RELATIONSHIP BETWEEN PLASMA-VOLUME, CAROTID BARORECEPTOR SENSITIVITY AND ORTHOSTATIC TOLERANCE

1. Studies were carried out on 43 otherwise healthy patients referred for investigation for attacks of syncope of unknown cause and on six h ealthy volunteers. 2. Plasma volume was determined by Evans Blue dye d ilution and blood volume was estimated using haematocrit. Carotid baro receptor sensitivity was determined from the changes in pulse interval in response to subatmospheric pressures applied to the neck overlying the carotid sinuses, and orthostatic tolerance was assessed as the ti me to presyncope in a test of head-up tilt, followed by the addition o f graded lower body suction. 3. Eight patients and one volunteer faint ed during head-up tilt alone, 23 patients and two volunteers fainted d uring tilt with lower body suction at -20 mmHg and 12 patients and thr ee volunteers either fainted during suction at -40 mmHg or tolerated t he entire procedure. 4. Although plasma and blood volumes were higher in males than females, the values normalized for either body weight or for calculated lean body mass were not different between male and fem ale patients and asymptomatic volunteers. The subjects showing the gre atest resistance to syncope were found to have significantly larger pl asma and blood volumes (P<0.0001) and significantly smaller barorecept or sensitivities (P<0.0002) than those who fainted earlier. 5. There w as a highly significant positive correlation in ah subjects between or thostatic tolerance (time to onset of syncope) and plasma and blood vo lumes (r=0.60, P<0.0001; r=0.53, P<0.0002), and highly significant neg ative correlations between time to syncope and baroreceptor sensitivit y (r=-0.61, P<0.0001) and between baroreceptor sensitivity and plasma and blood volumes (r=-0.54, P<0.0001; r=-0.31, P<0.03). 6. These resul ts show that tolerance to orthostatic stress is favoured by large plas ma and blood volumes and a low sensitivity of the carotid baroreceptor -heart rate reflex.