2019A05 Discuss the cerebral effects of prolonged anaesthesia in the steep head down position.

List:

· Effects of hydrostatic pressure change

· Effects of respiratory embarrassment

· Effects of general anaesthesia

Effects of hydrostatic change:

Background

· Monroe-Kellie doctrine:

o Cranium has fixed walls and one major outlet

o Increase in one intracranial substance must come at the expense of another or else pressure rises drastically

· Volume buffering:

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

o Arterial blood -> circulation: minimal

o Brain CSF -> spinal CSF: slower response, higher capacity

o Brain: no ability

Hydrostatic changes

· Example: laparoscopy, say head -10mmHg (13.5cm)

· Direct effects:

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

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

· Compensatory effects:

o CSF displaced -> ↓CSF volume (compensation)

Overall effects

· Mild ↑ICP

· Mild ↑IOP

· Mild ↓ cerebral perfusion pressure

Effects of respiratory embarrassment:

Mechanism

· Compression of diaphragm by abdominal viscera and fat

· Greatly exacerbated in obesity

↑PaCO₂

· ↓Thoracic compliance

· ↓V_T for a given pressure gradient

· ↓V_A (= RR x V_T – V_D)

· ↑PACO₂, ↑PaCO₂

· ↓pH in CSF and brain ECF (CO₂ crosses blood-brain barrier)

· ↑Activity nNOS, ↓activity L-Ca²⁺ channel

· Cerebral vasodilatation

· ↑CBV

· +/- ↑ICP (once volume buffering exhausted)

↓PaO₂

· ↓FRC

· Alveolar collapse

· ↑Shunt

· ↓PaO₂

· ↓DO₂

· Risk of CNS ischaemia

· Reduction in CMRO₂ under GA is protective

Effect of prolonged general anaesthesia:

· Drowsiness (drug accumulation)

· Nausea and vomiting

· Respiratory depression (↓response to ↓PaO₂ and ↑PaCO₂)

· Delirium, cognitive dysfunction (↑risk if steep Trendelenburg, deep anaesthesia)

(note effects are drug-dependent)