Human Splanchnic and Forearm Vasoconstrictor Responses to Reductions of Right Atrial and Aortic Pressures

Abstract

Regional vascular responses to gradual reductions in right atrial pressure and aortic pressure were investigated in nine 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 two 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. We concluded that low-pressure baroreceptors, presumably in the cardiopulmonary region, initiate splanchnic and forearm vasoconstriction with more pronounced vasoconstriction occurring in the forearm.KEYS WORDS peripheral circulation heart rate lower body negative pressure blood pressure regulation forearm blood flow splanchnic blood flow low-pressure baroreceptors • A major cardiovascular adjustment to moderate hemorrhage is increased sympathetic outflow to the heart and various vascular beds. Traditionally this adjustment has been associated with carotid sinus and aortic baroreceptors (1) which clearly play important roles in regulating arterial blood pressure. More recently, however, stretch receptors in the cardiopulmonary region (low-pressure baroreceptors) have been implicated in the mediation of reflex responses to hemorrhage in dogs, cats, and rabbits (2-4). In humans, simulation of hemorrhage by mild degrees of lower body negative pressure can evoke marked forearm vasoconstriction without significant changes in heart rate, aortic mean pressure, aortic pulse pressure, or maximal rate of rise of aortic blood pressure (dP/dt max) (5). It appears that reduced pressures in the cardiopulmonary region accompanying lower body negative pressure must be the stimulus for the reflex vasoconstriction. Previous studies in man (6, 7) have implicated low-pressure baroreceptors in the reflex release of forearm vasoconstrictor tone accompanying increases in thoracic blood volume induced by postural or respiratory maneuvers.We attempted to determine whether the vasoconstriction seen in forearms during a mild degree of lower body negative pressure, which was insufficient to measurably affect arterial blood pressure, was evide...

Cardiovascular Adjustments to Gravitational Stress

“…Applying LBNP at -40 to -50 mmHg causes a decrease in central venous pressure similar to that during a change from the supine to upright position (37, 167,244,250,355).…”

“…The effects of postural changes on blood flow to the resting arm and leg are directionally similar to the changes in overall systemic flow . Increasing degrees of venous pooling during LBNP cause a gradual and sustained reduction of forearm flow (167,231,271,362). The relative importance of muscular and cutaneous vasoconstriction is controversial.…”

“…Progressive peripheral pooling during LBNP gradually reduces splanchnic flow. Flow rates at -40 mmHg are less than 70% of control (1,167) .…”

“…Peripheral and splanchnic arteriolar constriction compensate for the decrease in systemic flow. Progressive peripheral venous pooling eventually causes tachycardia, and the heart rate changes are closely linked to changes in arterial pulse pressure (167 ........ 100 60 dial contractile state does not change until tachycardia develops. Thus the primary cardiac response is a negative Starling effect.…”

“…Vasoconstriction is manifest before heart rate or cono -20 -40 mmHg o -20 -40 mmHg tractility is measurably affected. Forearm flow and conductance decrease significantly before splanchnic flow is affected (1,167). The differential effects on limb flow and splanchnic flow are probably related to a predominance of low-pressure baroceptor effects at Inild degrees of LBNP and increasing effects on t he carotid sinus with progressive pooling and decreasing arterial pressures and pulse pressure (1,167) .…”