Dry soda lime markedly degrades sevoflurane during simulated inhalation induction

Abstract

We have investigated gas composition during simulated inhalation induction with sevoflurane to elucidate possible mechanisms of incidental prolonged induction times and airway irritation. Using a circle system, 8% sevoflurane in oxygen 6 litre min-1 was washed into an absorbing canister filled with fresh soda lime containing 2.9% KOH (Draegersorb, 'D') or no KOH (< 0.01%, Sofnolime, 'S'). The absorbent was dried by oxygen 20,000 litre before every second experiment. Maximum soda lime temperatures attained after 4-6 min were 107 degrees C using dry D and 62 degrees C (61 degrees C) with dry S. Temperature did not increase with fresh soda lime. With dry soda lime, sevoflurane was not detected at the T-piece for 3 min and reached 6-7% within 6-10 min. After 1 min, we detected methanol and compound A (CH2F-O-C(= CF2) (CF3)). Total amounts over 20 min were: methanol 1125 mg (D dry), 334 mg (S dry) and < 5 mg (fresh soda lime); compound A 148 mg (D dry), 13 mg (S dry) and 3-8 mg (fresh); and fluoride 8.5 mg (D dry), 3.3 mg (S dry) and 1 mg (fresh). Formaldehyde was detected only with dry lime (D > 2.5 mg, S > 0.6 mg). In summary, the use of moist soda lime is of crucial importance during inhalation induction. With dry soda lime, the patient may inhale potentially toxic degradation products in significant amounts. Sevoflurane degradation is aggravated by a high KOH content of the lime. The observed airway irritation may be caused by formic acid, which is generated in isomolar concentrations with methanol (Cannizzaro reaction). The amount of compound A found with dry KOH-containing lime is unlikely to be noxious.