ObjectiveTo evaluate the effect of rescuer team size on objective skill measures of basic life support (BLS) and advanced life support (ALS) using high-fidelity canine CPR simulation. DesignProspective, experimental study. SettingVeterinary clinical simulation center. SubjectsForty-eight Reassessment Campaign on Veterinary Resuscitation CPR-certified veterinary students. Measurements and Main ResultsFive groups of participants each conducted 3 CPR simulations in configurations of 4, 6, and 8 rescuers. Simulations represented a shock patient declining into asystole, followed by ventricular fibrillation and return of spontaneous circulation. Resuscitation efforts were video-recorded to evaluate BLS and ALS tasks. Mean (+/- SD) was derived and data were compared among team sizes using ANOVA and Tukey's post hoc analysis. Significance was set at P < 0.05. Among teams of 4, 6, and 8 rescuers, time to first chest compression (13 s [+/- 6], 9 s [+/- 2], 8 s [+/- 4]; P = 0.24) and positive-pressure breath (101 s [+/- 37], 56 s [+/- 15], 67 s [+/- 24]; P = 0.05) were not significantly different. Chest compression (100/min [+/- 5], 108/min [+/- 6], 107/min [+/- 6]; P = 0.12) and ventilatory rates (9/min [+/- 1], respectively, P = 0.52) were not significantly different. Time without chest compressions/total length of CPR was not significantly different (72 s [+/- 16], 61 s [+/- 16], 54 s [+/- 8]; P = 0.15). Capnography and ECG monitoring were used by all teams. Time to first vasopressor administration was significantly different among team sizes (268 s [+/- 70], 164 s [+/- 65], 174 s [+/- 34]; P = 0.04), with vasopressors being most quickly administered by teams of 6 rescuers. Time to electrical defibrillation was not significantly different (486 s [+/- 45], 424 s [+/- 22], 488 s [+/- 181]; P = 0.57). Incorrect ALS interventions occurred in 60%, 0%, and 40% of CPR events in 4, 6, and 8 rescuer teams, respectively. ConclusionsAlthough the achievement of BLS tasks was comparable in teams of 4 rescuers, teams of 6 rescuers may be preferable based on differences in the rate of guideline-incompliant treatments and ALS task efficiency. Teams of 8 rescuers were neither more efficient nor more accurate at conducting BLS and ALS tasks.