Pentobarbitone Distribution and Enzyme Induction

Sir,—Thank you for the opportunity of replying to the letter from Drs Bradshaw and Maddison. We can assume only that these workers have been unable to confirm our observations because of some fundamental difference in technique. The difference is not immediately apparent from the information provided in their letter, but several possibilities exist. First, our patients, who were not premedicated, were ventilated mechanically throughout the investigation. Second, in each instance supramaximal stimuli were applied to the ulnar nerve at a frequency of 50 Hz for 1 s at 12-s intervals throughout the study to elicit the tetanic responses of the adductor pollicis muscle. Third, the patient's lungs were ventilated with halothane 2% for at least 5 min before the administration of the first dose of neostigmine 2.5 mg i.v., which antagonized the tetanic transmission block and abolished the tetanic fade. Both the transmission block and the fade re-appeared after the second dose of neostigmine was given 2-3 min later during the continued administration of halothane. Since our original observation, we have confirmed that this phenomenon is a consistent finding. A further possibility is that our recording system may be more sensitive than that used by Drs Bradshaw and Maddison, as exemplified by the fact that we have been able also to demonstrate the weak neuromuscular blocking action of halothane in anaesthetized man (Hughes and Payne, 1977b).