Medical procedures often involve thin, tubular instruments, such as catheters, which require precise sizing for patient safety and procedural effectiveness. The size of these instruments is frequently measured using a unit abbreviated as ‘Fr,’ which stands for the French catheter scale, also known as the Charrière scale. This scale provides a common language for manufacturers and medical professionals to describe the physical dimensions of these devices. Accurate sizing is paramount because small variations in diameter can impact patient comfort, the risk of tissue damage, and the device’s functionality.
Understanding the French Catheter Scale
The French catheter scale is a system designed to measure the outer diameter of thin, flexible medical instruments. Developed in the 19th century by Joseph-Frédéric-Benoît Charrière, the French unit (Fr) directly relates to the instrument’s diameter in the metric system.
One French unit is defined as equal to one-third of a millimeter in diameter. To determine the outer diameter of an instrument in millimeters, one divides the French size number by three. For example, a device labeled 9 Fr has an outer diameter of exactly 3 millimeters.
This relationship establishes a linear connection between the French number and the physical size of the instrument. The French scale is straightforward: a higher French number always corresponds to a larger outer diameter of the catheter or tube.
The French scale measures only the external dimension, which interacts with the body’s structures, not the internal channel’s size, which can vary based on the device’s design.
Comparing 16 Fr and 18 Fr Sizes
Yes, an 18 Fr catheter is larger than a 16 Fr catheter. The French scale’s direct proportionality means that any increase in the Fr number represents a physically wider instrument. This difference can be quantified using the conversion rule of 1 Fr equaling 1/3 millimeter.
A catheter sized at 16 Fr has an outer diameter of approximately 5.33 millimeters. Conversely, a catheter sized at 18 Fr has an outer diameter of exactly 6.0 millimeters. Although the difference is only 0.67 millimeters, this variation is significant within the human body’s passageways.
The choice between these two sizes is often made based on the desired flow rate and the need to minimize trauma. An 18 Fr catheter offers a wider channel, which can provide better drainage capacity and approximately 19% less resistance to fluid flow compared to a 16 Fr size.
However, the slightly smaller 16 Fr catheter is often preferred for routine procedures because it causes less potential irritation or injury to the delicate lining of the urethra or other vessels.
Medical Devices That Use the French Scale
The French scale is broadly applied across a variety of tubular medical devices where the external measurement is the primary determinant for use. The most common application is with urinary catheters, such as Foley and intermittent catheters, which are inserted into the bladder for drainage.
Sizing ensures the catheter passes through the urethra without trauma while providing effective fluid removal.
The scale is also used for nasogastric and feeding tubes, which deliver nutrition or medication into the stomach through the nose. For these devices, the Fr size dictates the ease of insertion and the rate at which thicker fluids can be administered.
Interventional radiology utilizes the French scale for vascular sheaths and guide catheters, which are temporarily placed into blood vessels to allow other instruments to pass through.
Precise sizing is necessary for these instruments to achieve therapeutic goals while navigating the body’s intricate anatomy. A tube that is too large can lead to damage and pain, while one that is too small may not function effectively.
Therefore, the French number provides practitioners with the necessary technical detail to select the optimal tool for a specific patient and procedure.