Effects of i.v. midazolam on upper airway resistance

We have measured changes in supraglottic airway resistance (Rsg) produced by midazolam. Ten subjects were studied on two occasions, receiving in a random order either midazolam 0.1 mg kg-1 or placebo. Supraglottic pressures were measured using a balloon-tipped catheter and air flow with a pneumotachograph. Rsg were calculated at a flow rate of 0.3 litre s-1 during inspiration. No changes in Rsg and no apnoeic events were noted following placebo injection. Mean Rsg increased from 0.23 (SEM 0.07) kPa litre-1 s to 1.29 (0.38) kPa litre-1 s 5 min after injection of midazolam (P less than 0.01), and remained increased significantly for 20 min. Twenty-two apnoeic events were recorded in six subjects, including 11 obstructive events. We conclude that midazolam in sedative doses increases Rsg markedly and induces central apnoea during the first few minutes after i.v. administration and this is followed by obstructive apnoea.