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Infusion pumps: why they’re actually not pumping

We are familiar with infusion pumps: they deliver fluids from various containers (bags, bottles, syringes) into veins and arteries. Every hospital uses them, particularly Intensive Care Units, where one pump is generally needed for each drug. But are infusion pumps actual pumps?

These devices often have enough power to pump fluids, including relatively viscous ones; the fluid is pumped through tiny catheters into patients’ blood vessels. Sometimes this occurs with very high flow rates of up to 2 or 3 litres per minute (say, for emergency blood transfusions). But, in many cases, the devices do not pump. So what are they doing instead?

A brief history of infusion pumps

Imagine that you hang a bottle on a drip stand above the patient, connect an infusion set to it and then attach the other side to a patient’s vein, via a cannula. If we now opened the clamp on the tube that connects the cannula to the bottle, allowing the liquid to flow, the bottle would empty itself quite quickly under gravity. In emergencies, paramedics might squeeze the bags to get fluid into the patient more quickly. But, in most situations, the flow needs to be slowed down and controlled.

Historically, medical professionals have monitored flow rates by using drip chambers to count a pump’s drips per minute. Roller clamps have then been used to slow down the natural flow that occurs under gravity. This works well when flow accuracy is not too much of an issue. But, with drugs that need a more controlled flow rate – so as to not overdose or underdose the patient – this gravity system needs regular checking by the nurse or doctor. It would mean counting drops and continuous adjustments by nurses, which is very time consuming.

diagram of how a standard gravity drip infusion pump works

Later on, drop counters were developed to do this counting for healthcare professionals, and alerting them to any change in flow. However, nurses would still then have to adjust the roller clamp manually, so the process remained quite labour intensive.

Eventually, in the 1960s, infusion pumps were invented. These new pumps were capable of keeping a constant and controlled flow without any manual intervention.

In Europe, there are two main types of infusion pumps:

• Large Volumetric Pumps (LVPs), delivering fluid from bags or bottles, just like the drip shown below

• Syringe pumps, delivering smaller volumes of fluids through a syringe

diagram of the different types of infusion pumps including syringe and large volumetric

Large Volumetric Pumps

These pumps are positioned above the patient’s head, meaning intervention is necessary in order to reduce the flow that naturally would occur from gravity (free flow). This free flow can be controlled by mechanisms such as ‘peristaltic fingers’.

These infusion systems still have drip chambers and roller clamps, and can be used just like a standard gravity drip. However, generally a roller clamp is only used to stop the flow when nurses load the bag, or control it if the pump fails for any reason.

Diagram of how a large volumetric pump works

Syringe pumps

Drugs can also be administered through a syringe. You can imagine how hard it is to push a syringe’s plunger manually; it feels stiff and can lead to finger pain for nurses administering the drug. On the contrast, if left to gravity, syringes can run empty in a very short time. Depending on the height of the syringe relative to the patient, the gravity of the fluid can be strong enough to suck the plunger into the syringe (with nothing pushing it), delivering the fluid. This process is called syphoning.

Diagram of how a syringe pump works

That’s where syringe pumps play an important role. They hold the syringe plunger in place, not allowing it to syphon. A set of ‘syringe grippers’ then move at a tightly controlled speed to control the syringe plunger – and thus the flow rate – while the ‘syringe clamp’ holds the syringe body in place.

An in-depth diagram of syringe pump components

So, there you have it. Infusion pumps don’t pump but rather, limit the flow that would naturally occur due to gravity.

The name infusion pumps will stick, it is historic. But, technically, we might want to think of them more as ‘infusion control valves’.

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