The dynamic response of the cardiopulmonary parameters to passive head-up tilt.

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Abstract
Transitional changes in stroke volume, heart rate, and cardiac output resulting from head-up tilt were measured in healthy male subjects, using impedance cardiography. Respiratory frequency, tidal volume, ventilation rate, end-tidal O2 and CO2 tensions, O2 uptake, CO2 output, and exchange ratio were also determined breath-by-breath. When the subject was tilted upward from a supine to an upright position, the stroke volume was decreased while the heart rate was increased. The time required to attain the steady state was shorter in the stroke volume change than in the heart rate. In addition, since the heart rate was not increased so high as to be inversely related to the stroke volume, the cardiac output decreased by tilting upward. The ventilation rate remained almost unchanged within a tilt angle of 60° or so. Thus, both the O2 and CO2 fractions in expired air showed transitory changes. As the original body position was restored, the cardiac output was increased to the initial level, the tidal volume showing a transient increase. During the recovery process, PCO2 in expired air increased and PO2 decreased, showing that the change in cardiac output exceeded that in ventilation in quantity. The change in ventilation, moreover, preceded that of expired PCO2 by ca. 10 sec, suggesting that the enhancement of ventilation occurred through some agent (s) other than the chemical drive.