Do anxiety or hypocapnia predispose to apnoea after induction of anaesthesia?

We have studied the incidence of apnoea after induction of anaesthesia in patients allocated randomly to receive a standardized dose of either propofol or etomidate. We measured anxiety before operation with a standard questionnaire and end-tidal carbon dioxide concentration from a mask during breathing 35% oxygen, before induction of anaesthesia. Respiration was measured by pneumotachograph and impedance pneumograph. There was no significant relationship between anxiety score and end-tidal carbon dioxide concentration before operation. Patients given propofol (n = 26) received a median dose of 157 mg over 70 s, and 17 became apnoeic (median duration 24 s, quartile values 0, 76). Apnoea was more severe in patients whose preoperative end-tidal carbon dioxide value was less than the median value (median duration of apnoea 61 s compared with 10 s; P < 0.05). Patients given etomidate (n = 25) received 16.2 mg in 57 s, which was a significantly smaller fraction of the calculated dose requirement, and had significantly less apnoea: eight became apnoeic (median duration 0 s, quartile values 0, 23 s). There was no relationship between apnoea and end-tidal carbon dioxide concentration in these patients. Anxiety did not relate to the incidence of apnoea with either induction agent. We conclude that apnoea after induction of anaesthesia with propofol is more likely if hypocapnia is present but we could not relate apnoea or hypocapnia to anxiety in the ward before operation.