Effect of passive hyperventilation on seizure duration in patients undergoing electroconvulsive therapy

The influence of end-expired carbon dioxide concentration (E'CO2) on seizure duration was studied in 30 depressed patients undergoing electroconvulsive therapy (ECT) on three separate consecutive occasions. Unpremedicated patients breathed 100% oxygen before the induction of anaesthesia with methohexitone and the provision of partial neuromuscular blockade with suxamethonium. They then received no further ventilation, 10 breaths, or 20 breaths of hyperventilation with 100% oxygen from a non-rebreathing system with a high fresh gas flow. Transcutaneous oxyhaemoglobin saturation (SaO2) was monitored continuously and E'CO2 was measured before and after the seizure. Observation of an isolated forearm was used to time the seizure duration. SaO2 remained greater than 90% in all patients. Passive hyperventilation with 20 breaths significantly reduced E'CO2 and prolonged seizure duration. E'CO2 before the seizure and change in E'CO2 during the seizure did not correlate significantly with seizure duration.