The flow of fluids through an IV catheter can be described by Poiseuille’s Law. It states that the flow (Q) of fluid is related to a number of factors: the viscosity (n) of the fluid, the pressure gradient across the tubing (P), and the length (L) and diameter(r) of the tubing.
Tubing diameter: An important and frequently cited relationship is that of the tubing’s radius. Doubling the diameter of a catheter increases the flow rate by 16 fold (r4). The larger the IV catheter the greater the flow.
Fluid Viscosity: Flow is inversely proportional to the viscosity of the fluid. Increasing viscosity decreases flow through a catheter. Viscosity of commonly infused intravenous solutions range from 1.0 centiPoise to 40.0 cP (Reference: viscosity of water is 1.002 cP).
Viscosity of common infusions:
- 1.0 cP Lactated Ringers
- 4.0 cP Hetastarch
- 40.0 cP 5% albumin
Of note, the viscosity of blood increases with increasing hematocrit and decreasing temperature, hence one the rationales for fluid warmers: Warming and diluting blood prior to administration increases flow rates.
Pressure: Increasing Pressure further maximizes flow as described by Poiseuille’s Equation. This can be achieved by elevating fluid containers (IV bags). Utilizing pressure bags or pressurized infusion devices are additional ways to increase the pressure gradients to optimize flow rates.
To maximize flow, an ideal rapid infusion system would consist of the largest diameter and shortest length tubing possible. Infused fluid should be of the lowest possible viscosity and it should be delivered under maximum possible pressure.
Poiseuille’s law: Application