The relationship between IV catheter gauge and flow rate seems to be widely understood, but there could be standard practices in your fluid administration that are making your sweet 16 gauge in the AC perform more like a 20 in the hand. The limitation isn’t with the catheter itself, but rather the different equipment we regularly use as part of our standard care practice. Williams et al. (2016) studied the effects of different IV products (needleless connectors and extension tubing) from various manufacturers we regularly use when administering blood and fluid through a Belmont rapid infuser with different catheter gauges. The authors connected various pieces of equipment, including needleless connectors and extension tubing, to different catheter gauges and observed up to an 84% decrease in flow rate in large-bore catheters.
The flow rate is drastically reduced when a needleless connector is placed on a cordis (Williams et al., 2016).
Those little hubs that revolutionized the industry by allowing us to access the IV line without a needle also put a big speed bump in the way of our fluids. The design itself varies among manufacturers but the effect on laminar flow is consistent. There are many factors that affect laminar flow in fluids or smooth, consistent flow in a single direction. Luckily, we have a simple equation to explain this: ΔP=8Qμl/πr4. Yikes.
A needleless connector can divert flow and introduce significant turbulence (Williams et al., 2016).
The opposite of laminar flow is turbulent flow, so it might be easier to imagine the factors that contribute to turbulent flow including anything that alters the path of our fluid. Think of a river with large boulders, twists, and turns compared to a straight canal made of concrete. It is much easier for the water to flow down the canal without any interruptions. We also have to consider the viscosity of the fluid which can magnify these effects when we infuse fluids like blood.
Effects of Length
Imagine you have a 50-foot garden hose and a 100-foot garden hose both attached to a spigot that supplies an identical amount of water pressure to each. The laws of fluid mechanics dictate that the flow rate varies inversely with length, which means the 50-foot hose will have almost double the flow rate of the 100-foot hose. Put into fluid administration terms, the same volume in half the time when using half the IV tubing length. This doesn’t mean you can actually double your fluid administration rates as there will be other maximum limits in the system, notably the flow rate of the catheter.
Adding on multiple pieces of equipment can compound the decrease in flow, affecting larger gauges the most (Williams et al., 2016).
What Does All of This Mean?
There are many standard products we use every single day when initiating IV access on our patients that have the potential to limit the function of a large-bore catheter when resuscitating. These effects become even more pronounced when infusing thicker fluids like blood. The items include, but are not limited to needleless connectors, IV tubing length, stopcocks, and infusion ports on the IV tubing. It is easy to imagine how the use of just a few of these items can drastically lead to lower infusion rates if the right combinations are used. Learn the equipment you have and what steps you need to take to mitigate these effects. In a situation where rapid fluid administration is needed, this may involve not using or removing the needless connector, utilizing IV tubing without multiple access ports, and/or decreasing the length of IV tubing to suit your situation. It may even be worth considering attaching your infusion line directly to the catheter hub.
Thank you to the authors for bringing light to a subject that is commonly overlooked in our daily practice. They do a great job detailing the effects of various products on the market in different situations and as such I will not go into depth on each of the items reviewed. Please check out their article below for more information and feel free to reach out to me if you have any questions.
- Williams, J. H., Foushee, M., Bortsov, A., & McGraw, K. A. (2016, October 26). Goodbye, Large-Bore IV Access. Retrieved from https://www.anesthesiologynews…