Transcranial Doppler assessment of the lower cerebral autoregulatory threshold

Continuous transcranial Doppler ultrasonography of the middle cerebral arte ry (TCD-MCA) has been proposed as a method of identifying the lower cerebra l autoregulatory threshold. This study investigated the relationship betwee n continuous TCD-MCA and cerebral blood flow (CBF) in sheep Arterial blood pressure intracranial pressure, CBF and left TCD-MCA were measured in 12 an aesthetized and ventilated merino sheep. Cerebral perfusion pressure (CPP) was reduced by haemorrhagic hypotension. Measurements were recorded continu ously and breakpoint thresholds calculated by an analysis of variance. The TCD-MCA systolic velocity breakpoint (50 +/- 1.5 mmHg) did not significantl y differ from the lower limit of autoregulation, identified by the CBF brea kpoint (50 +/- 1.8 mmHg). The TCD-MCA diastolic velocity breakpoint occurre d at a significantly higher level of CPP (64 +/- 2 mmHg) (P < 0.01). The re lationship between TCD-MCA flow velocity and CBF thresholds has been descri bed. Early divergence of flow velocity may represent a compensatory mechani sm to maintain CBF.