ASSESSMENT OF CARDIAC-OUTPUT DURING EXERCISE - TRANSTHORACIC IMPEDANCE VS HEART-RATE ALONE

Previous investigators demonstrated divergent results, when comparing impedance to other established methods for the assessment of cardiac o utput. Cardiac output is defined as stroke volume multiplied by heart rate. Heart rate is easy to measure and the main determinant of cardia c output during exercise under physiologic conditions. Therefore, card iac output (CO) by tetrapolar impedance cardiography (TIC) was compare d to thermodilution (TD) in 35 patients (P). In a second step it was e xamined how heart rate alone would perform as a predictor of CO under exercise. Measurements were done at rest, 25, 50, 75, and 100 w. Resul ts were corrected for hematocrit (HCT). There was no correlation betwe en CO by TD and by TIC at rest (r = -0.006). Under exercise correlatio n was somewhat better with r = 0.45 despite correction for HCT. Mean v alues were similar, but scatter was wide. Relative increase in CO by b oth methods was also correlated (r = 0.46), while heart rate alone cor related best to TD (r = 0.65). Transthoracic impedance cardiography is not a reliable technique to measure absolute values of cardiac output at rest. During exercise large scatter limits this method to the meas urement of CO in larger groups. At exercise heart rate alone appears t o be a better indicator of increase of cardiac output than impedance c ardiography.