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ICP Physiology for Anesthesiology: CPP, Monro–Kellie, Herniation [ Medical Education ](https://mdster.com/blog?category=medical-education) ICP Physiology for Anesthesiology: CPP, Monro–Kellie, Herniation ================================================================== A high-yield clinical guide to intracranial compliance, perfusion pressure, and early recognition of impending brain catastrophe in the OR and ICU [ MDster Editorial Team ](https://mdster.com/about) · Jul 05, 2026 · 8 min read · 146 [ Reviewed by Dr. Ali Ragab, MBBCH, MSc, MCAI ](https://mdster.com/medical-reviewers/dr-ali-ragab) [Editorial Policy](https://mdster.com/editorial-policy) | [Corrections Policy](https://mdster.com/corrections) [ Board Review ](https://mdster.com/blog?tag=board-review) [ Anesthesiology ](https://mdster.com/blog?tag=anesthesiology) [ Critical Care ](https://mdster.com/blog?tag=critical-care) [ Neuroanesthesia ](https://mdster.com/blog?tag=neuroanesthesia) [ Cerebral Physiology ](https://mdster.com/blog?tag=cerebral-physiology) ![ICP Physiology for Anesthesiology: CPP, Monro–Kellie, Herniation](https://mdster.com/storage/blog/images/icp-physiology-for-anesthesiology-cpp-monro-kellie-herniation.jpg) Share this article Share this post On this page 1. [ Start with the right mental model ](#start-with-the-right-mental-model) 2. [ Monro–Kellie is the frame ](#monro-kellie-is-the-frame) 3. [ Compliance is easy to worsen in the OR ](#compliance-is-easy-to-worsen-in-the-or) 4. [ CPP is the number the neuron sees ](#cpp-is-the-number-the-neuron-sees) 5. [ Herniation: recognize patterns before the scan is back ](#herniation-recognize-patterns-before-the-scan-is-back) 6. [ Clinical Pearl ](#clinical-pearl) 7. [ What this changes in anesthetic practice ](#what-this-changes-in-anesthetic-practice) 8. [ Key Takeaways ](#key-takeaways) 9. [ Conclusion ](#conclusion) 10. [ Frequently Asked Questions ](#blog-faqs) 11. [ References ](#references-heading) On this page 1. [ Start with the right mental model ](#start-with-the-right-mental-model) 2. [ Monro–Kellie is the frame ](#monro-kellie-is-the-frame) 3. [ Compliance is easy to worsen in the OR ](#compliance-is-easy-to-worsen-in-the-or) 4. [ CPP is the number the neuron sees ](#cpp-is-the-number-the-neuron-sees) 5. [ Herniation: recognize patterns before the scan is back ](#herniation-recognize-patterns-before-the-scan-is-back) 6. [ Clinical Pearl ](#clinical-pearl) 7. [ What this changes in anesthetic practice ](#what-this-changes-in-anesthetic-practice) 8. [ Key Takeaways ](#key-takeaways) 9. [ Conclusion ](#conclusion) 10. [ Frequently Asked Questions ](#blog-faqs) 11. [ References ](#references-heading) You are inducing anesthesia for an acute subdural evacuation. The patient arrives somnolent and hypertensive, then one generous induction dose drops the MAP and suddenly the threatened brain is underperfused. That is why ICP physiology matters in anesthesiology: neurons do not care whether you intended to lower pressure if you simultaneously destroyed cerebral perfusion. [\[1\]](#cite-1 "Reference [1]") Start with the right mental model --------------------------------- ### Monro–Kellie is the frame The intact skull is a near-fixed box containing brain, CSF, and blood. When one compartment expands, another must shrink first or ICP rises. Early compensation comes mainly from shifting CSF and venous blood out of the cranial vault; later, that reserve runs out. [\[2\]](#cite-2 "Reference [2]") The board-relevant point is that ICP is not linear. At first, added volume causes little pressure change. Once compliance is exhausted, the curve turns steep, so a small extra volume from edema, hemorrhage, venous congestion, or obstructed CSF flow can produce a dramatic ICP jump. [\[3\]](#cite-3 "Reference [3]") That is also why ICP is not just a number. A patient with an ICP of 18 mmHg and falling compliance may be in more danger than a patient with the same ICP and preserved reserve. Trends, exam, CT, and physiology matter together. [\[4\]](#cite-4 "Reference [4]") ### Compliance is easy to worsen in the OR Anesthesiologists alter intracranial dynamics all the time. Reduce venous drainage, raise intrathoracic pressure, or provoke coughing and you can worsen ICP before the next scan is even done. ENLS emphasizes head elevation, midline positioning, minimizing noxious stimuli, and adequate analgesia and sedation for exactly that reason. [\[5\]](#cite-5 "Reference [5]") - Keep the head above 30° and the neck midline when possible to improve venous outflow. [\[5\]](#cite-5 "Reference [5]") - Do not accept coughing, bucking, shivering, or ventilator dyssynchrony in the patient at risk of intracranial hypertension. [\[5\]](#cite-5 "Reference [5]") - Remember that airway-pressure effects are variable: PEEP is not automatically forbidden, but in injured brains higher intrathoracic pressure can impair cerebral venous return and reduce CPP if hemodynamics are marginal. [\[6\]](#cite-6 "Reference [6]") CPP is the number the neuron sees --------------------------------- CPP = MAP − ICP is a board favorite because it is clinically ruthless. If ICP is 25 mmHg and MAP falls from 90 to 60 mmHg after induction, CPP falls from 65 to 35 mmHg. The ICP did not worsen, but brain perfusion did. [\[1\]](#cite-1 "Reference [1]") Current adult severe TBI guidance recommends treating ICP above 22 mmHg because values above that level are associated with increased mortality. The recommended CPP target is 60–70 mmHg, and aggressive attempts to push CPP above 70 mmHg with fluids or vasopressors are not advised. [\[7\]](#cite-7 "Reference [7]") Common exam and bedside traps: - Do not confuse blood pressure with cerebral perfusion. A normal-looking MAP may still be inadequate if ICP is high. [\[1\]](#cite-1 "Reference [1]") - Do not celebrate an ICP-lowering drug if it causes hypotension. Propofol can reduce cerebral metabolic demand and ICP, but bolus dosing can also depress circulation and must be countered if CPP falls. [\[5\]](#cite-5 "Reference [5]") - Do not forget that herniation and elevated ICP are related but not identical. A pressure gradient can drive tissue shift even before you have a monitor number. [\[5\]](#cite-5 "Reference [5]") Herniation: recognize patterns before the scan is back ------------------------------------------------------ Herniation syndromes reflect compartmental pressure gradients that shift tissue and compress the brainstem, cranial nerves, or cerebral vasculature. Clinically, rising ICP may present with headache, vomiting, pupillary change, and altered mental status. Cushing physiology can occur, but the full triad of hypertension, bradycardia, and irregular respirations is late and often incomplete. [\[5\]](#cite-5 "Reference [5]") Use this bedside pattern-recognition table. [\[5\]](#cite-5 "Reference [5]") SyndromeWhat shiftsHigh-yield bedside clueSubfalcineCingulate under falxMidline shift; think leg-predominant weakness from ACA compromiseUncal/transtentorialMedial temporal lobe through tentorial notchDecreased consciousness with ipsilateral dilated pupil and contralateral weaknessCentral transtentorialDownward diencephalon/midbrain shiftProgressive decline in consciousness, then posturing and midposition/fixed pupilsTonsillarCerebellar tonsils through foramen magnumIrregular breathing or apnea and rapid brainstem collapse ### Clinical Pearl > New anisocoria in a somnolent patient is herniation until proved otherwise. Do not wait for the full Cushing triad before you escalate. [\[5\]](#cite-5 "Reference [5]") For the anesthesiologist, uncal herniation is the syndrome you must spot fastest. The classic pattern is decreasing consciousness, an ipsilateral blown pupil, and contralateral weakness. If that appears around induction, transport, or positioning, treat it as a brain emergency, not a documentation exercise. [\[5\]](#cite-5 "Reference [5]") What this changes in anesthetic practice ---------------------------------------- The first job is to stop secondary injury. Avoid hypoxemia, avoid hypotension, avoid hyperthermia, and avoid anything that impedes venous drainage. These are basic moves, but they are exactly the moves that preserve CPP while definitive therapy is arranged. [\[5\]](#cite-5 "Reference [5]") In practical terms: - Minimize noxious stimulation during airway management and ongoing ventilation. [\[5\]](#cite-5 "Reference [5]") - Maintain oxygenation, ventilation, and circulation together; do not fixate on a single ICP value while MAP drifts down. [\[5\]](#cite-5 "Reference [5]") - Use sedation and analgesia deliberately for comfort and ventilator synchrony. [\[5\]](#cite-5 "Reference [5]") - Correct hyponatremia and target normothermia, because both worsen the intracranial environment. [\[5\]](#cite-5 "Reference [5]") Hyperventilation remains a rescue maneuver, not routine maintenance. ENLS recommends mild hypocapnia, roughly PaCO2 32–35 mmHg, only in selected acute situations after other measures or during active herniation management, because prolonged hypocapnia can worsen ischemia through cerebral vasoconstriction. Think bridge, not destination. [\[5\]](#cite-5 "Reference [5]") Key Takeaways ------------- - **Monro–Kellie** means the intact skull has limited room for brain, blood, and CSF; when compensatory reserve is exhausted, small volume increases cause large ICP rises. [\[2\]](#cite-2 "Reference [2]") - The pressure–volume relationship is exponential, so falling compliance matters as much as the absolute ICP value. [\[3\]](#cite-3 "Reference [3]") - **CPP = MAP − ICP**. In anesthesia, a drop in MAP can be as dangerous as a rise in ICP. [\[1\]](#cite-1 "Reference [1]") - Current adult severe TBI guidance supports treating ICP >22 mmHg and targeting CPP 60–70 mmHg. [\[7\]](#cite-7 "Reference [7]") - Uncal herniation classically causes decreased consciousness, ipsilateral pupillary dilation, and contralateral weakness. [\[5\]](#cite-5 "Reference [5]") - Cushing triad is late. Do not wait for all three signs before you act. [\[5\]](#cite-5 "Reference [5]") Conclusion ---------- ICP physiology becomes manageable once you think in compartments, compliance, and perfusion. Protect venous outflow, protect MAP, and recognize herniation early; that is how anesthesiologists keep a threatened brain from becoming an unsalvageable one. [\[2\]](#cite-2 "Reference [2]") Frequently Asked Questions ---------------------------- ### What ICP threshold should trigger treatment in monitored adult severe TBI? Current Brain Trauma Foundation guidance recommends treating ICP above 22 mmHg, and ENLS defines intracranial hypertension as sustained ICP greater than 22 mmHg for more than 5 minutes. Clinical context, CT findings, and exam still matter. [\[7\]](#cite-7 "Reference [7]") ### Why is induction hypotension so dangerous when ICP is elevated? Because CPP equals MAP minus ICP. Even if ICP stays unchanged, a drop in MAP can sharply reduce cerebral perfusion and worsen secondary brain injury. [\[1\]](#cite-1 "Reference [1]") ### Is Cushing triad an early sign of impending herniation? No. Hypertension, bradycardia, and irregular respirations are classic but usually late, and all three are often not present together. [\[5\]](#cite-5 "Reference [5]") ### Which herniation syndrome classically causes a blown pupil? Uncal transtentorial herniation classically causes decreased consciousness with ipsilateral pupillary dilation and contralateral weakness. [\[5\]](#cite-5 "Reference [5]") ### Should hyperventilation be used routinely to control ICP? No. Mild hypocapnia is a temporizing rescue strategy in selected acute situations, but prolonged hyperventilation risks cerebral ischemia from vasoconstriction. [\[5\]](#cite-5 "Reference [5]") References (7) ------------------ 1. 1. [ pmc.ncbi.nlm.nih.gov/articles/PMC4239411 ](https://pmc.ncbi.nlm.nih.gov/articles/PMC4239411/) [↩](#cite-ref-1-1 "Back to text") 2. 2. [ pmc.ncbi.nlm.nih.gov/articles/PMC4971608 ](https://pmc.ncbi.nlm.nih.gov/articles/PMC4971608/) [↩](#cite-ref-2-1 "Back to text") 3. 3. [ pmc.ncbi.nlm.nih.gov/articles/PMC7201553 ](https://pmc.ncbi.nlm.nih.gov/articles/PMC7201553/) [↩](#cite-ref-3-1 "Back to text") 4. 4. [ pmc.ncbi.nlm.nih.gov/articles/PMC10481563 ](https://pmc.ncbi.nlm.nih.gov/articles/PMC10481563/) [↩](#cite-ref-4-1 "Back to text") 5. 5. [ www.neurocriticalcare.org/Portals/0/ENLS%205.0/ENLS%206.0/Protocol%20V6\_0\_Intracranial%20Hypertension%20and%20Herniation.pdf?ver=oGpOZQ2sI6-mgzJhEgTtTg%3D%3D ](https://www.neurocriticalcare.org/Portals/0/ENLS%205.0/ENLS%206.0/Protocol%20V6_0_Intracranial%20Hypertension%20and%20Herniation.pdf?ver=oGpOZQ2sI6-mgzJhEgTtTg%3D%3D) [↩](#cite-ref-5-1 "Back to text") 6. 6. [ pubmed.ncbi.nlm.nih.gov/30653472 ](https://pubmed.ncbi.nlm.nih.gov/30653472/) [↩](#cite-ref-6-1 "Back to text") 7. 7. [ braintrauma.org/coma/guidelines/severe-tbi ](https://braintrauma.org/coma/guidelines/severe-tbi) [↩](#cite-ref-7-1 "Back to text") Next step Build momentum in Anesthesiology with focused, exam‑style practice -------------------------------------------------------------------- - Airway, ventilation, and crisis drills - High‑yield anesthesia pharmacology made practical - Track weak topics and improve faster [ Start practicing ](https://mdster.com/user/dashboard) [ Explore Anesthesiology ](https://mdster.com/speciality/anesthesiology) [ View pricing ](https://mdster.com/pricing) [ Explore features ](https://mdster.com/features) No credit card required. Full access to all features\*. No commitment. Cancel anytime. \*AI SOE Examiner is limited to 10 cases monthly for Advanced & Bundle subscribers. 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