factors influencing the autoregulation of the cerebral...

Factors Influencing the Autoregulation of the Cerebral Blood Flow During Hypotension and Hypertension

GLENN W. KINDT, M.D., JULIAN R. YOUMANS, M.D., AND OTMAR ALBRAND, M.D. Medical College of South Carolina, Charleston, South Carolina

I T IS well known tha t during ranges of hypotens ion and hyper tension an auto- regulation of h u m a n cerebral blood flow

occurs, which keeps the cerebral perfusion relatively constant , s,19,32 This autoregulat ion accompanies such diverse conditions as hy- pertension due to toxemia of pregnancy, 2~ essential hypertension, 9,t3 the adnlinistration of norepinephrine, '6,29 and modera te hypo- tension due to spinal anesthesia '7 or arfonad administrat ion. 25,~[~ In the in tact cerebro- vascular system, autoregulat ion fails only with marked hypotension. 5,6

Despi te the autoregula t ion phenomenon, clinical observat ions have shown tha t it is very impor t an t to nminta in an elevated sys- temic blood pressure in pat ients with certain carotid and cerebral arterial diseases. 1 Far- ha t 4 has repor ted cases in which the neuro- logical deficit developing af ter aneurysnml surgery, angiography, and cranio tomy for tumor was improved by elevating the sys- temic blood pressures above normal. I t is reasonable to assume tha t these pat ients had an altered and p robab ly a compromised cerebral circulation. The i r improvement with induced hyper tens ion suggests t ha t cerebral blood flow was increased. I f this were true, it would app ea r t ha t the autoreg- ulation of cerebral blood flow, which has been proven so often in the normal situation, is not effective when the arterial tree is com- promised. Our exper iments have been di- rected toward a s tudy of this autoregulat ion phenomenon under conditions of a normal and an impaired arterial tree.

Ma te r i a l s and M e t h o d s

We chose goats as the experimental ani- nmls because their cerebrovascular sys tem is favorable for the s tudy of gross blood flow to the brain with e lectromagnet ic flow probes. The ver tebra l arteries are quite small and

Received for publication July 29, 1966. Revision received October 21, 1966.

ineffective. 2 The carotid arteries supply most of the blood to the brain, and c lamping theni above the mandibular and occipital arteries causes a lmost immediate death. 31 Thus a nearly total blood supply to the brain is con- tained within the easily-accessible carotid arteries, and makes theln a sat isfactory channel for the s tudy of changes in over-all cerebral blood flow.

Repea ted observat ions were made on 18 goats weighing 16 to 44 kg. After the animals had been heparinized and anesthet ized with sodium seeobarbital, a t r aeheos tomy was performed and breathing was controlled with a Bird respirator. The left femoral a r te ry was cannulated for cons tan t blood-pressure recordings using a S t a t h a m strain gauge. The right femoral a r te ry and vein were also ean- nulated so tha t blood could be removed or reinfused as needed during the hypotens ive study. Cisternal pressure m e a s u r e m e n t s were made through an 18-gauge spinal needle connected to a S t a t h a m strain guage. The carotid arteries were exposed f rom the bi- furcation of the braehioeephalie a r te ry to the base of the skull. The carot id sinuses were denervated and all branches of the carotid arteries were l igated and divided. S ta tham Medicon flow probes were placed on each carotid ar tery and connected to a dual- channel Medicon flow meter . All the mea- surements were recorded with a Honeywel l eight-channel Visicorder.

Dur ing the first port ion of the exper iment the animals were normotensive. One carotid a r te ry was occluded and the blood flow in the opposite carotid ar tery was recorded. This was repeated with the occluded and moni- tored vessels reversed. In the second pa r t of the study, each animal was made hyper- tensive with norepinephrine, and hypoten- sive by allowing it to bleed f rom the femoral ar tery. Blood flow measurements were re- corded with bo th carot id arteries open and with a l ternate occlusion of each carotid a r te ry during the blood pressure al terat ions.

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Two pa t ien t s who were having carot id- a r t e r y blood-flow studies were also observed in a l imited manner during hypotension and hyper tens ion.

Resul t s

Animal Studies. When one carot id a r te ry was occluded, there was the expected p rompt increase in blood flow through the eontra- l a te ra l caro t id a r te ry (Fig. 1). There was l i t t le change in the systemic blood pressure. The average flow rate in each carot id a r t e ry wi th bo th carot ids open was 9~ ml per

minute. The average increase in blood flow in the ipsi la teral caro t id a r t e ry following con- t ra la te ra l carot id occlusion was 48.4%. Single carot id occlusion resulted in a ~6% re- duct ion in the to ta l b lood flow to the head as compared to t h a t wi th both carot ids open. The eis ternal CSF pressure remained within normal l imits t h roughou t the studies.

When hyper tens ion was induced by in- fusion of norepinephr ine , with both carotid ar ter ies open (Fig. ~), tile carot id a r te ry flow rate remained re la t ive ly constant or fell sl ightly, despi te the marked increases in the

FIG. 1. A recording of the effect of unilateral occlusion of each carotid artery. A prompt increase in eontralateral flow occurs as the flow in the occluded carotid falls to its respective baseline. (BP--femoral artery blood pressure; LCF--left carotid blood flow; RCF--Right carotid blood flow.)

FIG. 2. A recording of the effect on rigilt (RCF) and left carotid artery flow (LCF) with blood pressure (BP) eleva- tion. In this instance there is a slight decrease in carotid flow as the blood pressure is elevated.