Prebypass muscle relaxant: to the patient or the pump?

The objective of this study was to assess the difference in muscular relaxation, produced by administration of the prebypass relaxant dose to the patient or in the bypass circuit. This prospective study was conducted on 100 patients scheduled to undergo elective coronary artery bypass grafting. All patients received 2 mg of vecuronium as prebypass relaxant, with neuromuscular junction monitoring using an accelograph. The patients in Group A (n=50) received the prebypass relaxant dose directly through the central venous cannula during heparinisation while group B patients (n=50) received it into the bypass circuit after initiation of bypass. Further doses of the relaxant were administered, if the train of four ratio was more than 10% at anytime after 5 min of bypass. The train of four ratio before heparinisation was similar in both groups, 12+/-6% in Group A and 13+/-5% in Group B, but after the prebypass relaxant dose, it was 1+/-2% in Group A and 15+/-3% in Group B (P<0.05). On bypass, in Group A it was 3+/-2%, 5+/-2%, 6+/-3% and 6+/-3% at 1, 5, 10 and 15 min at normothermia, while in Group B it was 29+/-6%, (P<0.01) at 1 min, 19+/-7%, (P<0.05) at 5 min, 13+/-6% at 10 min and 3+/-2% at 15 min. Additional doses of relaxant on bypass were required in 48 patients in Group B and none in Group A (P<0.01). It is concluded that the degree of muscle relaxation is significantly more profound when the prebypass relaxant dose is given to the patient directly before initiating the bypass, than when it is added to the bypass circuit after initiating the bypass. Time taken is longer and dose needed is larger, to produce the same effect.