Physiological and pathophysiological cerebrovascular regulation monitored by transcranial doppler

Abstract

Transcranial Doppler ultrasonography (TCD) became available in the middle of the 1980s as a new technique for examining cerebral circulation. With this technique it is possible to measure the velocity of blood flow in major cerebral arteries. The purpose of this thesis was to use this method for evaluating changes in cerebral perfusion m physiological and patho-physiological conditions.

The observation that some previously healthy young persons had developed a stroke in association to physical exercise (effort-stroke) inspired Paper 1. In that study, young and healthy subjects performed dynamic cycle exercises with increasing loads during simultaneous recordings of blood flow velocity in the middle cerebral artery (VMCA). We found unexpected changes in VMCA during simultaneous recordings of increases in heart rate and systemic blood pressure. For this reason we studied the effect of increases in blood pressure induced by sympathetic stimulation (Paper II) and by pain (Paper III). Some of the questions arising from Paper I could then be answered, but several new questions arose.

To answer these, we began Paper IV, which included a new series of dynamic physical exercises as well as a modified experimental protocol with additional parameters. In that study, we monitored VMCA and ipsilateral blood flow in the common and internal carotid arteries and end-tidal pC02. To investigate further some of the circulatory mechanisms involved in effort-stroke, we also evaluated patients with heart failure while they were performing physical exercises (Paper V). The studies showed that TCD is a useful method for monitoring changes in cerebral blood flow. No other non-invasive cerebro-circulatory technique can register such changes at intervals of less than a few minutes. If the diameter of the examined artery remains unchanged, the changes in velocity registered will be proportional to the changes in volume flow. One of the studies showed that changes in blood velocity in the middle cerebral artery satisfactorily reflect changes in volume flow during dynamic physical exercise. The study also indicated that no changes in the diameter of the recorded artery occurred during the exercise. However, we also found that the artery can change its diameter. This was observed only during pathophysiological conditions, such as attacks of Horton's headache and unphysiological situations, like those following the administration of nitroglycerin and during peroperative direct electrical sympathetic stimulation at the T2-level. Cerebral blood flow (CBF) has been traditionally considered to remain constant at a level of 50 ml per 100g brain tissue per minute. Our studies suggest that CBF increases by at least 10 - 15% during moderate dynamic physical exercise. This implies that the CBF in healthy humans is not, and does not need to be, so precisely regulated as was previously thought.