The effects of 1-hydrazinophthalazine on cerebral blood flow, vascular resistance, oxygen uptake and jugular oxygen tension in hypertensive subjects

Cerebrovascular aspects of antihypertensive treatment

Although the treatment of hypertension clearly benefits the brain in most patients, there are, however, unfortunate exceptions. Overzealous blood pressure lowering especially, and sometimes conservative blood pressure lowering, occasionally compromise the supply of blood to the brain to such an extent that neurological dysfunction or death results. Despite an awareness of this problem for more than a decade, the number of reports of such cases is increasing. An understanding of the problem requires detailed knowledge of both the pathophysiology of the cerebral circulation in hypertension and the cerebrovascular effects of antihypertensive drugs. If antihypertensive treatment, in particular emergency blood pressure lowering, is to always be safe, thought must be given to the cerebrovascular effects of the drugs to be used. This topic is discussed in relation to the observed (i.e., experimentally determined) and inferred (i.e., from clinical observation) effects of antihypertensive drugs and treatment on the cerebral circulation, especially with regard to autoregulation of cerebral blood flow.

Dihydralazine induces marked cerebral vasodilation in man

Dihydralazine is widely used for acute control of hypertension. In experimental studies it seems to dilate cerebral resistance vessels and increase intracranial pressure. However, the effect on cerebral blood flow (CBF) in man has been little studied. Measurements of CBF were performed with the i.v. xenon-133 technique in seven young, normotensive volunteers before and 15, 60 and 180 min after 6.25 mg i.v. dihydralazine, corresponding approximately to 0.1 mg kg-1 body weight. For comparison the CBF reactivity to inhalation of 5% CO2 in air was investigated. Dihydralazine increased CBF throughout the period of study, in median 16, 27 and 23% at the three periods of measurements, respectively. The arterial blood pressure remained unchanged, whereas heart rate increased significantly. During CO2 inhalation, CBF increased on average 29%. Thus, the cerebral vasodilation exerted by a small i.v. dose of dihydralazine was of the same order of magnitude as the effect of 5% CO2 inhalation. These results in normal subjects should be extrapolated to diseased persons only with extreme caution. Still, the very marked and long lasting vasodilation observed suggests that dihydralazine, from a theoretical point of view, in certain clinical situations may be harmful.

Autoregulation of cerebral blood flow. Studies during drug-induced hypertension in normal subjects and in patients with cerebral vascular diseases

Autoregulation of cerebral blood flow (constant perfusion despite variations in blood pressure) was studied in 15 control subjects and in 24 patients with cerebral vascular lesions. Thirteen patients were studied within the first 2 days of an episode of cerebral ischemia ("acute" group); 11 were studied from 15 days to several months after an ischemic episode ("chronic" group). Measurements of regional cerebral blood flow (rCBF) were made by recording extracranially in four to six locations the clearance of 85 Kr dissolved in saline and injected into the internal carotid artery. Blood flow values were calculated with a digital computer, using a compartmental analysis of the clearance curves. After measuring rCBF in the resting state, the blood pressure was increased by 30 to 40 mm Hg by the intravenous infusion of angiotensin amide (Hypertensin, Ciba). While the blood pressure was kept constant at the new level, a second series of measurements was made. Autoregulation was present in all normal subjects: no significant differences were found in average or compartmental flow rates between values obtained at normal and at hypertensive levels of blood pressure.

Loss of autoregulation was demonstrated in 10 of 13 patients of the "acute" group and in two of 11 patients of the "chronic" group. These data support previous observations that autoregulation is impaired during the first few days after an ischemic episode. The three patients of the "acute" group with a preserved autoregulatory response to hypertension had only transient ischemic attacks. In each of the other subjects of this group loss of autoregulation was found in one to three different regions of the brain. Our results suggest that regional loss of autoregulation is found frequently near the periphery of the ischemic brain. Impairment of autoregulation is one of the types of derangement of cerebral circulation that occurs after an ischemic episode.