A World Without Pumps: Medicine

Delivering Wellness Without Pumps: One Drop at a Time

In a world without pumps, wellness comes slowly. It drips, slower than grains of sand in an hourglass.

Without pumps, development of medicine relies on outdated manufacturing processes; delivery of medical advancements grinds slowly and imperfectly; and the manufacturing of life-saving medical devices stalls as patients anxiously wait for life-saving solutions that seldom arrive.

Fortunately, today we live in a time when pumps do play an essential role in diagnosing and treating patients. Pumps are now central to the development of ever-evolving, sophisticated medical devices that keep us healthy and help us lead better lives.

In the world of medicine, we can’t live without pumps.

As children, we learned that the heart is an organ that pumps blood throughout our body. But the heart—which functions like a reciprocating pump, or a class of positive-displacement pumps that include the piston pump, plunger pump, and diaphragm pump—isn’t the only pump found genetically in our human system. Our lungs operate like diaphragm pumps, and digestion relies on pumps, as does our esophagus, and our intestines function like a peristaltic pump.

Our human pumps resemble mechanical pumping systems. Some compare man-made and nature-made to using a propeller to make an airplane fly rather than a pair of wings.

Many of us are familiar with simple devices labeled “pumps”: blood pressure pumps, angioplasty, air pumps, and others that compress air but do not pump a liquid. But when we talk about delivering precise and repeatable flow in many medical devices and fluid-dispensing applications, we look at positive displacement metering pumps.

These enable liquids—ranging from test samples to reagents and wash fluids—to be transferred, dispensed, or metered, depending on the application. As devices are designed to process smaller and smaller volumes of fluids, the requirements for precise metering pumps become greater. Because of the large number of different pumps available, proper pump selection is extremely important to achieve efficient flow control in medical devices that handle fluids.

A variety of methods are available to accurately meter fluids in medical device applications. Selecting the appropriate pump depends on the control and accuracy required for the device. Recent pump designs have combined the advantages of traditional diaphragm liquid-metering pumps with electronically controlled stepper-motor drive technology, broadening the fluid-metering options available to medical device designers. Below, we describe how some of the pumps used in medicine and medical devices work.

Positive Displacement
Because many medical devices pump fluids, fluid transfer is sometimes accomplished using either centrifugal or positive displacement pumps. Centrifugal pumps transfer energy to a fluid through a spinning impeller, converting the impeller energy to fluid pressure, which moves the fluid. Because these types of pumps are pressure and fluid dependent, they are not typically used for metering because they can’t maintain precise flows under changing inlet and discharge conditions. But they do provide high flow rates at low pressures. Typical applications for centrifugal pumps in medical devices would include heating or cooling equipment or patients, as well as filtration of water or other fluids.

Positive displacement pumps trap a fixed volume of fluid and move this fluid with gears, pistons, diaphragms, vanes, or other devices. These pumps, which typically operate at lower speeds, are less sensitive than centrifugal pumps to changes in discharge and suction conditions. Flow is regulated by adjusting speed and displacement. These features make positive displacement pumps an obvious choice for metering fluids. Additional applications for these pumps would be to precisely dose specific reagents into cell samples to look for certain reactions. These are typically related to instruments for DNA sequencing, lab screening for cancer or processes.

One of the earliest medical devices still in use today is the syringe with hypodermic needle. Although syringes were used experimentally on animals in the mid 1600s, routine use with human patients began in the mid 1800s.

Syringes
One of the earliest medical devices still in use today is the syringe with hypodermic needle. Although syringes were used experimentally on animals in the mid 1600s, routine use with human patients began in the mid 1800s.

Syringes consist of three main components: a barrel, plunger, and piston. Early syringes were made of glass or metal and have remained relatively unchanged except for materials. We’ve all been subjected to hand-powered syringes when we receive an injection, but syringes are also used for injecting fluid into an intravenous tube. This is usually done manually, but an IV infusion pump is often used in hospitals. The infusion pump injects a precise amount of fluid into a patient’s IV line over an extended period.

The syringe pump is the simplest form of the piston pump, which is designed to meter up to the volume of one full stroke of the syringe. By accurately stepping the piston on a syringe pump, precise dispense rates can be obtained in microliters.

Rotary Metering Pumps
Gear-type rotary pumps use gear teeth, lobes, or vanes to trap a fixed volume of fluid. A rotary motion carries the fluid from the inlet to the outlet of the pump. In a gear pump, for example, two meshing gears rotate in a closed cavity with minimal clearance between the gear teeth and the pump casing. Fluid captured between each tooth and the casing at the inlet port is carried to the outlet port. By maintaining precise volumes between the teeth and low leakage rates between each tooth and the casing, a gear pump can provide accurate metering. It’s easy to achieve flow control by controlling the rotational speed of the gears. The accuracy of the control speed of the motor often determines the pump’s flow accuracy. Lobe and vane pumps operate similarly but substitute smooth lobes or vanes for the gear teeth.

Rotary pumps that use gears or lobes provide low pulsation and continuous flow. They can produce high pressures and handle high viscosities. These pumps require no valves and can handle shear-sensitive fluids. However, rotary pumps have surfaces that can rub and wear, and they require dynamic seals, which can also wear out and leak.

Peristaltic Pumps
Peristaltic pumps can accurately meter very low flows down to fractions of a milliliter. These pumps require no valves and have a seamless, sterilizable flow paths in which the fluid never contacts the pump. They can handle particulates and are easy to maintain.

Reciprocating Pumps
Reciprocating pumps displace a fixed volume of fluid through the reciprocating motion of a piston, a diaphragm, or a bellows. In the simplest example, a piston is drawn back in a closed chamber, creating a vacuum that draws in a fixed volume of fluid. The piston then moves forward and expels the fluid. In this way, accurate flow control can be achieved by controlling either the stroke length of the piston or the piston’s stroking speed.

Diaphragm Metering Pumps
Diaphragm metering pumps compensate for some of the disadvantages of piston-style pumps by replacing the piston with a flexible diaphragm. Because clamping around the edge seals the diaphragm, the pump uses no dynamic seals, which can wear. This eliminates leakage and contamination of the pumped fluid.

Liquid Diaphragm Pumps
Liquid diaphragm pumps use an eccentric to move a diaphragm up and down inside a chamber. On the down stroke, liquid is drawn into the chamber through a nonreturn valve. The valve closes as soon as the diaphragm starts to move upward. This movement compresses the liquid and forces it out of the chamber through another nonreturn valve, thus producing flow. This pumping concept is effective for handling either liquid or gases.

Traditional Diaphragm Liquid-Metering Pumps
Flow control is normally exercised by changing the pump stroke or, alternatively, by changing the rotation speed of the motor. In the case of ac motors, rotation is normally from 2800 to 3200 rpm, but with dc motors, rotation speed can range from 1200 rpm to 4000 rpm. For relatively low flow rates, stepper motors have been used, which have speeds up to approximately 300 rpm. Conventional diaphragm metering pumps are reliable, self-priming, and chemically resistant.

Other Pumps
Certain pumps that we’re familiar with don’t move liquid but are also part of the medical pump ecosystem. A pump familiar to us, for example, is the blood pressure cuff pump (a sphygmomanometer). A generation or two ago, blood pressure was taken using what may seem to be crude instruments now. A doctor or nurse wrapped a sleeve around the patient’s upper arm, placed the end of a stethoscope on the patient’s arm, and repeatedly squeezed a bulb that pumped air into the sleeve. As the sleeve inflated, it applied pressure (indicated by a pressure gauge) to the arm.

The pressure restricts blood flow in the arm and the doctor or nurse use a stethoscope to listen for blood to begin flowing and for when it stabilized, while simultaneously monitoring the pressure. They then record the pressures—systolic and diastolic, both expressed as millimeters of mercury (mm Hg). This manual procedure is still used today but is now usually automated. The difference is that with automated machines, sensors measure your systolic and diastolic blood pressure.

When open-heart surgery, heart and lung transplants, and other serious and deeply invasive procedures are being conducted, they require stopping a patient’s heart, lungs, or both. In these cases, a cardiopulmonary bypass machine (CBM) is used. This machine receives blood from the patient’s circulatory system through a sterile tube. Once in the CBM, the blood is oxygenated and pumped back into the patient’s body. The CBM is operated by a perfusionist, who monitors the patient’s vital signs and adjusts the machine to ensure blood is pumped at the precise rate and pressure to sustain the patient through surgery. Centrifugal pumps are also used to circulate chilling agents through blankets and wraps to bring the patient’s body and blood temperature down to a level low enough to perform the operation.

Another device is an intra-aortic balloon pump (IABP), typically used during angioplasty, which controls the flow of blood through the aorta. A catheter places a deflated bladder into the aorta, near the heart. The deflated bladder allows the heart to pump blood out to the patient’s body. When the heart relaxes, the IABP pumps gas into the catheter to inflate the bladder. The inflated bladder restricts flow to keep more blood in the heart and to widen narrowed or obstructed arteries or veins. Air pumps have found their way to numerous applications from hospital settings and even to homes. The CPAP—Continuous Positive Airway Pressure-air pump—is one example.

Healthcare Without Pumps
Because our bodies contain pumps, obviously we cannot live without them. But if we had to live without man-made pumps the medical community, as we know it, would be back in the dark ages—or worse. The only medicines we could take would be those that can be taken orally or those applied to the affected area, and forget about most modern diagnostic instruments.

About all healthcare workers would be able to do is take a patient’s pulse and temperature. The instructions would be to take two aspirin and not call anyone in the morning.

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