ATROPINE-INDUCED HEART-RATE CHANGES - A COMPARISON BETWEEN MIDAZOLAM-FENTANYL-PROPOFOL-N2O AND MIDAZOLAM-FENTANYL-THIOPENTONE-ENFLURANE-N2O ANESTHESIA

Atropine-induced heart rate (HR) changes were studied in 19 patients (ASA physical status I) during anaesthesia maintained predominantly with propofol-N2O or thiopentone-enflurane-N2O. Ten patients (Group A) received midazolam (0.07 mg · kg-1), fentanyl (1 μg · kg-1), propofol (2 mg · kg-1) and succinylcholine (1 mg · kg-1). Following tracheal intubation, anaesthesia was maintained with propofol (6 mg · kg-1 · hr-1), N2O (67 per cent) and O2 (33 per cent). In nine patients (Group B) thiopentone (4 mg · kg-1) was substituted for propofol and anaesthesia maintained with N2O (67 per cent) O2 (33 per cent), and enflurane (0.5 per cent inspired concentration). The study was non-randomised because Group B patients were only included if HR before administration of atropine < 90 beats · min-1. IPPV was performed in all patients using a Manley ventilator (minute vol. 85 ml · kg-1; tidal vol. 7 ml · kg-1). Ten minutes after tracheal intubation, incremental doses of atropine (equivalent cumulative doses: 1.8, 3.6, 7.2, 14.4, 28.8 μg · kg-1) were administered at two-minute intervals and HR responses calculated during the last 45 sec of each intervening period. No differences were observed between the groups following 1.8 and 3.6 μg · kg-1 atropine, but propofol-N2O anaesthesia was associated with reduced responses (P < 0.01) following 7.2, 14.4 and 28.8 μg · kg-1 atropine. These results suggest that there is a predominance of parasympathetic influences during propofol-N2O anaesthesia compared with thiopentone-enflurane-N2O anaesthesia. © 1990 Canadian Anesthesiologists.