Flow rates are, in theory, determined by Poiseuille’s Law, which states that the flow rate depends on fluid viscosity, pipe length, and the pressure difference between the ends of the pipe .
Of course we won’t be calculating this during a resuscitation! Nor would it be useful if we did: the equation assumes laminar flow, whereas turbulent flow is more likely. Nor is it practical to look up the viscosity of crystalloid/blood/plasma, which also dramatically impacts flow rates.
Instead, remember this equation: Larger + shorter = faster
And keep in mind the following:
In practice, our friends in Australia actually put common catheters to the test, and provided these helpful results:
Or, as a picture:
Note, these flow rates were achieved using crystalloid. Blood will be slower due to higher viscosity.
