Influence of respiration on stroke volume determined by impedance cardiography

The conventional method of calculating stroke volume (SV) with impedance has been to measure changes in thoracic impedance for only those beats of the impedance cardiogram (IC) which cross on a specific baseline. This severely limits the number of usable beats since respiration produces oscillation of the IC around this baseline. This study investigated the influence of respiration on SV calculated independent of the baseline in seven women, 20-44 years old, in each of three postures: seated, supine, and standing. SV was determined in three continuous respiratory cycles from the following beats: 1. all; 2. all inspiratory (I); 3. I on baseline; 4. all expiratory (E); 5. E on baseline; 6. all end-E; 7. end-E on baseline (conventional). The absolute values for SV, heart rate (HR), and cardiac output (Q) were posture dependent. SVs did not differ among the respiratory phases in the standing and seated postures; in the supine posture, the smallest SV occurred at end-E on baseline. HR varied with respiration in both the standing and seated postures but not in the supine posture. Despite significant changes in HR in the seated and standing postures, Qs did not differ among the respiratory phases. Supine Qs were significantly smaller during end-E due primarily to a small SV. Consequently, when reporting Q values for supine subjects, it is important to designate the respiratory phase and baseline criteria. Our results indicate that SVs calculated independent of the baseline in general agree with the more conventional method of calculation. Therefore, we recommend the calculation of SV independent of the baseline. The reliability of the data will be increased by increasing the number of observations and SVs can be determined under circumstances where it is difficult to obtain end-E beats on the baseline.