So far, evidence is scarce regarding the impact of rewarming temperature on bleeding after cardiac surgery under cardiopulmonary bypass (CPB). We seek to evaluate how rewarming temperature affects postoperative bleeding in patients undergoing mitral valve repair.

Clinical, anesthesia and CPB data were analyzed for 379 adults undergoing isolated mitral regurgitation repair. Platelet levels (1000/μL) were divided into three groups: low, 53–164 (n = 128); medium, 165–208 (n = 126); and high, 209–906 (n = 125). The primary endpoint was chest drainage volume (mL) at 24 postoperative hours. To delineate the degree and impact of rewarming, a variable “rewarming/cooling ratio” was defined as: peak rewarming temperature (°C) divided by nadir cooling temperature (°C). General linear models were used to evaluate factors associated with the volume of 24-h chest drainage.

Mean flow rate was 2.7 ± 0.1 L/min/m², nadir core temperature 30.7 ± 1.3°C and peak rewarming temperature 36.2 ± 0.5°C. The “rewarming/cooling ratio” averaged 1.133 ± 0.044.

Postoperatively, nadir platelet count was 115 ± 46 k/μL, and chest drainage averaged 344 ± 181 mL at 24 h. Two patients underwent re-exploration for bleeding (0.5%). Transfusion was required in 85 patients (22.4%).

Gender (F = 20.585, p < 0.001), platelet count (F = 3.875, p = 0.024), fibrinogen (F = 4.241, p = 0.040), and chest incision (F = 58.097, p < 0.001), rather than the rewarming temperature (F = 2.322, p = 0.128), were factors significantly associated with 24-h chest drainage.

A significant interaction existed between platelet levels and “rewarming/cooling ratio” (F = 3.717, p = 0.025), i.e., with a higher “rewarming/cooling ratio”, 24-h chest drainage tended to increase in the low platelet group, and to decrease in patients with high and medium platelet levels.

In this series of patients undergoing isolated mitral regurgitation repair, gender, platelet count, fibrin and chest incision, rather than rewarming temperature, were independent factors significantly associated with postoperative bleeding. The impact of rewarming temperature on postoperative bleeding was modified by preoperative platelet levels. It is advisable to avoid excessive cooling during CPB and keep the cooling temperature at maximally allowable level.