A Foley catheter is a flexible medical device designed for continuous urinary drainage from the bladder. This tube is temporarily inserted through the urethra to manage conditions like urinary retention, to monitor output during surgery, or for patients who cannot use a toilet independently. Catheters are manufactured in a wide range of sizes to ensure proper fit and effective function. The size selection is a precise medical decision, as using a catheter that is too small or too large can lead to discomfort or potential complications.
Understanding the French Scale
Catheters and other tubular medical instruments are sized using a standardized system known as the French scale, often abbreviated as Fr. This system, also called the Charrière size, measures the outer diameter of the catheter tube. The larger the French number, the larger the outer diameter of the catheter.
One French unit (1 Fr) is defined as one-third of a millimeter (0.33 mm). This conversion means that multiplying the diameter in millimeters by three will yield the French size. For example, a catheter measuring 1 millimeter in diameter would be equivalent to 3 Fr.
16 Fr vs. 18 Fr: The Direct Comparison
Applying the French scale directly answers the question of which catheter is physically larger: an 18 Fr Foley catheter has a larger outer diameter than a 16 Fr Foley catheter. The 16 Fr catheter has an outer diameter of approximately 5.3 millimeters (mm). In comparison, the 18 Fr catheter is approximately 6.0 mm in diameter. This difference of about 0.7 mm is a significant physical change in the context of the narrow urethra.
Why Different Sizes Matter
The selection between a 16 Fr and an 18 Fr catheter is a decision based on balancing drainage effectiveness with patient comfort. The smallest size that will adequately drain the urine is generally preferred to minimize the risk of urethral trauma or irritation. Using a catheter that is unnecessarily large can increase the risk of bladder spasms and damage to the urethral lining.
A larger diameter, such as 18 Fr, offers a greater internal capacity, which is sometimes necessary for certain clinical situations. For example, a larger bore allows for better drainage when the urine contains thick sediment or blood clots. Larger sizes are also frequently used for continuous bladder irrigation, where a three-way catheter is needed to flush the bladder.
For routine drainage of clear urine in adults, a smaller size like 14 Fr or 16 Fr is often the standard choice. However, if a patient experiences frequent catheter blockage or requires drainage following certain surgical procedures, the increased capacity of an 18 Fr may be necessary.