Hemodynamic responses to passive body tilts.
To investigate the relationship between the effects of gravity and hemodynamic responses to body tilts in the early steady-state, twelve healthy young adult males were passively tilted from the supine control position (SCP) to the 15 degrees, 30 degrees and 60 degrees head-up position (HU), and then the -15 degrees and -30 degrees head-down position (HD). Cardiac output (Q), stroke volume (SV), acceleration index (ACI) of the heart, thoracic fluid volume (TFV), blood pressure (BP), heart rate (HR), total peripheral vascular resistance (TPR) and ECG were measured. In the postural changes from SCP to the 15 degrees, 30 degrees and 60 degrees HU, the SV gradually decreased and the Q also decreased. The HR and TPR gradually increased. On the contrary, in the -15 degrees and -30 degrees HD, the SV increased and the Q increased and the HR and TPR decreased. The ACI increased in the HD, but decreased in the HU. The mean BP increased in the HU, but decreased in the HD. The TFV increased in the HD, and decreased in the HU. The average QRS vector was 56.1 degrees on the SCP and it increased in the HU, but decreased in the HD. In conclusion, the changes of hemodynamic parameters except in BP were linearly related to the sine of the tilt angle in regardless of HU or HD.