Cardiac responses to head up tilt during early extrauterine life: relevance of active acquisition of erect posture.

Mammals must adapt to gravity on passing from the intrauterine to the extrauterine environment. In order to evaluate the cardiovascular effects of gravity in the first phases of extrauterine life, the effects of passive orthostatism on the cardiac filling volume were investigated through longitudinal haemodynamic studies in 14 normal healthy males before (6 months) and after (18 months) acquiring the ability to stand. Left ventricular diameter (by echocardiographic measurement), arterial blood pressure (by sphygmomanometry) and heart rate were measured in the supine and upright position at both ages. At 6 months the left ventricular end diastolic volume was not modified by a change in posture [supine 6(SEM 3) ml, upright 6(3) ml], so heart rate was minimally altered [supine 128(9), upright 130(11) beats.min-1] and blood pressure remained stable [supine 74(6), upright 73(5) mm Hg]. After the acquisition of the erect posture (18 months) left ventricular end diastolic volume was reduced [supine 14(3), upright 8(2) ml], heart rate increased [supine 110(11), upright 127(12) beats.min-1] and blood pressure remained constant [supine 80(6), upright 79(7) mm Hg]. The assumption of the erect posture therefore represents a phase when, for the first time in the natural history of the cardiovascular system, translocation of intravascular volume from the cardiopulmonary area to the periphery stimulates nervous and humoral responses to control the dynamics of body fluids and arterial blood pressure in a gravitational environment.