✓ Application of Burton's concept of the critical closing pressure to experimental data on brain-blood flow in the monkey suggests that perfusion pressure, not vascular bed resistance, is the primary variable affecting cerebral blood flow. Perfusion pressure for the cerebral circulation is the mean arterial pressure minus the critical closing pressure (MAP — CCP). Vasomotor tone and intracranial pressure are the major determinants of the critical closing pressure. Changes in either of these variables, therefore, affect perfusion pressure and flow. Data on brain-blood flow at fixed vasomotor tone obtained over wide pressure ranges show little change in vascular bed resistance despite significant changes in flow. The diameter of resistance vessels probably does not change significantly throughout the normal physiological range of cerebral blood flow. The limits of the critical closing pressure in the anesthetized monkey are from 10 to 95 mm Hg. Using these limits, and beginning with the average values for MAP and CCP in 11 awake monkeys breathing room air, the authors present theoretical flow curves in response to changes in intracranial pressure and mean arterial pressure that closely approximate the data reported in man.
Four examples wherein the critical closing pressure was measured directly. Reversal of flow occurs when the diastolic part of the above DPFR crosses the vertical zero-flow lines. The CCP2 values between 60 and 90 mm Hg are seen. Case 1 is from Early, et al.,2 and is in an awake, sitting, hyperventilated animal. The following values apply to animals 2, 3, and 4.
DPFR's in four anesthetized animals to show range of extrapolated critical closing pressures. Central operating points are shown. Each DPFR represents data obtained between a 2.5- to 8-second period. The limits of arterial blood gas determinations are as follows.
PaO2 (mm Hg)
PaCO2 (mm Hg)
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