Of Size, Syrup & Saline - @ JEMS.com

Of Size, Syrup & Saline

The Science Behind Fluid Flow


Howard Rodenberg, MD, MPH, Dip(FM) | | Monday, December 3, 2007

In my last column, we discussed a principle of physics known as Poiselle's Law. In establishing my argument that virtually all breath sounds are related to airflow and that the sound ("pitch") of the airflow is related to airway diameter, we used a very simplified version of this tenet. We now need to examine the law in its entirety, as the dictum has implications beyond breath sounds. In fact, Poiselle's Law controls virtually everything we know about giving IV fluids.

The flow of any substance through a tube is related to a number of factors, all of which are succinctly described by Poiselle's Law. We ended our previous discussion by noting that airflow was related to airway diameter in the bronchial tree. The complete version of Poiselle's Law indicates that tubular flow is proportionately related to not only the diameter of the tube, but also to the viscosity of the substance flowing through the tube and the pressure gradient that causes the directional flow.

Let's start with the impact of catheter diameter on IV flow. As we've noted in our prior discussion, Poiselle's Law states that flow within a tube is proportional to diameter of the tube raised to the fourth power (that is, X4). To determine what this really means, we'll examine the impact on this rule on fluid flow through 14, 18 and 22-gauge catheters. But before we start with the math, you need to know just what needle and catheter gauge really means. (Don't worry. You weren't supposed to know this already.)

The gauge of an IV needle or catheter refers to the number of the devices you can line up side by side within the space of an inch. It's nothing more scientific than that. So you can lay fourteen 14-gauge needles side by side to make an inch, or twenty-two 22-gauge catheters. Technically, the distance we're measuring as a fraction of an inch really pertains to the lumen of the needle; the metal or plastic around the "gauge-sized" lumen adds an insignificantly small number to the total catheter size.

You may have also heard of French sizes of bladder catheters, chest tubes and central lines. As opposed to IV needle gauges, which assign higher numbers to smaller cannula, French sizes do the reverse. The size of the catheter rises with the number.

This is not some kind of Gallic trick. It turns out that French catheters are indeed named after their country of origin. But in the case of catheter sizes, the French actually seem to have gotten the whole numbering game right, just as they succeeded with the metric system.

(For the record, I'm not really a Francophile. While I think the French have gotten a lot of things right Catherine Denueve, for one and I do not make "Freedom Toast" for breakfast, patriots may rest assured that I haven't been won over. I never got the whole concept of the beret. It looks like something worn by Quisp, the propeller-topped alien on the 1960's cereal box. And I find the idea that the Maginot Line could keep out German airplanes just as curious. Which leads us to my theory that Belgium is the Official Dishrag of Western Europe, the Cincinnati Bengals of Warring States. There has to be a reason they always get invaded first. Probably something to do with the fact that it's hard to muster enthusiasm to defend white asparagus, lace tablecloths and a fountain of a small boy relieving himself. But I, as usual, digress.)

According to our definition, the diameter of a 14-gauge needle is 0.0714 inches. The diameter of an 18-gauge catheter is 0.0556 inches, falling to 0.0455 inches for a 22-gauge device. Do the math using Poiselle's Law, and we find that in raising the diameters to the fourth power (multiplying the diameter by itself four times), "flow" is 0.000026 through a 14-gauge catheter, 0.0000096 with an 18-gauge needle and 0.0000043 via a 22-gauge (bear with me for the moment, I'm using "flow" as an abstract concept). Assuming that all else is equal, it means that I can deliver 6 mL of fluid through a 14-gauge needle, or 2.25 mL via an 18-gauge line, in the same time it takes me to give 1 mL of fluid using a 22-gauge device. In realistic terms, this translates to 600 cc of fluid infused through a 14-gauge catheter in the same time it takes to push a mere 100 cc into a 22-gauge line. You can readily see that this is a non-linear relationship; the rise in flow does not proceed in a stepwise, orderly progression. Instead, flow increases exponentially as catheter size grows. Therefore, when we want to give lots of fluids, we start large-bore IV's. It's all due to the physics of Poiselle.

Let's switch gears for a moment and discuss the role of viscosity. Viscosity refers to the resistance of the substance to flow; its "stickiness," if you will. Things that flow easily, like water, are of low viscosity. Substances that resist easy flow, such as pancake syrup and ketchup, are of high viscosity (at least until you hold the bottle up to your eye to see what's taking so long). You can easily surmise that fluids of high viscosity are very resistant to flow through a tube such as an IV line; and that if you have any hope of making such a fluid flow, it can only do so through very large-bore cannulae.

We're fortunate in EMS to not have to deal with viscosity issues very much. There are some chemical differences in viscosity between normal saline and IV fluids with additives such as dextrose, lactate, or colloids, but not really enough to have a practical effect. However, the environment in which we use these fluids may certainly have an effect. Fluids become sluggish and freeze in cold weather; in the presence of heat, the bonds between molecules in the fluids may weaken. These effects are manifested as increases or decreases in the viscosity of the IV fluid, with resultant rises and falls in administration rates.

The pressure gradient between the two ends of the tube is another determinant of flow. All things, be they solid, liquids or gases, will flow down a gradient from higher to lower areas of pressure. The greater the pressure gradient, the faster the flow. The EMS implications of this principle are plain. If you want to increase your IV flow rates, put pressure on the IV bag. Many EMS services carry pressure infusers for this very purpose, and virtually all paramedics learn the trick of putting the BP cuff around the IV bag. However, this principle has a few curious implications for why we do what we've always done.

For example, have you ever wondered why we cannulate veins and not arteries? It's not just that veins are more superficial and easier to hit. It has much more to do with the fact that the pressure in the venous system is lower than that of the arterial tree, and the venous valves actively work to maintain vascular pressures as low as possible. Under conditions of shock, central venous pressure may actually be negative with respect to atmospheric pressure. As a result, when we place an IV cannula into a vein, it eagerly sucks in fluid (for what it's worth, the relatively negative pressure inside the vessel is also why veins collapse). If we were to cannulate an artery, within which the blood pressure exceeds the pressure upon the fluid to be infused, all we would get is an IV line full of pulsating arterial blood.

The pressure differential explains why, although we use gravity to establish a pressure differential when hanging IV fluids, a sure way to infuse fluids faster is to put the IV bag underneath the patient, using his weight to generate pressure on the bag (you can also just lay the IV bag on the ambulance floor and step on it). It's got nothing to do with gravity and everything to do with the pressure differential between the beginning and end points of flow.

Surprisingly, the length of the tube has nothing at all to do with flow. This may seem counterintuitive, for one would think that the longer the area of narrowing, the harder it would be for fluid to pass that obstacle. The fact is that it makes no difference at all; even a single fixed point of a narrowed channel reduces flow throughout the remainder of the system.

Amusing Medical Snippet of the Week

I recently delivered a talk at the annual Asthma Certification Course sponsored by the Allergy, Asthma and Immunology Section of the National Medical Association. Following a colleague's presentation on the diagnosis of asthma, a member of the audience opined that it's important to ask patients if they have difficulty breathing during sex.

I had to stifle a laugh as someone in the back row remarked, "Gee, I thought that meant you were doing it right."

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Related Topics: Industry News, Airway and Respiratory, Medical Emergencies, Research

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