HEMODYNAMIC-RESPONSE TO VASOPRESSIN IN DEHYDRATED HUMAN-SUBJECTS

Background: Despite the known potent vasoconstrictor effects of vasopr essin, the role of this hormone in the maintenance of blood pressure i s incompletely understood. In studies performed in animals with increa sed plasma vasopressin concentrations, several complex cardiovascular effects have been noted, including decreases in heart rate and cardiac output, which may account for a lack of effect on arterial pressure d espite the vasopressin-induced increase in total peripheral resistance . Only a few studies have been done to assess the cardiovascular effec ts of vasopressin in human subjects, and most of these have been limit ed to measurement of heart rate and arterial pressure only. The presen t study was designed to identify more fully the cardiovascular effects of vasopressin when plasma vasopressin concentrations are increased b y osmotic stimulation without the superimposition of major nonosmotic stimuli associated with severe volume depletion. Method: Studies were performed on 11 normal human subjects in supine and erect posture befo re and after 24 hours of fluid deprivation, and following administrati on of a selective V1 receptor antagonist, [d(CH2)5Tyr(RIE)]AVP, after dehydration. Cardiovascular parameters were measured noninvasively by thoracic electrical bioimpedance cardiography and blood samples for me asurements of plasma concentrations of vasopressin and other hormones affected by dehydration and differences in posture were collected for subsequent analysis. Results: After 24 hours of fluid restriction, pla sma osmolality was increased from 287 +/- 0.9 to 294 +/- 0.7 mosm/kg H 20 and plasma vasopressin concentrations (Pavp) were increased in both supine and erect posture. Mean arterial (MAP) and systolic blood pres sure (SBP) were reduced by fluid restriction but were higher in erect than in supine posture both before and after fluid restriction, Heart rate (HR), diastolic blood pressure (DBP), and systemic vascular resis tance (SVRI) were also higher in erect than in supine posture, while c ardiac index (CI), stroke index (SI), end-diastolic index (EDI), and a n index of total thoracic fluid content (TFC) were all reduced in erec t posture, both before and after dehydration. Plasma renin activity (P RA) and plasma norepinephrine concentrations (Pne) were increased in e rect posture, both before and after dehydration, but there was no effe ct of erect posture on plasma vasopressin concentrations (Pavp), eithe r before or after dehydration. Administration of the V1 receptor antag onist after dehydration had no effect on hemodynamic parameters other than small reductions in DBP and cardiac preload, Conclusion: It is co ncluded from these studies that small increases in Pavp associated wit h moderate dehydration do not play a role in the maintenance of arteri al pressure in normal human subjects in either supine or erect posture .