Solute removal during continuous renal replacement therapy in critically ill patients: convection versus diffusion

Zaccaria Ricci, Claudio Ronco, Alessandra Bachetoni, Giuseppe D'amico, Stefano Rossi, Elisa Alessandri, Monica Rocco, Paolo Pietropaoli, Zaccaria Ricci, Claudio Ronco, Alessandra Bachetoni, Giuseppe D'amico, Stefano Rossi, Elisa Alessandri, Monica Rocco, Paolo Pietropaoli

Abstract

Introduction: The best modality, for continuous renal replacement therapy (CRRT) is currently uncertain and it is poorly understood how transport of different solutes, whether convective or diffusive, changes over time.

Methods: We conducted a prospective cross over study in a cohort of critically ill patients, comparing small (urea and creatinine) and middle (beta2 microglobulin) molecular weight solute clearance, filter lifespan and membrane performance over a period of 72 hours, during 15 continuous veno-venous dialysis (CVVHD) and 15 continuous veno-venous hemofiltration (CVVH)sessions. Both modalities were administered based on a prescription of 35 ml/kg/h and using polyacrylonitrile filters.

Results: Median filter lifespan was significantly longer during CVVHD (37 hours, interquartile range (IQR) 19.5 to 72.5) than CVVH (19 hours, IQR 12.5 to 28) (p = 0.03). Median urea time weighted average (TWA) clearances were not significantly different during CVVH (31.6 ml/minute, IQR 23.2 to 38.9) and CVVHD (35.7 ml/minute, IQR 30.1 to 41.5) (p = 0.213). Similar results were found for creatinine: 38.1 ml/minute, IQR 28.5 to 39, and 35.6 ml/minute, IQR 26 to 43 (p = 0.917), respectively. Median beta2m TWA clearance was higher during convective (16.3 ml/minute, IQR 10.9 to 23) than diffusive (6.27 ml/minute, IQR 1.6 to 14.9) therapy; nonetheless this difference did not reach statistical significance (p = 0.055). Median TWA adsorptive clearance of beta2m appeared to have scarce impact on overall solute removal (0.012 ml/minute, IQR -0.09 to 0.1, during hemofiltration versus -0.016 ml/minute, IQR -0.08 to 0.1 during dialysis; p = 0.79). Analysis of clearance modification over time did not show significant modifications of urea, creatinine and beta2m clearance in the first 48 hours during both treatments. In the CVVHD group, the only significant difference was found for beta2m between 72 hours and baseline clearance.

Conclusion: Polyacrylonitrile filters during continuous hemofiltration and continuous hemodialysis delivered at 35 ml/kg/h are comparable in little and middle size solute removal. CVVHD appears to warrant longer CRRT sessions. The capacity of both modalities for removing such molecules is maintained up to 48 hours.

Figures

Figure 1
Figure 1
Kaplan-Meier analysis of circuit survival for continuous veno-venous hemofiltration (CVVH) and continuous veno-venous dialysis (CVVHD).
Figure 2
Figure 2
Time weighted average (TWA) clearance of (a) β2 microglobulin (beta2mic), (c) urea and (d) creatinine by convective and diffusive transport. (b) Beta2mic adsorptive clearance during continuous veno-venous hemofiltration (CVVH) and continuous veno-venous dialysis (CVVHD). Data are expressed as median and interquartile range. None of these comparisons reaches statistical significance.
Figure 3
Figure 3
Behaviour of (a) β2 microglobulin (beta2mic), (b) creatinine and (c) urea clearance over time for continuous veno-venous hemofiltration (CVVH) and continuous veno-venous dialysis (CVVHD). Beta2mic removal decreased significantly with respect to baseline during CVVHD at T4 (72 hours). Data are expressed as median and interquartile range. The asterisk indicates p < 0.05.

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