The effect of left ventricular (LV) venting on myocardial oxygen consumption (MVO2) during reperfusion after hypothermic cardioplegic arrest was tested using 29 dogs placed on cardiopulmonary bypass (CPB). During the first 20 minutes of reperfusion after 45 minutes of 20 degrees C arrest, group 1 (n = 10) had total venting with the LV systolic pressure (LVSP) maintained at 0 mm Hg. Group 2 (n = 10) had partial venting with the LVSP at 41.6 +/- 1.4 mm Hg (one-half perfusion pressure), while in group 3 (n = 9), the LVSP was kept just below CPB perfusion pressure (76.6 +/- 3.1 mm Hg) by occlusion of the LV vent. Next, there were 10 minutes of partial bypass (CPB flow reduced by 50%) followed by cessation of CPB. MVO2 expressed as ml O2/100 g LV/min (wet weight) during the first 20 minutes of reperfusion was 2.66 +/- 0.33 for group 1, 2.40 +/- 0.36 for group 2 and 4.62 +/- 0.53 for group 3 (p less than 0.05 vs groups 1 and 2). There were no significant differences in MVO2 in any of the groups during partial CPB compared with the period without CPB. These results demonstrate that effective LV venting reduces MVO2 during reperfusion after hypothermic ischemic arrest. However, there was no reduction in MVO2 during partial CPB compared with the initial period without CPB. Thus, volume loading of the left ventricle during reperfusion by failing to vent leads to increased myocardial oxygen demand, which may be detrimental in hearts that have sustained significant ischemia or when revascularization is incomplete.