Pressure-related flow rates for continuous renal replacement therapy in very small children: an in vitro study

OBJECTIVE: Continuous renal replacement therapy (CRRT) is extremely challenging in very small children, as most CRRT intravascular access devices are too large. We aimed to quantify flow rates through several alternative intravascular devices. DESIGN: Experimental in vitro study simulating CRRT. SETTING: Whole milk and equine blood were used as human blood substitutes due to similar viscosity. Milk under gravity pressure was run through a standard CRRT circuit. Equine blood was run through a working CRRT machine. SUBJECTS: Eight intravenous access devices used in paediatrics, with a variety of connectors. INTERVENTIONS: Devices were tested with milk for flow between 50 and 200 mm Hg pressure, and with blood for pressure at flows between 20 and 50 mL/min. MAIN OUTCOME MEASURES: Flows at each input pressure with milk, and pressures at each flow rate with blood. RESULTS: With both experimental systems, 8Fr and 6.5Fr haemodialysis catheters, and 18G and 20G cannulae allowed excellent low pressure flow rates. 5Fr triple central catheter, 5Fr and 4Fr umbilical venous catheter, and 5Fr haemodialysis catheters did not allow flows at reasonable pressures for CRRT. A three-way tap did not impede flows, but a needle-free valve did. Flows increased with pressure non-linearly, presumably due to increasing turbulence. CONCLUSIONS: In very small patients needing CRRT, where large haemodialysis catheters cannot be used due to the patient size, we advise the use of two 18G cannulae in different sites. A three-way tap can be added, but not any other connectors. In vitro this system gave suitable flow rates with some flexibility for in vivo variations.