2009A09 Discuss the physiological factors that determine intracranial pressure (ICP).
Describe how changes in posture affect ICP.



·         Intro

·         Monroe-Kellie doctrine

·         Factors affecting each component

·         Head down: effect and compensation

·         Head up: effect and compensation


Introduction: ICP


·   Force per unit area within the cranial vault


·   5-10mmg in the supine position, measured at the external auditory meatus


·   a) intracranial volume (fixed)

·   b) amount of brain, blood and CSF


Monroe-Kellie doctrine:


·   Cranium has fixed walls and one major outlet

·   An increase in one substance must come at the expense of another, otherwise pressure rises rapidly

Elastance curve

Volume buffering

·   Venous blood -> circulation: rapid response, lower capacity

·   Arterial blood -> circulation: minimal

·   CSF cranial -> spinal: slower response, higher capacity


Volume determinants:



·   ↑Volume: oedema, tumour, abscess

·   ↓Volume: atrophy

Cranial CSF


N.B.: a) X axis unit mmCSF not mmHg   b) Y axis zero point not at the X axis


·   ↑Volume: obstruction to circulation, Trendelenburg

·   ↓Volume: CPP <70mmHg -> no production, reverse Trendelenburg



·   ↑Volume:

o ↑Temp, seizure (↑CMRO2 -> vasodilatation)

o ↑PaCO2 (↑[H+] in CSF and brain ECF -> vasodilatation)

o ↓PaO2 (vasodilatation)

o ↑↑mAP?*

o Trendelenburg (↑venous pressure)

o Neck strictures (↑venous pressure)

·   ↓Volume

o ↓Temp

o ↓PaCO2

o ↓↓mAP?*

o Reverse Trendelenburg


(*The relationship between CBF and CBV is not clear to me)




·   Gynae laparoscopy

·   Say -10mmHg = -13.5cm

Direct effect

·   Add 10mmHg hydrostatic pressure

·   CNS arteries: 100 -> 110 mmHg (1.1x normal) -> small ↑arterial volume

·   CNS veins: 2-> 12mmHg (6x normal) -> large ↑venous volume


·   CSF displaced -> ↓CSF volume (compensation)

·   Parenchyma: no change

Overall effect

·   Mild ↑ICP




·   Shoulder surgery

·   Say +22mmHg = +30cm

Direct effect

·   CNS arteries: 100 -> 78mmHg (0.8x normal) -> small ↓ volume

·   CNS veins: 2 -> -20mmHg (-10x normal) -> large ↓ volume

·   CSF: cranial to spinal -> ↓ volume

·   Parenchyma: unchanged


·   Myogenic response: ↓mAP -> ↓stretch -> reflex cerebral vasodilatation -> ↑CBV -> ↑ICP

·   Central ischaemic response: ↓↓mAP -> CNS acidosis -> ↑SNS output -> ↑HR, ↑SVR, ↑mAP (often reflex bradycardia i.e. Cushing response)

Overall effect

·   ICP falls from +5mmHg to -10mmHg

·   CVP becomes the limiting factor for CPP (Starling resistor)



Addit: means of reducing ICP:


·   Head up 30° (↓CVP)

·   Loosen any neck strictures (↓CVP)


·   pO2 (normalise CBF)

·   ↓pCO2 (↓CBF

·   ↓Temp (↓CMRO2 -> ↓CBF) (note doesn’t improve outcomes after TBI)


·   Hypnotics: propofol, thiopentone (↓CMRO2)

·   Anti-epileptics: e.g. levetiracetam (↓CMRO2 if seizure)

·   Muscle relaxants: e.g. cisatracurium (↓coughing and Valsalva -> ↓CVP)

·   Osmotic agents: hypertonic saline, mannitol (↓parenchymal water)


·   External ventricular drain (↓CSF)

·   Decompressive craniectomy (note doesn’t improve outcomes after TBI)


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