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



·         Effects of hydrostatic pressure change

·         Effects of respiratory embarrassment

·         Effects of general anaesthesia


Effects of hydrostatic change:


·   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:


·   Compression of diaphragm by abdominal viscera and fat

·   Greatly exacerbated in obesity


·   ↓Thoracic compliance

·   ↓VT for a given pressure gradient

·   ↓VA (= RR x VT – VD)

·   ↑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)


·   ↓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)

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