Plasma cholinesterase and malignant hyperthermia.

Sir,—Humphrey, Downing and Brock-Utne (1980) are less than accurate in describing the destiny of expiration switched to inspiration in the Johannesburg Circuit. They vaguely refer to experimental evidence gathered "while studying a similar system"—evidence which simply does not tally with known pharmacokinetic principles. With a fresh gas inflow of 70mlkg~' min"' in a patient weighing 65 kg, the time constant is circuit volume (certainly no more than 6 litre) plus a large functional residual capacity of 3 litre, divided by the 4.5 litre min" ' gas flow; at most 2 min for 63% clearance and 6min for 95% clearance. There is no possibility that washout could take "more than lOmin" in the given circumstances. I have just seen the Johannesburg Switch performing well in Holland and it would be a pity if its clinical use is discredited unfairly. J. W. MOSTERT Pretoria, S. Africa