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

 

List:

·        Intro

·        Monroe-Kellie doctrine

·        Factors affecting each component

·        Head down: effect and compensation

·        Head up: effect and compensation

 

Introduction: ICP

Definition

·   Force per unit area within the cranial vault

Normal

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

Determinants

·   a) intracranial volume (fixed)

·   b) amount of brain, blood and CSF

 

Monroe-Kellie doctrine:

Description

·   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:

Parenchyma

(80-85%)

·   ↑Volume: oedema, tumour, abscess

·   ↓Volume: atrophy

Cranial CSF

(7-10%)

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

Blood

(5-8%)

·   ↑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 mAP and CBV is not clear to me)

 

Trendelenburg:

Example

·   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

Compensation

·   CSF displaced -> ↓CSF volume (compensation)

·   Parenchyma: no change

Overall effect

·   Mild ↑ICP

 

Erect:

Example

·   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

Compensation

·   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:

Physical

·   Head up 30° (↓CVP)

·   Loosen any neck strictures (↓CVP)

Physiological

·   ↑pO2 (normalise CBF)

·   ↓pCO2 (↓CBF

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

Pharmacological

·   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)

Psurgical

·   External ventricular drain (↓CSF)

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

 

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