Objective: Previous studies have reported that nadir oxygen delivery during cardiopulmonary bypass is associated with the occurrence of postoperative acute kidney injury (AKI). However, these measurements only considered the bottom point of the oxygen delivery (DO 2 ) but did not consider the duration of DO 2 . We aimed to examine whether the time-dose response of DO 2 during cardiopulmonary bypass can be used to estimate the risk for postoperative AKI.Methods: We evaluated 112 patients who underwent cardiac surgeries with cardiopulmonary bypass. We analyzed the perfusion parameters recorded every 20 seconds. To obtain time-dose response of DO 2 index (DO 2 i), the area under the curve was calculated as below the 300 mL/min/m 2 threshold, which accounts for depth and duration of cumulative oxygen debt. In addition, the cumulative time below DO 2 i 300 mL/min/m 2 was also calculated. Receiver operating characteristic analysis, univariate regression analysis, and multivariate regression analysis were used to evaluate associations between perioperative variables and postoperative AKI.Results: Patients who developed AKI had larger area under the curve below the 300 mL/min/m 2 threshold (1581 vs 632; P<.01) and cumulative time below DO 2 i 300 mL/min/m 2 (34.7 vs 15.3 minutes; P<.01). Nadir DO 2 i was not significantly different between the non-AKI and AKI groups (263.4 vs 247.0 mL/min/m 2 ; P ¼ .291). Conclusions:The time-dose response of DO 2 i during cardiopulmonary bypass is a better indicator than nadir DO 2 i in evaluating AKI risk. Maintaining DO 2 i levels >300 mL/min/m 2 may result in decreased risk for postoperative AKI.

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Our study indicated the presence of a compensatory mechanism in which increased blood flow of the microcirculation is in compensation for the lack of oxyhemoglobin delivery caused by hemodilution.