2.2 Cerebral Physiology, ICP, CBF, and Autoregulation

Cerebral Physiology, ICP, CBF, and Autoregulation

Quick Answer: CPP = MAP − ICP. Adult ICP is typically about 7–15 mmHg. Global CBF is about 50 mL/100 g/min. Intact autoregulation holds CBF nearly constant between roughly MAP 50–150 mmHg and shifts toward higher pressures in chronic hypertension. Hyperventilation vasoconstricts and is a bridge only. Cushing triad is late.

Every ICP crisis on this examination is a volume problem in a closed box. Independent OpenExamPrep material here covers cerebral physiology, intracranial pressure, cerebral blood flow, and autoregulation as listed among Principles of neurocritical care in the ABPN Content Specifications.

Monro–Kellie doctrine

The Monro–Kellie relationship states that the intracranial compartment is nearly incompressible. Brain tissue, blood (arterial plus venous), and CSF share a fixed volume inside the skull and rigid dura. If one compartment grows—hematoma, edema, hyperemia, or trapped CSF—another must shrink or ICP rises. Early compensation is displacement of CSF into the spinal subarachnoid space and venous-blood outflow. Once those buffers are exhausted, small added volume produces a large ICP jump. That is the pressure–volume curve: the flat (compliant) portion, then an exponential steep limb where compliance (Δvolume / Δpressure) collapses.

Normal adult ICP in the supine position is typically about 7–15 mmHg (many references phrase the same window as 5–15 mmHg). The Brain Trauma Foundation 4th edition treat threshold used when this physiology is applied to TBI is ICP 22 mmHg, with a CPP target of 60–70 mmHg—not because 21 mmHg is "healthy," but because that is the intervention cut in that guideline. Driving CPP above 70 mmHg with heavy fluid and pressors has been associated with more ARDS, so more is not automatically better.

CPP arithmetic you will be asked to do

Cerebral perfusion pressure is:

CPP = MAP − ICP

(If central venous pressure exceeds ICP, the downstream pressure is CVP rather than ICP; in most neuro-ICU stems ICP is the relevant downstream pressure.)

MAP = diastolic BP + 1/3 (systolic − diastolic).

Worked example 1. BP 140/80. MAP = 80 + (60 / 3) = 100 mmHg. ICP 18. CPP = 82 mmHg.

Worked example 2. BP 110/70. MAP = 70 + (40 / 3) = 83 mmHg. ICP 28. CPP = 55 mmHg. If the goal band is 60–70 mmHg, this patient is underperfused. Raising MAP with a vasopressor without lowering a herniating ICP is incomplete; osmotherapy, CSF drainage, and airway/ventilator toilet may drop ICP faster than a pure pressor strategy.

Worked example 3. ICP 24 mmHg, BP 160/90. MAP = 90 + (70 / 3) ≈ 113. CPP ≈ 89. Do not celebrate the CPP and ignore ICP 24 mmHg if you are in a treat-ICP protocol. High CPP does not cancel a mass-effect emergency.

Cushing triad—systemic hypertension, bradycardia, and irregular respiration—is a late medullary ischemic response. It is not the screening test for raised ICP. A new blown pupil or a steep rise on an EVD tracing should move you long before the pulse slows.

CBF, CMRO2, and flow–metabolism coupling

Normal global cerebral blood flow (CBF) is about 50 mL/100 g of brain/min (on the order of 15% of cardiac output, roughly 750 mL/min for an adult brain). Gray matter runs higher than white matter. When CBF falls toward 20 mL/100 g/min, electrical failure appears; irreversible injury clusters around 10 mL/100 g/min if that hypoperfusion lasts.

Cerebral metabolic rate of oxygen (CMRO2) is about 3.0–3.5 mL O2/100 g/min. In health, arterioles dilate when metabolism rises (seizure, fever) and constrict when metabolism falls (deep coma, hypothermia), a relationship called flow–metabolism coupling. Fever and convulsive or nonconvulsive seizures raise CMRO2 and can raise CBF and cerebral blood volume, which raises ICP. Cooling lowers CMRO2 by roughly 6–7% per 1 °C, which is why temperature control matters even when you are not inducing hypothermia as a therapy.

Autoregulation: the plateau and the right shift

Autoregulation is the myogenic and metabolic ability of cerebral arterioles to keep CBF nearly constant while MAP (more precisely CPP) varies across a window. In a previously normotensive adult that window is classically about MAP 50–150 mmHg. Below the lower limit, vessels are already dilated; CBF becomes pressure-passive and falls with MAP—ischemia. Above the upper limit, forced dilation produces hyperemia, capillary leak, and edema, the vascular story behind hypertensive encephalopathy.

Chronic hypertension shifts the entire plateau toward higher pressures. A MAP of 65 mmHg that was adequate last decade may now sit below that patient's lower limit. Dropping a long-standing hypertensive ICH patient to a "normal" textbook MAP can cause hypoperfusion even if the number looks pretty on the flowsheet. Acute brain injury, large infarct, SAH, and many anesthetics impair or abolish autoregulation; then every MAP change is a CBF change. Exam stems that say "assume intact autoregulation" want the 50–150 plateau. Stems that say "malignant MCA infarct" want you to assume the plateau is broken.

PaCO2, PaO2, temperature, and viscosity

PaCO2 is the most potent acute chemical controller of CBF. Over the usual clinical range (about 20–80 mmHg), CBF moves roughly 1–2 mL/100 g/min for each 1 mmHg change in PaCO2 (on the order of a few percent per mmHg). Hypercapnia dilates arterioles, increases cerebral blood volume, and raises ICP. Hypocapnia constricts, shrinks blood volume, and lowers ICP. That is why a few minutes of hyperventilation can buy time while you order mannitol, hypertonic saline, CSF drainage, or an OR for a hematoma.

Hyperventilation is a bridge only. The same vasoconstriction that drops ICP can drop CBF below ischemic thresholds. Avoid PaCO2 <25 mmHg. Do not leave a patient at a PaCO2 of 28 mmHg overnight as "ICP prophylaxis." After the crisis, return ventilation toward a normal PaCO2 (~35–45 mmHg) once other ICP measures are in place.

PaO2 has little effect on CBF until hypoxemia is severe (classically PaO2 below about 50–60 mmHg), when hypoxic vasodilation increases blood volume and can raise ICP. Fix the airway and the PaO2; do not treat that ICP spike with still more hyperventilation as the only move.

Viscosity and hematocrit sit in the background. Within a physiologic range, lower viscosity (hemodilution) can raise CBF, but extreme anemia reduces oxygen-carrying capacity. Polycythemia does the opposite. You will not titrate hematocrit solely to "optimize CBF" on this exam, but a stem that pairs a hematocrit of 18% with delayed cerebral ischemia after SAH is asking about delivery of oxygen, not about a second osmotic agent.

Put the levers together for a herniating patient: head-up, neck veins free, PaCO2 briefly lowered as a bridge, osmotherapy, CSF drainage if a ventricle can be tapped, treat fever and seizures that raise CMRO2, and support MAP so CPP does not collapse while ICP is high. Independent practice at /practice/abim-neurocritical-care should test whether you can compute CPP and choose a bridge rather than a lifestyle of hypocapnia.