The commonly quoted lower limit of cerebral autoregulation of 50 - 60 mmHg (mean arterial pressure, MAP) may be wrong. In a presentation to the Society of Neurosurgical Anesthesiology and Critical Care (October, 2007, San Francisco), John Drummond presented his review of about six studies of human cerebral autoregulation from which he has concluded that the lower limit of autoregulation in humans is at the lowest 70 mmHg and that it may be even higher (see Drummond JC, The lower limit of autoregulation: time to revise our thinking? Anesthesiology 1997;86:1431-1433). His chapter on "Cerebral Physiology and the Effects of Anesthetics and Technique" in Miller RD (ed): Miller's Anesthesiology, sixth edition, 2005 page 817 has been rewritten to reflect his current thinking.
Do not confuse the lower limit of autoregulation with the ischemic threshold. The cerebral blood flow (CBF) at which the brain becomes ischemic is considerably lower than the lower limit of autoregulation. However once the lower limit of autoregulation is reached, CBF falls as MAP is lowered further. Remember also that individual patients follow their own unique autoregulation curve and not the ideal. The untreated hypertensive patient for example typically has a lower limit of autoregulation that is much higher than a normotensive patient.
As an example, Larsen et al studied two female and 10 male healthy volunteers aged 21- 61 years (median 30) to determine the lower limit of cerebral autoregulation by both a Fick technique and by V(mean) as determined by transcranial Doppler (TCD) insonation of the middle cerebral artery. They found that the mean value of the lower limit of autoregulation was 79 mmHg by Fick and 91 mmHg by TCD. Of greater interest was the range of values, 53 - 113 mmHg by Fick and 41 - 108 by TCD (Transcranial Doppler is valid for determination of the lower limit of cerebral blood flow autoregulation. Stroke 1994;25:1985-1988). Considering the consequences of cerebral ischemia, it would seem prudent to be cognizant of the latest thinking with respect to cerebral autoregulation.
David S. Smith, M.D., Ph.D.