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Original article
Pressure-related flow rates for continuous renal replacement therapy in very small children: an in vitro study
  1. Isabella Stevens-Harris1,2,
  2. Dusan Raffaj1,2,3,
  3. Patrick Davies1,2
  1. 1 University Of Nottingham, Nottingham, UK
  2. 3 Public Health, Saint Elizabeth College of Health and Social Work, Bratislava, Slovakia
  3. 2 Paediatric Critical Care Unit, Nottingham Children's Hospital, Nottingham, UK
  1. Correspondence to Dr Patrick Davies; Patrick.Davies{at}


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.

  • Intensive Care
  • Paediatric Practice
  • Nephrology
  • Neonatology

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  • Contributors IS-H completed the experimental work and drafted the paper. DR advised on study design and edited the paper. PD conceived the study, advised on study design, edited the paper and is guarantor.

  • Funding This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors

  • Provenance and peer review Not commissioned; externally peer reviewed.

  • Data sharing statement Source data is available on request from the corresponding author.

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