Chest
Volume 102, Issue 2, August 1992, Pages 448-455
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Clinical Investigations
Evaluation of Cardiac Output by Thoracic Electrical Bioimpedance during Exercise in Normal Subjects

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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<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.

Chest 1992; 102:448-55)

Section snippets

METHODS

Eleven healthy male volunteers, ages 21 to 40 years, who were hospital personnel or respiratory therapy students, were recruited for the study. Their physical characteristics are shown in Table 1. Cardiac output was determined simultaneously by both CO2 rebreathing and thoracic impedance techniques at rest and during exercise with the subjects seated on a cycle ergometer (Lode Model Corival 400, Groningen, Netherlands).

Measurements of ventilation, gas exchange, and cardiac output by the CO2

RESULTS

The coefficients of variation for the same day in the three subjects at the different levels of exercise and the intrasubject coefficients of variability for the same subjects on different days are shown in Table 2. The coefficient of variation was highest for the resting determinations done on different days. The intersubject variability was somewhat greater than the intrasubject variability. Table 3 compares the individual data of cardiac output obtained by both methods in all subjects at

DISCUSSION

Of the many methods estimating cardiac output, invasive methods such as the Fick, dye dilution, and thermodilution have become standard in the clinical setting. However, noninvasive methods are more appropriate in the exercise laboratory and in instances where more extensive procedures are unwarranted or contraindicated. The CO2 rebreathing technique (indirect Fick) has been shown to agree well with invasive methods of cardiac output estimation.21, 22, 23, 24, 25 The added advantage of using

ACKNOWLEDGMENTS

We thank Marcia Gofsky of Cardiostat Inc for the loan of the Bomed NCCOM-3 equipment used in the study, Nancy Hanes and Colleen Kennell for their help with the statistical analysis and the preparation of the figures and tables, and Dr. Richard Pardy for his review of the manuscript.

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    Manuscript received March 11; revision accepted January 21.

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