Cardiac index (CI) and mean arterial pressure (MAP) are concurrent determinants of renal perfusion. Hypotension is associated with acute kidney injury (AKI), but clinical trials focused solely on raising intraoperative MAP showed no benefit. Whether CI provides useful clinical information is controversial. We evaluated the association between AKI and low CI during periods of hypotension in cardiac surgery. Length of stay (LOS) was a secondary outcome.

In adults undergoing coronary artery bypass (CAB) surgery with cardiopulmonary bypass, MAP and CI were recorded every minute. Duration of exposure to eight joint ranges of MAP (< or ≥65 mm Hg) and quartiles of CI were calculated. Logistic regression estimated odds ratios (ORs) with 95% confidence intervals for AKI adjusted for all covariates, time in each joint MAP/CI range, and duration of hypotension.

Among 1272 participants (67 [50–90] yr, 21% female), 379 (30%) were exposed to ≥5 min of joint hypotension/low CI (CI≤2 L min⁻¹ m⁻²). Joint hypotension/CI≤2 was associated with an 11% increased risk of AKI (adjusted OR=1.11 per 5-min, 95% confidence interval: 1.02–1.22, P=0.021) independent of all covariates. Adjustment for duration of hypotension had no impact on results, and hypotension was not associated with AKI in adjusted models. Hypotension/CI≤2 was associated with increased ICU LOS (adjusted time ratio = 1.04 per 5-min, 95% confidence interval: 1.02–1.07, P=0.002) and hospital LOS (adjusted time ratio = 1.02, 95% confidence interval: 1.00–1.03, P=0.009); hypotension/CI>2 was not.

Joint exposure to hypotension/CI≤2 during cardiac surgery was associated with AKI and increased LOS, whereas exposure to hypotension/CI>2 was not. Prospective interventional trials are needed to evaluate whether the relationship between CI and AKI is indeed causal and whether CI-guided therapy can help reduce AKI.