Pharmacodynamics of doxacurium during cardiac surgery with hypothermic cardiopulmonary bypass

To determine the characteristics of neuromuscular block produced by two and three times the 95% effective dose (ED95) of doxacurium in patients undergoing coronary artery surgery with hypothermic cardiopulmonary bypass.

In a prospective non randomized study, ten patients received doxacurium 0.05 mg.kg-1 (Group 1) and ten others received 0.075 mg.kg-1 (Group 2) with midazolam and sufentanil. The mechanomyographic response of the adductor pollicis muscle after supramaximal train-of-four (TOF) stimulation of the ulnar nerve was recorded intraoperatively and postoperatively. Additional doxacurium (10% of the initial dose) was administered until sternal closure whenever the first twitch (T1) had recovered to 25% of control.

The onset time (time to maximal T1 depression) of doxacurium was 390 +/- 148 sec in Group 1 and 370 +/- 74 sec in Group 2 (P = 0.71). The clinical duration of neuromuscular block (time to 25% T1 recovery) was 165 +/- 90 min in Group 1 and 258 +/- 86 in Group 2 (P = 0.03). On arrival to recovery room the mean T1 was 57 +/- 23% in Group 1 and 24 +/- 21% in Group 2 (P = 0.003); the mean T4/T1 ratio was 0.25 +/- 0.15 for five patients of Group 1 with four responses to TOF stimulation and 0.10 for the only patient of Group 2 with four twitches.

In contrast with findings in patients without cardiac disease, this study shows comparable onset times of doxacurium with doses of two and three times ED95. The clinical duration of doxacurium is 60 to 100% longer than previously reported in noncardiac surgery.