Cerebral Autoregulation

🔹 Introduction

Cerebral autoregulation (CA) is the brain’s intrinsic ability to maintain relatively constant cerebral blood flow (CBF)despite fluctuations in mean arterial pressure (MAP).

This mechanism is crucial for preserving cerebral perfusion, avoiding ischemia during hypotension, and preventing hyperemia or increased intracranial pressure (ICP) during hypertension.

🔹 Normal Cerebral Blood Flow

• CBF ≈ 50 mL/100 g/min
• Brain weighs ~1.4 kg → Total CBF ≈ 700–900 mL/min
• Brain receives ~15–20% of cardiac output

🔹 Concept of Autoregulation

Autoregulation is the ability of cerebral vessels to vasodilate or vasoconstrict in response to changes in MAP to maintain constant CBF.

🎯 Autoregulatory Plateau (Normal Range)

• MAP 50–150 mmHg (in normotensive individuals)
• Within this range: CBF remains relatively constant

🔸 Beyond the Limits:

• MAP <50 mmHg → Cerebral hypoperfusion → ischemia
• MAP >150 mmHg → Loss of vasoconstriction → hyperperfusion, edema, risk of hemorrhage

🔹 Cerebral Perfusion Pressure (CPP)

CPP=MAP−ICP

• Normal CPP: 60–100 mmHg
• CBF is directly related to CPP, only when autoregulation is impaired

🔹 Mechanisms of Autoregulation

Cerebral autoregulation is multifactorial, involving:

🔸 1. Myogenic Response

• Stretch-sensitive ion channels in vascular smooth muscle
• ↑ MAP → vasoconstriction
• ↓ MAP → vasodilation

🔸 2. Metabolic Regulation

• Local metabolic byproducts (CO₂, H⁺, adenosine) cause vasodilation
• Increased metabolism → ↑ CBF

🔸 3. Neurogenic Factors

• Sympathetic and parasympathetic tone modulates cerebral vascular resistance

🔹 Factors Affecting Autoregulation

🔹 Static vs Dynamic Autoregulation

🔹 Shift in Autoregulation Curve

🔸 Chronic Hypertension

• Rightward shift → Tolerates higher MAP, but ischemia at normal BP

🔸 Head Injury / SAH / Stroke

• Impaired autoregulation → CBF becomes pressure-dependent
• Prone to secondary brain injury with BP changes

🔹 Impact of Anesthetic Agents

🔹 Clinical Relevance in Anesthesia

✅ Neuroanesthesia

• Maintain MAP within autoregulatory limits
• TBI patients often lose autoregulation → need CPP-targeted management

✅ Stroke/SAH

• Avoid hypotension → worsens penumbra ischemia
• Avoid hypertension → risk of rebleed

✅ Anesthesia choice

• Use agents that preserve autoregulation in neurosurgical patients

🔹 Monitoring Autoregulation

• Transcranial Doppler (TCD): Real-time flow velocity changes
• Near-infrared spectroscopy (NIRS): Measures cerebral oxygenation
• PRx (Pressure Reactivity Index): Used in ICU; CPP-guided therapy

🧠 Viva Tip

Q: What happens to autoregulation in traumatic brain injury?

A: It is often impaired. CBF becomes directly pressure-dependent, so hypotension can cause ischemia and hypertension can worsen edema/ICP.

🔍 Summary Table

🔍 References

1. Miller’s Anesthesia, 9th ed. – Ch. on Neurophysiology
2. Cottrell & Young’s Neuroanesthesia
3. BJA Education – Cerebral Autoregulation
4. StatPearls – Cerebral Autoregulation
5. Brash Biophysics of ICP and CBF regulation