CHRONIC CEREBRAL HYPOTENSION INDUCES A DOWNWARD SHIFT OF THE CEREBRALAUTOREGULATION - A HYPOTHESIS BASED ON TCD AND OPG-GEE STUDIES IN AMBULATORY PATIENTS WITH OCCLUSIVE CEREBROVASCULAR-DISEASE

In order to establish whether or not the cerebral autoregulation is st ill effective at blood pressure rates in those who experience a chroni c cerebral hypotension 37 patients were subjected to noninvasive CO2 e nhanced transcranial Doppler (TCD) and ocular pneumoplethysmographic ( OPG-GEE) studies of the middle cerebral and ophthalmic arteries. The p atients demonstrating a combination of a unilateral patent internal ca rotid artery (ICA diameter reduction less than 49%) and a contra-later al ICA occlusion based on duplex scan examinations were examined. The cerebral blood flow velocities at normocapnia (BFV) and pulsatility in dices IPI) were measured by TCD, An equivalent of the reserve capacity of the cerebral autoregulation was estimated by quantification of the CO2 enhanced cerebral vasomotor reactivity (VMR). Cerebral blood pres sure equivalents such as the OSP/BSP ratio and the ophthalmo-brachial systolic pressure (OBSP) index were estimated by measuring the ocular systolic blood pressure (OSP) and the systemic systolic blood pressure (BSP). The TCD and OPG-GEE data are both in the normal range distally to show a significant reduction in both OSP/BSP ratio's and OBSP indi ces. if the VMR was in the normal range, distally to an occluded ICA ( n=25), the pressure drop was mild and the BFV at rest was in the norma l range. Nevertheless, the waveforms were more damped as seen from the reduction in pulsatility indices but the reduction did not reach stat istical significance when compared to the PI values distally to a pate nt ICA. When the VMR was reduced distal to an occluded (ICA (n=12) bot h TCD and OPG-CEE data showed a severe reduction in cerebral blood pre ssure equivalents, BFV's and PI's when compared to the patent side. It can be concluded that the collateral circulation in most patients is not adequate to restore a completely normal blood pressure in the cere bral territory distal to the occluded ICA. If the pressure drop is sev ere enough, the cerebral autoregulation is exhausted and the VMR is su bsequently reduced. However, if the pressure drop is mild the VMR rema ins within normal limits. These findings indicate that the cerebral au toregulation is effective in patients who experience a mild chronic hy potension.