THE EFFECTS OF TWENTY DEGREE HEAD-UP TILT UPON THE CEREBRAL CIRCULATION OF PATIENTS WITH ARTERIAL HYPERTENSION BEFORE AND AFTER SYMPATHECTOMY 1

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Abstract
Cerebral blood flow was measured (N2O technic) before and after 20 min. in a 20[degree] head-up tilt in order to observe the effect of a slight elevation of the head and shoulders upon the cerebral circulation of hypertensive patients following sympathectomy. This slight degree of tilt could be tolerated for 20 min. as early as 2 weeks after sympathectomy. Measurements were made in 15 patients who had bilateral thorocolumbar sympathectomy and in 18 with essential hypertension. These results were compared with observations in normotensive patients previously reported (Jour. Appl. Physiol. 2(6): 317. 1949). "Effective" cerebral arterial pressure was reduced in all these groups by this slight elevation; cerebral blood flow was not reduced; therefore, cerebral vascular resistance was reduced. Unexplained was the increase in O2 uptake of the brain when patients with essential hypertension were tilted. Symptoms of cerebral anoxia during tilt were observed in these patients but not in the sympathectomy group in whom the O2 uptake was unchanged. O2 content of jugular venous blood was reduced in both groups while semi-erect. This suggests that, although the cerebral vascular resistance was diminished, the increased tone was only partially relaxed. Failure of the cerebral vasculature to relax sufficiently to keep jugular O2 content unchanged might be considered as evidence suggesting the presence of significant amts. of pressor substances even in the patients who have had a sympathectomy. One patient with moderate hypertension was studied with this test before and 2 weeks, 14 mos., and 24 mos. after sympathectomy. Postural hypotension of a moderate degree with this test was observed over the 2-yr. period. In the most recent test cerebral blood flow was unchanged when "effective" cerebral arterial pressure was reduced 38%, cerebrovascular resistance having decreased equally with the arterial pressure. Thus this patient appeared to be able to dilate the cerebral vasculature sufficiently to keep blood flow unchanged but only at the cost of a greater extraction of O2 from the jugular venous blood.