Supplementary Material for: Impact of continuous renal replacement therapy initiation on urine output and fluid balance: A multi-centre study.

Introduction The effect of continuous renal replacement therapy (CRRT) on renal function is poorly understood. However, the initiation of CRRT may induce oliguria. We aimed to investigate the impact of CRRT commencement on urine output (UO). Methods Retrospective cohort study in two intensive care units. We included all patients who underwent CRRT and collected data on hourly UO and fluid balance before and after CRRT commencement. We performed an interrupted time series analysis using segmented regression to assess the relationship between CRRT commencement and UO. Results We studied 1057 patients. Median age was 60.7 years (IQR, 48.3 – 70.6) and the median APACHE III was 95 (IQR, 76 – 115). Median time to CRRT was 17 hours (IQR, 5 - 49). With start of CRRT, the absolute difference in mean hourly urine output and mean hourly fluid balance was -27.0 mL/hr (95%CI -32.1 to -21.8; p value < 0.01) and – 129.3 mL/hr (95%CI -169.2 to -133.3), respectively. When controlling for pre-CRRT temporal trends and patient characteristics, there was a rapid post initiation decrease in urine output (-0.12 mL/kg/hr; 95%CI -0.17 to -0.08; p value <0.01) and fluid balance (-78.1 mL/hr; 95% CI -87.9 to -68.3; p value <0.01), which was sustained over the first 24 hours of CRRT. Change in urine output and fluid balance were only weakly correlated (r -0.29; 95% CI -0.35 to -0.23; p value <0.01). Conclusion Commencement of CRRT was associated with a significant decrease in urine output that could not be explained by extracorporeal fluid removal.