Upper Threshold of Cerebral Autoregulation
The upper limit of cerebral autoregulation in normotensive adults is approximately 150 mm Hg mean arterial pressure (MAP), though individual variability exists and this threshold shifts rightward in chronic hypertension. 1, 2
Classical Autoregulation Thresholds
In normotensive adults, cerebral blood flow (CBF) remains constant across a MAP range of approximately 60–150 mm Hg, with 150 mm Hg representing the upper autoregulatory threshold above which CBF increases passively with rising pressure. 1, 2
Above 150 mm Hg MAP, the cerebral vasculature loses its ability to constrict further, leading to hyperperfusion, disruption of the blood-brain barrier, and potential hypertensive encephalopathy. 1, 2
The lower limit of autoregulation averages 60 mm Hg MAP in normotensive adults, though recent monitoring data demonstrate wide interindividual variability ranging from 40–90 mm Hg in adults. 3, 4
Individual Variability and Modern Monitoring
Substantial intersubject variability exists in both the upper and lower limits of autoregulation, challenging the traditional teaching of fixed thresholds at 50 and 150 mm Hg. 3, 4
Near-infrared spectroscopy (NIRS)-based autoregulation monitoring in critically ill patients has documented individualized autoregulation curves with median lower limits of 86.5 mm Hg (IQR 74–93.5) and upper limits of 93.5 mm Hg (IQR 83–99), though these values reflect pathologic states rather than normal physiology. 5
During cardiopulmonary bypass, targeting MAP within individualized autoregulation limits (measured under normocapnic conditions before bypass) is recommended when technical expertise is available, rather than relying on population-based thresholds. 6
Chronic Hypertension and Autoregulatory Shift
In chronic hypertension, the entire autoregulatory curve shifts rightward toward higher arterial pressures due to structural remodeling and functional changes in cerebral resistance vessels. 1, 2
This rightward shift means that chronically hypertensive patients tolerate acute blood pressure elevations better but have impaired tolerance to acute hypotension, as their lower limit may be 80–90 mm Hg MAP rather than 60 mm Hg. 1, 2
The upper limit in chronic hypertension may extend beyond 150 mm Hg MAP, providing relative protection against hypertensive encephalopathy at pressures that would cause breakthrough hyperperfusion in normotensive individuals. 1, 2
These adaptive changes are partially reversible after chronic antihypertensive treatment, though the time course of reversal varies. 1
Mechanisms of Autoregulation
The upper autoregulatory threshold reflects the maximum capacity for myogenic vasoconstriction in response to rising transmural pressure, beyond which passive distension occurs. 1, 2
Both myogenic (pressure-dependent smooth muscle contraction) and metabolic mechanisms contribute to autoregulation, with modulation by sympathetic nervous activity and the renin-angiotensin system. 1, 2
Sympathetic stimulation shifts the autoregulatory curve toward higher pressures, effectively raising the upper limit, while renin-angiotensin antagonism shifts it toward lower pressures. 1
Clinical Implications for Blood Pressure Management
Acute Ischemic Stroke (No Reperfusion Therapy)
Permissive hypertension up to 220/120 mm Hg (MAP ~153 mm Hg) is recommended for the first 48–72 hours, as autoregulation is impaired in the ischemic penumbra and systemic pressure supports collateral flow. 7
This strategy deliberately operates near or above the traditional upper autoregulatory limit because the ischemic brain has lost normal autoregulation and requires pressure-dependent perfusion. 7
Intracerebral Hemorrhage
For spontaneous ICH with systolic BP 150–220 mm Hg, target 130–150 mm Hg systolic (approximately MAP 90–110 mm Hg) within 1 hour, well below the upper autoregulatory threshold to minimize hematoma expansion. 8
When elevated intracranial pressure is present, maintain cerebral perfusion pressure 60–80 mm Hg even while lowering systemic MAP. 6, 8
Cardiopulmonary Bypass
Maintain MAP 50–80 mm Hg during bypass, avoiding MAP >80 mm Hg as vasopressors to increase pressure above this range are not recommended (Class III). 6
This conservative upper target of 80 mm Hg during bypass reflects concerns about impaired autoregulation under non-physiologic perfusion conditions. 6
Common Pitfalls
Assuming fixed thresholds of 60 and 150 mm Hg apply universally ignores individual variability, chronic hypertension, and pathologic states that alter autoregulation. 3, 1, 2
Failing to recognize that autoregulation is lost or impaired in acute stroke, severe head injury, space-occupying lesions, and neonatal asphyxia leaves the brain vulnerable to pressure-passive injury at both extremes. 2
In chronic hypertension, acutely lowering MAP to "normal" levels (e.g., 70–80 mm Hg) may cause cerebral hypoperfusion because the patient's lower autoregulatory limit has shifted rightward. 1, 2
Hyperventilatory hypocapnia can restore autoregulation in some disease states where it is otherwise impaired, providing a potential rescue strategy. 2