KAPLAN, V., G. B. BUCKLAR, and K. E. BLOCH. Noninvasive Monitoring of Cardiac Output during Exercise by Inductance Cardiography. Med. Sci. Sports Exerc., Vol. 35, No. 5, pp. 747–752, 2003.
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.
To evaluate the cardiovascular response to exercise and to determine factors that might limit performance of physical work, the measurement of stroke volume and cardiac output during exercise would be desirable. However, the techniques currently available for this purpose are either invasive requiring insertion of intravascular catheters, such as thermodilution or the direct Fick method, or depend on instrumentation of the airway and require specialized breathing maneuvers, such as alveolar gas exchange based methods (14).
Inductance cardiography, also named thoracocardiography, is a noninvasive technique for monitoring changes in stroke volume and cardiac output that does not depend on instrumentation of the airway (2,17). A transducer placed transversely around the chest at the level of the heart records variations of the cross-sectional area it encircles (19). The signal is mainly modulated by respiration but to a minor degree (approximately 5% of the total amplitude) by cardiac activity as well (5,6). By digital band-pass filtering and ensemble averaging, the signal with the ECG R-wave as a trigger, respiratory and body-motion–related signal components are suppressed and relatively pure cardiogenic curves can be obtained (7). The amplitudes of these curves track changes in stroke volume (2,17). After an initial calibration by an independent method, cardiac output can be measured by inductance cardiography as the product of heart rate and stroke volume. Accuracy of inductance cardiographic derived cardiac output has been validated by simultaneous thermodilution measurements in critically ill patients (4,17), by echocardiography in normal subjects (5), and by thermodilution in juvenile piglets and dogs (3,17). Furthermore, comparisons of inductance cardiography to impedance cardiography, a technique based on a different physical principle (i.e., transthoracic electrical impedance), have revealed similar trends of changes in cardiac output after subcutaneous injection of terbutaline in resting normal subjects (16).
As inductance cardiography allows monitoring of changes in cardiac output unobtrusively in a nearly hands-off fashion, it has the potential to serve as a valuable tool for application during exercise tests. However, performance of the technique in this setting has not been evaluated. Therefore, the purpose of this study was to investigate whether inductance cardiography can be employed to monitor changes in cardiac output during upright bicycle exercise in healthy volunteers. To avoid the need for invasive instrumentation, we compared inductance cardiography to two noninvasive reference methods that derive cardiac output from alveolar gas exchange based on an indirect Fick principle: a carbon dioxide rebreathing technique (12,14) and an indirect oxygen uptake technique (18).