Increase in transcranial Doppler pulsatility index does not indicate the lower limit of cerebral autoregulation

Transcranial Doppler pulsatility index was reported clinically to increase when cerebral perfusion pressure decreased, hypothetically marking the lower limit of cerebral autoregulation. We sought to investigate the relationship between pulsatility index, cerenbrovascular resistance, and cerebral perfusion pressure in various states of autoregulation in an animal model of moderate intracranial hypertension.

Eight New-Zealand White Rabbits were studied with basilar artery blood flow velocity (ultrasound Doppler) and cortical blood flow (laser Doppler) monitored continuously during subarachnoid saline infusion to increase intracranial pressure (< 55 mm Hg). Four animals demonstrated a stable cortical blood flow, and four demonstrated decreasing blood flow when cerebral perfusion pressure decreased.

Pulsatility index showed the same pattern of increase when cerebral perfusion pressure decreased, independent on whether cortical blood flow was stable or falling. The percentage rate of increase in the pulsatility index was not different in autoregulating and non autoregulating animals. The rate of decrease in cerebrovascular resistance was significantly lower (p < 0.05) in non-autoregulating than in autoregulating animals.

The increase in transcranial Doppler pulsatility index when cerebral perfusion pressure falls cannot be interpreted as a phenomenon able to mark the lower limit of cerebral autoregulation.