Medical Catheters and Plastics – Part II

In the last blog (Part I of the series), we looked at the definition, application areas and the properties required for catheters. We shall continue the series with the focus on the size of catheters. We will look at the selection of size and effect of catheter dimensions on the flow of fluids through them.

Catheter size

The ‘French’ scale is used to denote the size of a catheter, the Fr (French) number divided by three is the diameter (D) of the catheter in millimeters (mm), i.e.

The French size was devised by Joseph Charriere, a 19th century Parisian surgical instrument maker [18].

The circumference of catheters, C, is only slightly (4.7%) greater than its calculated French size. An increasing French size corresponds to a greater diameter of the catheter; however, the size only corresponds to the external size of the catheter, so the effective volume of the catheter depends on its wall thickness and the lumens (size and geometry).

Table 1 gives some dimensions of catheters in different units of measurement.

Table 1: Catheter size conversion between French sizes to millimeters and inches

In most cases for diagnostic or interventional catheters, sizes between 5 and 7 Fr are used. A catheter is usually in the range of 100–125 cm (40”–50”) in length. Cardiac catheters can range from the simplest cylindrical tubes to more complicated structures. The degree of complexity depends upon the nature of the application of the catheter.

Flow through catheters

When catheters are used to pump fluids of different sorts through them, as in a catheter used to deliver medicinal fluids, the flow rates that can be achieved assume significant importance. The Hagen-Poiseuille equation from fluid mechanics describes the flow of a liquid in a circular orifice thus:

Where,

Q is the volumetric flow rate

ΔP is the pressure drop along the length of the tube

r is the radius of the tube

μ is the viscosity of the fluid being transported through the tube, and

L is the length of the tube.

Since the Hagen-Poiseuille equation applies to flow through rigid tubes, it can be used to describe the flow through vascular catheters, and understand how the dimensions of a catheter can influence the flow rate. The effect of the inner radius of the catheter on the flow rate assumes significance as the flow is directly related to the fourth power of the radius. A change in the catheter diameter can have a profound influence on the flow rate through it. To cite an instance, doubling the inner radius of a catheter, can increase the flow rate through it by sixteen-fold. According to the equation, the influence of length on the flow rate is significantly less than its radius; however, the flow is inversely proportional to the length of the catheter, and it is important to take that into consideration in catheter design. The fluid viscosity is also inversely proportional to the flow rate. Hence, increasing viscosity will decrease the flow through the catheter. The viscosity of commonly used infusions used in intravenous injections range from 1 centipoise (cP) to 40 cP. The viscosity of water at room temperature is ~ 1cP, plasma is mostly water but contains other components such as proteins, electrolytes and other macromolecules, and as a consequence the viscosity of plasma at 37˚C is about 1.8 to 2 times that of water. The viscosity of plasma forms a part of the viscosity of blood which is further determined by red blood cells. The concentration of the red blood cells in the blood, known as hematocrit, has a very strong impact on the viscosity of the blood. At 37˚C, the viscosity of blood is estimated to be between 3 and 4 cP.

If you have any other questions or would like to suggest topics for us to write about, please feel free to contact us at info@polymerupdateacademy.com

Author
Ajay D Padsalgikar (Ph.D. - California, USA)
Trainer, Polymerupdate Academy