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

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

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