Defining oliguria during cardiopulmonary bypass and its relationship with cardiac surgery-associated acute kidney injury

Abstract

Background: While urine flow rate ≤0.5 ml kg-1 h-1 is believed to define oliguria during cardiopulmonary bypass (CPB), it is unclear whether this definition identifies risk for acute kidney injury (AKI) . The purpose of this retrospective study was to evaluate if urine flow rate during CPB is associated with AKI.

Methods: Urine flow rate was calculated in 503 patients during CPB. AKI in the first 48 h after surgery was defined by the Kidney Disease: Improving Global Outcomes classification. Adjusted risk factors associated with AKI and urine flow rate were assessed.

Results: Patients with AKI [n=149 (29.5%)] had lower urine flow rate than those without AKI (P<0.001). The relationship between urine flow and AKI risk was non-linear, with an inflection point at 1.5 ml kg-1 h-1 Among patients with urine flow <1.5 ml kg-1 h-1, every 0.5 ml kg-1 h-1 higher urine flow reduced the adjusted risk of AKI by 26% (95% CI 13-37; P<0.001). Urine flow rate during CPB was independently associated with the risk for AKI. Age up to 80 years and preoperative diuretic use were inversely associated with urine flow rate; mean arterial pressure on CPB (when <87 mmHg) and CPB flow were positively associated with urine flow rate.

Conclusions: Urine flow rate during CPB <1.5 ml kg-1 h-1 identifies patients at risk for cardiac surgery-associated AKI. Careful monitoring of urine flow rate and optimizing mean arterial pressure and CPB flow might be a means to ensure renal perfusion during CPB.

Clinical trial registration: ClinicalTrials.gov NCT00769691 and NCT00981474.

Keywords: acute kidney injury; cardiac surgery; cardiopulmonary bypass; oliguria.

© The Author 2016. Published by Oxford University Press on behalf of the British Journal of Anaesthesia. All rights reserved. For Permissions, please email: journals.permissions@oup.com.

Figures

Fig 1

Patient flow diagram outlining recruitment and final enrolment.

Fig 2

Distribution of urine flow rate during CPB for (a) all patients and (b) box and whisker plot of urine flow rate during CPB for patients with and without postoperative AKI. In panel b, the horizontal line in the shaded box represents the median value and the shaded box represents the IQR. The error bars below and above the shaded area represent ±1.5× the IQR; points beyond the error bars are outliers. Patients with cardiac surgery–associated AKI had a lower urine flow rate than did patients without AKI (1.14 ml kg−1 h−1; IQR 0.66–2.16 vs 1.62 ml kg−1 h−1; IQR 1.08–2.82; P<0.0001).

Fig 3

Neither CPB flow nor MAP differed between patients who did and did not develop AKI. CPB flow was positively correlated with urine flow rate [(a) r=0.12 (95% CI 0.03–0.21); P=0.0109] but did not differ between patients with and without AKI [(b) 2.07 ± 0.20 vs 2.06 ± 0.19 L min−1 m−2, P = 0.37]. The MAP during CPB (mmHg) was positively correlated with urine flow rate [(c) r = 0.16 (95% CI 0.07–0.24); P = 0.0003) but there was no difference in mean MAP between patients with and without AKI [(d) 74 ± 8.1 vs 73 ± 8.2 mmHg; P = 0.22).

Fig 4

Box and whisker plot of urine flow rate (ml kg−1 h−1) vs the deviation of MAP at, below, or above the LLA (grey box) in 5-mmHg BP intervals. These data were obtained from 50 patients in part 1 of the study. The horizontal line in the middle of each box represents the median and the length of the box represents the IQR. The error bar shows the 10–90% CI. When MAP was ≥20 mmHg below the LLA, urine flow rate was lower than it was at the LLA (P = 0.049; green box). When MAP was ≥40 mmHg above the LLA, urine flow rate was higher than it was at the LLA (P = 0.029; orange box: P < 0.0001, red box: P = 0.0008).