The M number was designed to describe the pressure-flow characteristics of a vascular cannula, catheter or vascular access device. This allows comparison of different catheters and to help select appropriate catheters for a procedure.


Variables affecting catheters include length, internal diameter, wall thickness, the number and placement of side holes.


Parameters use to calculate the M number:

(1) viscosity of whole blood in poise

(2) the pressure drop through the catheter in mm Hg

(3) the blood flow in mL per minute



• The viscosity of blood is about 0.03 poise (3 centipoise) when the hematocrit is 41%. The viscosity at other hematocrits is discussed in Chapter 3.

• De is the effective diameter.

• L is the catheter total wetted length in cm.


LDe =

= (7.161 * (10^6)) * ((viscosity)^(-0.25)) * (pressure in mm Hg) * ((blood flow)^(-1.75))


M number =

= LOG10(LDe)


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