Catheter Size and Fluid Resus

A review on why short and fat is a good attribute for intravenous catheters and some other things about flow.

Q (flow), P(pressure), r (radius), L (length), η (viscosity)

Radius is the most important factor as it is to the 4th. A doubling of the internal diameter increases the flow rate by a factor of 16. A decrease in length of the catheter and a decrease in viscosity of the fluid will increase the flow rate. Viscosity change is important with products such as blood (i.e. blood is mixed with saline and warmed in a rapid infuser, yes there are other reasons a rapid infuser uses saline and warming). An increase in the pressure increases flow (i.e. pressure infusion bag).

Some numbers:

  • 18g x 30 mm: flow rate = 105 ml/min
  • 16g X 30mm: flow rate = 220 mL/min
  • 14g x 30mm: flow rate = 295 ml/min
  • 16g x 200 mm (distal lumen of triple lumen catheter): flow rate = 60 ml/min
  • 8.5F x 100 mm (introducer catheter): flow rate = 233 ml/min with pressure bag at 300mmHg flow rate = 444ml/min

So what have we learned….short and fat is good (14g X 30mm or 8.5 ID cath) and pressure bags at 300mgHg dramatically increase the flow rate. I could not find a flow rate for 14 gauge iv catheter with pressure bag (in vivo data) although it likely will be greater than the 8.5F introducer catheter.

NOTE: The flow rates were taken from different manufacturers websites. Flow rates are relatively close to each other as reported by different manufacturer data. Most studies in literature are experimental flow whereas manufacturer data is based on in vivo studies.

The ICU book 3rd ed., Marino

McPherson D, Adekanye O, Wilkes AR, Hall JE. Fluid flow through intravenous cannulae in a clinical model. Anesth Analg. 2009 Apr;108(4):1198-202.

Stoneham MD. An evaluation of methods of increasing the flow rate of i.v. fluid administration. Br J Anaesth. 1995 Sep;75(3):361-5.

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