[Changes in the cardiac dynamics in passive tile to the supine position--impedance cardiography measurements]

Passive tilt is an effective means to manipulate cardiodynamic processes. While there is a large amount of literature available concerning cardiac function with upright tilt, passive tilt into the supine position is poorly investigated. There is evidence that adapting processes to the new position do not show analogy in both conditions. In this study we investigated cardiodynamic changes occurring with passive-graded tilt from the upright position to 20 degrees headdown tilt. Twenty healthy students of both sexes participated in the study. Using impedance cardiography combined with phonocardiography it was possible to acquire stroke volume, heart rate, Heather index, and systolic time intervals. The whole procedure was repeated to test the reliability of impedance data. There was no steady augmentation of stroke volume. Between 90 degrees and 60 degrees stroke volume remained stable and showed only little change between 60 degrees and 30 degrees. Between 30 degrees and the supine position there was a steep increase of stroke volume. Tilting into the upright position apparently has a different effect. The onset of heart rate decline preceded the increase of stroke volume. It is concluded that only little changes of stroke volume/preload are necessary to induce vagal response. In the 30 degrees position both cardiac output and Heather index were minimal; this may be due to decreased sympathetic output. Systolic time intervals showed volume dependancy. Correlations between the first and second part of the study were high for all variables.