Introduction: Cardiac output (CO) responds to exercise is a key determinant of cardiac performance. Exercise CO is estimated by measuring peak oxygen uptake (VO2) during cardiopulmonary exercise testing (CPET). However, maximal VO2 uptake consists of a central (CO) and a peripheral (AVDO2) component contributing to a different extent to exercise capacity. Hence, the assessment of exercise CO is an important complement to peak VO2, but clinically practical and reliable methods are missing. This study evaluates the test-retest reliability of CO assessment during CPET using the Non-Invasive Cardiac Output Monitor (NICOM) in healthy volunteers.
Hypothesis: NICOM works reproducible and can reliably be used for CO assessment during CPET.
Methods: 25 healthy volunteers (mean age 46 years) were enrolled. All subjects underwent echocardiography to exclude structural heart disease and two identical graded bicycle CPET (T1 and T2) separated by one week using a spiroergometry system. Bioreactance cardiography (NICOM®, Cheetah Medical) was added for CO assessment. In brief, four skin patches are used to apply an electric current to the thorax and process the output signals to assess the phase shift. Since phase shifts are highly correlated with the aortic blood flow, the stroke volume (SV) can be estimated. Reproducibility was assessed by using intraclass correlation coefficient (ICC).
Results: The mean (SD) peak heart rate at maximal work load was 165 (±15) bpm and 161 (±16) bpm at T1 and T2, and the mean respiratory exchange ratio was 1.13 (±0.06) and 1.14 (±0.05), respectively. The mean peak SV was 104.2 (±26.8) ml at T1 and 103.2 (±27.5) ml at T2, respectively, whereas the mean peak exercise CO was 18.37 (±6.68) L/min at T1 and 17.56 (±6.46) L/min at T2, respectively. The ICC for CO and SV was 0.78 [0.57-0.90] and 0.79 [0.57-0.90], respectively.
Conclusion: The NICOM method showed a good test-retest reliability for estimating cardiac output at maximal exertion. The assessment of exercise CO by NICOM may complement peak VO2 in CPET. However, the results cannot necessarily be translated to disease states.