IV Catheter Size: How much of a difference does it make?
Resuscitation protocols almost ubiquitously state the need for two large-bore IVs for rapid fluid administration. The fact that more fluid flows through a larger tube shouldn’t surprise anyone. It’s the reason that straws at fast food restaurants are so much wider than normal straws — larger diameter means more gulp per suck. It’s also the same reason that firemen use larger attack lines (ie. ‘deuce-and-a-half’; 2.5″) for commercial fires as compared to residential fires. Big fires need big water.
If you’re like me, however, you’ve probably wondered how much of a difference it really makes to use an 18ga catheter over a 20ga catheter. After all, from initial inspection they look pretty similar in size. In my experiences as a prehospital provider, I considered the 18ga IV to be fairly routine. Sure, it’s a relatively large-bore catheter (and it’s a little more painful to insert), but once it’s in, I knew that I would have great access for anything the patient required. Once I started working in the hospital, however, I noticed that 20ga catheters became more common. Frequently I would be asked to help place a ‘large-bore’ IV so that a patient could receive CT contrast. The implication was that I would place a 20ga catheter, which I found to be somewhat paradoxical.
Just so that we’re talking the same language, let’s review some of the factors to influence flow rate. Feel free to skip this section if the theory doesn’t appeal to you. The clinical stuff is down below.
Let’s assume a perfect scenario, where fluid of the same consistency (gas or liquid) flows through a perfectly straight pipe. As Isaac Newton reminds us, “For every action, there is an equal and opposite reaction.” In other words, whenever something is pushed, it is also pulled. When you sit in a chair, gravity is pulling your butt into the seat, and the seat is pushing up onto your butt. In our pipe and fluid example, one force is pushing the fluid through the pipe, and the other force is preventing the fluid from moving. When considering fluids, the difference in pressure between the beginning of the tube and the end of the tube is what drives the flow forward. The flow is slowed, however, by resistance from the walls of the tube.
In perfect circumstances, when a fluid flows in a laminar (“straight”) manner through a tube, the rate of flow is dependent upon the amount of resistance it encounters. Close to the sides of the tube, there is a lot of resistance from the fluid ‘rubbing’ onto the walls. This makes the flow very low. As you examine the fluid farther and farther from the wall, the flow will increase because the contribution of wall resistance is smaller.
This flow relationship in the pipe ends up looking like a parabola. To demonstrate this principle in action, I have drawn you a picture of laminar flow through a tube.
With a smaller diameter tube, the contribution of resistance will be higher than a really large tube, where the center of flow is very far from the wall.
The second thing to consider is the length of the tube. The longer a tube, the more time that fluid will be in contact with its walls, which contributes to resistance. This is the same reason why you don’t see any 50 ft USB cables. The flow through the wire (electricity) is also impacted by resistance. Since USB uses a relatively low voltage, there isn’t enough driving force to get a quality signal more than about 16 feet without significant signal degradation (according to USB specifications).
The final two things to consider are viscosity and turbulence.
Turbulence is unavoidable in real life. In the human body, we don’t see perfectly straight pipes like the ones shown above. Our vessels are dynamic, and differ in size and shape along their course. When turbulence does occur, the flow through the pipe is no longer nice and straight. Rather, it becomes chaotic, and disrupts the normal layering of flow. This creates additional resistance. Thus, factors that increase turbulence also increase resistance to flow.
The conversion of laminar flow to turbulent flow can be predicted using Reynold’s number, and takes into account viscosity (‘stickiness’) of fluid, and the speed (velocity) of the fluid. Viscosity can be easily understood by comparing water to maple syrup. Water flows easily and is very ‘thin’ as compared to maple syrup, which has a thick and sticky quality, which holds its flow together.
When the speed of the fluid is high, this promotes increased turbulence. Likewise, when the viscosity of the fluid is low, turbulence is promoted.
Turbulence may also be affected by the diameter of the tube. If you try to push 100cc of fluid through a coffee straw, and then try to push 100cc of fluid through a garden hose, you will find that you must push the fluid through the coffee straw at a much faster speed in order to finish at the same time as the garden hose. As we said earlier, when the speed of the fluid is high, turbulence is promoted. Thus, a larger pipe is inherently less prone to turbulent flow when the same volume of fluid must be infused over the same period of time.
Having considered the factors that influence flow, we can examine how they interact with one another.
As with all things in physics, this equation does not perfectly approximate the real world. It assumes no turbulence, only simple laminar flow. Nonetheless, it gives us valuable information about the variables which contribute most to flow.
- Pressure Difference: Flow of a fluid is driven from high pressure to low pressure. When you pop a bottle of champagne, the fluid inside the bottle always flows outward, because of this pressure difference. Thus, if you increase the pressure difference (ie. squeeze / apply a pressure sleeve to your IV bag), you will increase the flow. Importantly, this pressure difference can also be increased by increasing the height of the bag above the patient. Since the potential energy of the fluid in the bag is related to its height above ground, the greater the difference in height between the bag and the patient, the more kinetic energy will be able to contribute to flow.
- Radius of Tube: This is huge. In the equation, the flow is related to the radius to the fourth power. This means that the flow exponentially increases by tube radius. For example, if you double the radius of the tube, you increase the flow rate 16x.
- Viscosity of Fluid: If you increase the viscosity, you decrease the flow rate. “Sticky” fluids tend to flow less readily than thin fluids (maple syrup vs. water). This is why it is so hard to push an amp of D50. It requires much more pressure to make D50 flow through the cannula at the same rate as water. As we discussed above, a thin fluid is more prone to turbulence, however, which can also contribute to resistance. In the case of Poiseuille’s Law, turbulence is not factored in.
- Length of Tube: The longer the tube, the lower the flow rate. This is because the fluid will be in longer contact with the walls of the tube, allowing resistance to play a larger role in the overall flow.
The influence of these variables is, of course, theoretical and mathematical. Do they pan out in actual practice?
Reddick et al. published a simple experiment in 2011 to examine flow rates with different intravenous devices. In general, they found good agreement with Poiseuille’s Law. I have abridged their data to include peripheral IVs only (the stuff that you all are going to use most frequently).
The best comparison is between the 14ga and 16ga catheters, as they have exactly the same length. Despite all other variables being equal, the catheter gauge alone improved flow by 65%! This is the difference between 4.2 min for your liter bag to empty and 6.4 min for your liter bag to empty (and we’re not even considering the additional application of pressure). They also interestingly found that the addition of a needle-free connector (“bung”) reduced flow by 40% with peripheral IVs…
When things are so small, it’s hard to imagine how big the difference between two gauges is. So I took the internal diameters of the most common gauges of angiocath, and blew them up to a size that we could understand better. In the diagram below, the diameter of a 24 gauge catheter is equivalent to a 6ft man.
When viewed this way, we can imagine a tremendous difference between these catheter gauges! I mean, a 14 gauge is like…
So, in conclusion, there appears to be a significant difference between catheters, even when they are only one size larger or smaller. If you want to get big water into your patient, observe the following recommendations:
- Gauge is king. Choose the fattest tube you can stick into the patient.
- When it comes to catheter length, it’s not the size… it’s how you use it. Shorter catheter lengths mean less resistance.
- Consider plugging your drip set directly into the catheter hub instead of using a needle-free adapter or saline lock. The aforementioned study reports significant loss of flow through these types of connectors.
- Optimize the pressure difference between the bag and the vein. Place the bag high above the patient and use a compression sleeve, if indicated.
