Human splanchnic and forearm vasoconstrictor responses to reducations of right atrial and aortic pressures

Regional vascular responses to gradual reductions in right atrial pressure and aortic pressure were investigated in 9 men. In each study, lower body negative pressure was applied in a ramp of -1 mm Hg/min for 40-50 minutes. During the range from control to -20 mm Hg, right atrial pressure (4 studies) fell from 4.2 mm Hg to -0.6 mm Hg; heart rate was slightly reduced (2 beats/min), and aortic mean pressure and pulse pressure (6 studies) were unchanged. The maximal rate of rise of aortic pressure showed no consistent trends. Forearm blood flow (30 studies) fell with the onset of lower body negative pressure and reached 67% of the control value by -20 mm Hg. Splanchnic blood flow (14 studies) was significantly reduced by -7 mm Hg and fell to 89% of control by -20 mm Hg. During the range from -20 to -50 mm Hg, right atrial pressure continued to fall. Aortic mean pressure fell slightly or was unchanged in four subjects and fell dramatically at -35 mm Hg in 2 subjects. Aortic pulse pressure began to fall at about -20 mm Hg and fell linearly thereafter. Heart rate paralleled aortic pulse pressure (r=-0.86 to -0.93). Forearm blood flow fell to 55% and splanchnic blood flow fell to 65% of control at -50 mm Hg. Thus, significant vasoconstriction occurred without measurable change in arterial blood pressure. It is concluded that low pressure baroreceptors, presumably in the cardiopulmonary region, initiate splanchnic and forearm vasoconstriction with more pronounced vasoconstriction occurring in the forearm.