ABC | Volume 113, Nº2, August 2019

Original Article Coll et al Non-invasive cardiac output measurement Arq Bras Cardiol. 2019; 113(2):231-239 Jones et al. 26 tested the test-retest reliability in a healthy study population. 22 healthy adults performed twice a symptom-limited exercise test. Standard cardiorespiratory data were measured via spiroergometry and the hemodynamic response was monitored via TB using the NICOM ® system. The authors state that TB allows good test-retest reliability for hemodynamic measurement at rest as well as under submaximal and maximal exertion. This particular study was the first to confirm that TB might be a feasible test method. Noteworthy, the study was performed under tightly controlled research conditions. Overall, three visits were necessary to determine individual cardiorespiratory capability and to perform both exercise tests. Furthermore, to exclude confounders, certain inclusion criteria had to be fulfilled (e.g., non-smokers, empty stomach for > 2 h, no vigorous exercise 24 h before testing, no alcohol or caffeine consumption). Such scientific testing conditions are often difficult to guarantee in daily clinical routine. Thus, it remains unclear, if TB is an appropriate examination procedure not only in a research setting but also in daily clinical routine. Contrary to CO based on heart rate (HR) and stroke volume (SV), cardiac power output (CPO) indicates the overall function of the heart. 27 CPO is the product of the CO and mean arterial pressure (MAP) and therefore is a measure of cardiac pumping. 28 Peak cardiac power output (CPO peak ), the CPO achieved during maximal stress, is a major determinant of exercise intolerance and performance in cardiac patients and healthy persons, respectively. 29,30 Worth mentioning, CPO measuring can improve medical management 31,32 and risk stratification 33-35 in cardiac patients. In chronic heart failure, CPO is a powerful and independent predictor of survival outcome. 35 CPO also reflects cardiovascular adaptions and training status in athletes. 6 In fact, compared to non-athletes, 36 CPO is higher in athletes. 3,37 Thus, CPO might be an additive performance diagnostic parameter, which could help to guide training modalities. 37,38 Like other established measures of exercise capacity, CPO cannot be predicted from resting cardiac parameters. 3 Under this background, the aims of the present study were: 1) to evaluate the test-retest reliability of TB in healthy adults during the daily clinical routine, and 2) to assess the relationships between CPO and resting measures of cardiac structure and function as well as traditional cardiopulmonary exercise parameters. Here, we applied a progressive statistic approach to provide thresholds above which effects might be meaningful and to present CO and CPO values that may be used as reference values in future studies. Methods Participants In the study, 25 test persons were included into the study. All participants had no history of cardiovascular or pulmonary diseases, no cardioactive medication, a blood pressure of ≤ 140/90 mmHg, a body mass index < 25, a normal electrocardiogram, and a normal echocardiogram at the time of inclusion. Study design This study is a prospective non-interventional diagnostic single-center study. Participants were recruited in a cardiologic and internal medicine facility. The study was approved by the ethics committee of the University Witten/ Herdecke and written informed consent was obtained. A standard echocardiogram was performed to exclude structural heart diseases and to investigate the relationships between established echocardiographic parameters and cardiopulmonary and hemodynamic values. Heart size, wall thickness, systolic, and diastolic function were all in physiological limits. All participants underwent two cardiopulmonary exercise tests separated by on week. During testing, TB using the NICOM TM device was applied. Transthoracic echocardiography Echocardiography was performed to assess cardiac structure and function using a standard ultrasound system (Vivid 7, General Electric, Milwaukee, Wisconsin). A complete transthoracic study was performed, including 2D, M-mode, spectral, and color Doppler techniques according to current recommendations and guidelines. 39,40 Standard parameters were: interventricular septal wall thickness in diastole, left ventricle end-diastolic diameter, left ventricular posterior wall thickness in diastole, and fractional shortening. Left ventricular ejection fraction was measured by means of modified biplane Simpson´s method. Doppler tissue imaging was performed at the junction of the septal and lateral mitral annulus in apical 4-chamber view to determine peak mitral annular velocity during early filling (E`) and the ratio between early mitral inflow velocity and mitral annular early diastolic velocity (V). Cardiopulmonary exercise testing A symptom-limited incremental exercise test was performed in a seated position on a cycling ergometer (ec‑3000, customed GmbH, Germany). The tests were performed by trained personal. After 5 min of rest, participants started at 0 W and the workload increased every 2 min by 25 W (standard WHO protocol). HR, blood pressure on the right armusing a sphygmomanometer, and a 12-lead electrocardiogram were obtained at rest and each stage as well as for 3 min post-exercise. The respiratory gas analysis was performed using a spiroergometry system (Cortex Metalyzer®3B, Leipzig, Germany, softwareMetasoft studio 5.1.2 SR1). Ventilatory oxygen consumption and standard gas exchange data were measured breath-by-breath and averaged over 30 s. The following standard parameters were measured: Time to exhaustion, maximumworkload, ventilatory anaerobic threshold (VAT) and peak oxygen uptake (VO 2peak ). The anaerobic threshold was determined using the V-slope method. 41 The submaximal load was determined as the second last completed incremental. VO 2peak was defined as the highest VO 2 observed during testing. Thoracic bioreactance TB (NICOM®, Cheetah Medical, Portland, Oregon, USA) was added for noninvasive hemodynamic monitoring during rest and exercise. The examination was performed according to the manufacturer's protocol, as described previously. 2,21,42 232