The relationship between commencement of continuous renal replacement therapy and urine output, fluid balance, mean arterial pressure and vasopressor dose

Background and objectives: The effect of initiating continuous renal replacement therapy (CRRT) on urine output, fluid balance and mean arterial pressure (MAP) in adult intensive care unit (ICU) patients is unclear. We aimed to evaluate the impact of CRRT on urine output, MAP, vasopressor requirements and fluid balance, and to identify factors affecting urine output during CRRT.

Design: Retrospective cohort study using data from existing databases and CRRT machines.

Setting: Medical and surgical ICUs at a single university-associated centre.

Participants: Patients undergoing CRRT between 2015 and 2018.

Main outcome measures: Hourly urine output, fluid balance, MAP and vasopressor dose 24 hours before and after CRRT commencement. Missing values were estimated via Kaplan smoothing univariate time-series imputation. Mixed linear modelling was performed with noradrenaline equivalent dose and urine output as outcomes.

Results: In 215 patients, CRRT initiation was associated with a reduction in urine output. Multivariate analysis confirmed an immediate urine output decrease (–0.092 mL/kg/h; 95% confidence interval [CI], –0.150 to –0.034 mL/kg/h) and subsequent progressive urine output decline (effect estimate, –0.01 mL/kg/h; 95% CI, –0.02 to –0.01 mL/kg/h). Age and greater vasopressor dose were associated with lower post-CRRT urine output. Higher MAP and lower rates of net ultrafiltration were associated with higher post-CRRT urine output. With MAP unchanged, vasopressor dose increased in the 24 hours before CRRT, then plateaued and declined in the 24 hours thereafter (effect estimate, –0.004 μg/kg/ min per hour; 95% CI, –0.005 to –0.004 μg/kg/min per hour). Fluid balance remained positive but declined towards neutrality following CRRT implementation.

Conclusions: CRRT was associated with decreased urine output despite a gradual decline in vasopressor and a positive fluid balance. The mechanisms behind the reduction in urine output associated with commencement of CRRT requires further investigation.