Assessment of the hemodynamic response to single passive head up tilt by non-invasive methods in normotensive subjects.

The hemodynamic response to passive upright tilt in thirty-five normal subjects was assessed by combined electrocardiography, differentiated electrical impedance cardiography, phonocardiography and non-invasive blood pressure measurement. The increase in transthoracic impedance and decrease in estimated stroke volume were accompanied by a cardio-acceleratory and vasopressor response. The increase in heart rate did not compensate for the decrease in estimated stroke volume and the estimated cardiac output decreased. Diastolic and mean blood pressure, however, increased. The relative systolic time interval was largely increased. Most of these changes returned to base-line on resuming the supine position. This test procedure and the non-invasive methods used offered a suitable setting for the within-subject assessment of postural changes in cardiovascular function. Certain methodological aspects were found to affect the interpretation of the impedance cardiographic data. Pre-systolic low amplitude-high velocity changes in transthoracic impedance, especially when occurring in conditions where strict respiratory control cannot be sustained, can complicate the delineation of the systolic time intervals and the correction of respiration-related fluctuations in the signal's base-line. One further subject is presented with imminent syncope on tilting, in order to highlight this intrinsic but unpredictable morbidity as a limiting factor in the application of this study procedure.