Central venous catheters tend to be triple lumen (16G and two 18G) or quadruple lumen (14G, 18G, 16G, 18G).
Gauge system: (Stubbs Gauge-Peter Stubbs, English engineer)
Needle size is described by a wire gauge classification which originally referred to the number of times a wire could be drawn through a draw plate. This system is also used for cannulae.
Laminar flow (smooth and without eddies) is governed by the Hagen-Poiseuille equation.
Q = flow
n = viscosity of the fluid
pi = 3.1415926
r = radius of the tube
P = pressure gradient
L = length of the tube
Thus, it can be seen that flow is directly proportional to the radius4 and is inversely proportional to the length. Thus, greatest flow is seen in shorter, wider cannulae.
14G 250-360 ml/min
16G 130-220 ml/min
18G 75-120 ml/min
(Jean Poiseuille 1797-1869, French physiologist, Gotthilf Hagen 1797-1884, German engineer)