Older patients have a higher cardiac surgery-associated acute kidney injury (CSA-AKI) related mortality. Low oxygen delivery (DO₂) during cardiopulmonary bypass (CPB) is a risk factor for CSA-AKI, but critical DO₂ thresholds for older patients are unknown. This study investigated critical DO₂ thresholds for CSA-AKI in patients ≥70 years undergoing on-pump cardiac surgery.

Patients were enrolled from July 2015 until August 2017. CPB data from 432 patients were collected, and DO₂ values were calculated per minute. The primary outcome was CSA-AKI. The association between DO₂ and CSA-AKI was analysed with multivariable regression analysis. Multiple DO₂ thresholds were analysed. The association between CSA-AKI and the area below the DO₂ thresholds (DO₂ deficit) was evaluated, as was the association between frailty and CSA-AKI.

CSA-AKI occurred in 63 (14.6%) patients. Mean and nadir (lowest) DO₂ values were lower in patients with CSA-AKI (283 vs 312 ml/min/m²; P-value <0.001 and 238 vs 270 ml/min/m²; P-value <0.001, respectively). The adjusted relative risk for CSA-AKI was 1.006 [99% confidence interval (CI) 1.001–1.012] per ml/min/m² nadir DO₂ decrease. The critical DO₂ threshold was 270 ml/min/m² [adjusted relative risk 2.06 (99% CI 1.33–2.80)]. The DO₂ deficit below 270 ml/min/m² was associated with CSA-AKI [adjusted relative risk 2.84 (99% CI 1.87–3.81)]. No association between frailty and CSA-AKI was found (P = 0.82).

Low DO₂ increased the risk for CSA-AKI in older patients who had cardiac surgery. A critical DO₂ threshold of 270 ml/min/m² was applicable for frail and non-frail patients. The efficacy of a DO₂ >270 ml/min/m² to reduce CSA-AKI in older patients needs further evaluation.