 max was accompanied with increase in ventilation (49%), pulmonary diffusing capacity (45%) and cardiac output (5%).

Serial observations of arterial and mixed-venous blood gases after step change in ventilation

In 22 dogs, subjected to a step change in ventilation, serial changes in blood gas composition and lactate and pyruvate concentrations of arterial as well as mixed venous blood were studied. The change of PaCO2 was approximately 20 mm Hg both in hypo- and hyperventilation. During hypoventilation the difference in various forms of CO2 between arterial and mixed venous blood showed first a downward shift and then gradually increased, whereas during hyperventilation they progressively increased and reached a constant level within 10-20 min. This difference was assumed to be mainly due to more efficient CO2 elimination through lung ventilation in hyperventilation as compared with CO2 accumulation from tissue metabolism in hypoventilation. In vivo buffer slopes for CO2 during hypoventilation were about half those in vitro, whereas during hyperventilation both slopes were approximately the same. In vivo arterial buffer slope was higher during hypoventilation and lower during hyperventilation as compared to that of mixed venous blood in the respective state of ventilation.