Infusion Pumps - Clinical Tree

Background

History And Evolution Of Infusion Pumps

In 1638 a British physician named William Harvey first described the circulatory system and found that the heart pumps blood throughout the body by continuous circulation. Before this, most people believed that blood flowed in arteries and veins like “human breath,” essentially just moving back and forth in the body.

This opened the way for Sir Christopher Wren, the famous English architect, to fashion a quill and porcine bladder system ( Fig. 16.1 ) in 1658 to test the effects of infusing wine, ale, and opium. His work in this area has earned him the distinction of being the “father of modern intravenous (IV) infusion.”

Fig. 16.1, A depiction of the first intravenous injection of opium with a quill and bladder.

The simplest infusion device is not too different from Wren’s work – a simple IV bag with tubing connected to an IV catheter in a patient. These were the first types of infusions used in the 1900s. To understand the factors that affect the flow rate of an infusion, it is important to first review the principles of fluid dynamics.

Understanding Flow

The rate of flow is described by the following equation:

$Q = Δ P / R$

where Q is the flow, ΔP is the change in pressure, and R is the resistance. To understand the flow, each part should be reviewed. Change in pressure is given by the following formula, known as Pascal’s law:

$Δ P = ρ \times g \times (h_{2} - h_{1})$

The change in pressure depends on the density (ρ), which is approximately 1 kg/L for most liquids; the force of gravity (g); and the difference in the height between the fluid reservoir and the infusion location, known as head height (h ₂ −h ₁ ).

Resistance is a much more difficult aspect to deal with, because resistance in a pipe can change based on a multitude of factors in a number of complex relationships. However, if constant laminar flow is assumed, this simplifies the relationship. Resistance (R) to constant laminar flow is given by the following equation:

$R = \frac{8 η L}{π r^{4}}$

In this expression, η is the viscosity of the liquid, L is the length of tubing, and r is the radius. Thus the radius of the tubing has the greatest effect on resistance (i.e., radius raised to the 4th power). Doubling the radius decreases the resistance 16 times. It should also be noted, however, that viscosity and tube length play a critical role in resistance and thus in the flow rate. These equations can be combined together to form Poiseuille’s law for laminar flow in a pipe:

$Q = \frac{Δ P}{R} = \frac{π ρ g (Δ h) r^{4}}{8 η L}$

Thus, all of the factors affecting the rate of flow in a gravity device are illustrated by Poiseuille’s law: density (ρ), gravity (g), difference in height (Δh), tubing radius ( r ), fluid viscosity (η), and tube length (L). These factors are components of all infusions, and the clinician should remain cognizant of these at all times.

Gravity-Driven Infusions

Manual Flow Regulators

In a gravity-driven infusion, head height is the primary factor that creates a pressure gradient to generate flow. The flow rate can be approximated by counting the drip rate and taking into account the number of drops per milliliter, known as the IV drop factor, which depends on the size of the drip chamber orifice. To control the flow rate of a gravity-driven infusion, the resistance of the system can be adjusted using a roller clamp or other manual flow regulator. It is important to remember that as the height of the fluid changes, the head height changes, and the flow rate will change as well. Thus the roller clamp would need to be adjusted for the infusion to maintain a constant flow rate ( Fig. 16.2 ). Manual flow regulators, such as the Dial-a-Flow, are available to the clinician. These devices can improve the accuracy of the flow rate compared to the roller clamp.

Fig. 16.2, Roller clamp infusion controller.

Electronic Flow Regulators

The next step in the progression of infusion devices was the addition of a controller that adjusted the resistance based on the desired flow rate, rather than having a clinician monitor the drip rate ( Fig. 16.3 ).

Fig. 16.3, The Drip-Eye drip counter regulated gravity controller (Parama-Tech Co., Fukuoka, Japan).

Mechanism of Operation

Electronic flow regulators primarily operate by using a drip counter. An optical sensor is placed on the drip chamber to count the drops as they fall and thereby block the sensor. The drops have a uniform size based on the temperature, surface tension, and pressure. Because the temperature and pressure remain relatively constant, the viscosity and size of the drip chamber have the greatest role in determining the flow rate.

Considerations

Advantages

Electronic gravity-flow devices, such as drop counters, have several advantages over standard IV systems. First, they can minimize clotting because the controllers have flow sensors that can detect a pressure change that could have resulted from clotting, especially at low flow rates. Second, the addition of a flow controller dramatically decreases the incidence of unintended rapid infusion. Third, it can help minimize dry IV lines by monitoring the flow and stopping the infusion before the line runs completely empty. This prevents the infusion of air into the patient’s vasculature.

An additional advantage of regulated gravity devices is a low driving force, as the device is driven by the pressure at head height. A low driving force decreases the severity of infiltrations compared with infusion pumps that operate at higher pressures.

One of the main attractions of this class of device is its lower cost. Although more expensive than a simple IV bag and administration set, it requires less clinician supervision. This allows a single individual to monitor several infusions. In addition, these controllers are relatively small and are substantially less expensive than most positive-pressure infusion pumps.

Disadvantages

Regulated gravity devices have several distinct drawbacks in their operation. Many of these controllers are designed such that they limit the flow when the tubing is clamped. However, if the tubing is released from the flow controller during administration, unintended free flow of the solution is still possible. In addition, in the programming of drip counters, the viscosity of the fluid, or the number of drops per milliliter, must be included in setting the infusion. This typically requires a look-up chart or similar table, which can be a source of error in programming an infusion. Because different-sized drip chambers are available, the correct chamber size also must be selected when programming the controller. The low driving force of the system also results in some drawbacks. The limited pressure confines infusions to low-pressure target vasculatures (the venous system) and limits the attainable flow rate for viscous fluids.

Positive-Pressure Pumps

External sources of pressure can create larger pressure gradients to pump fluid than gravity alone. Positive-pressure pumps have many advantages over gravity-fed controllers. These pumps can maintain relatively good accuracy at low flow rates compared with gravity-fed controllers. This is due to the design of their pumping mechanisms; gravity controllers restrict nearly all flow in the tubing at low flow rates. In addition, positive-pressure pumps can reduce the number of minor occlusions with highly viscous solutions and provide faster infusion rates than gravity-fed systems. Drawbacks due to the high-pressure generation capability include delays in detection of occlusion at low flow rates and the potential for more severe infiltrations. A summary of infusion pump pros and cons is presented in Table 16.1 .

Table 16.1

Infusion Pump Pros and Cons

Pump Type	Pros	Cons
Regulated gravity-induced flow	Detects clotting, infiltrations, and occlusions Prevents dry IVs Low driving force and decreased severity of infiltrations Low cost Requires less clinical supervision than a gravity IV	No free-flow clamping Need to know fluid viscosity Needs to be the correct chamber for specific size drip Low pressure, slow infusions Max infusion rate depends on fluid viscosity, head height, and other factors
Elastomeric reservoir	Low cost Portable and discreet	Low accuracy Flow rate not easily adjustable
Spring-powered passive syringe pumps	Low cost Can be used with most syringes	Low accuracy Requires special microbore tubing
Peristaltic pumps	Similar accuracy to drip counters Positive pressure can be used to infuse in invasive lines Wide range of infusion rates, up to 1200 mL/h Fewer programming errors than drip counters	Pulsatile flow Tubing deforms over time, leading to less precise doses Requires specific tubing set High cost Not as accurate for low flow rates
Cassette (volumetric) pumps	Similar accuracy to drip counters and peristaltic pumps Similar function to peristaltic pumps but often easier to load because of the cartridge	More difficult to prime More expensive sets because of the cartridge Pulsatile flow Not as accurate at low flow rates
Syringe pump	Highest accuracy Able to function with many third-party syringes Ability to infuse highly concentrated drugs with accuracy at low flow rates	Limited volume (typically 50–60 mL) Susceptible to start-up delays if mechanical slack is not removed Longer time to determine an occlusion has occurred because of low flow rates

There are many different methods to generate positive pressure. Passive devices use elastic or spring-powered mechanisms and do not require external sources of power, while electric large volume and syringe pumps use electromechanical energy to propel fluid.

PASSIVE MECHANICAL PUMPS-ELASTOMERIC RESERVOIR

Mechanism of Operation

Elastomeric reservoir “pumps” use one of the simplest methods of generating positive pressure: a balloon-like reservoir that exerts constant pressure on the medication within the balloon. The fluid is passed through a small flow restrictor, and, as specified by Poiseuille’s law, the flow (Q) is proportional to the radius to the fourth power (r ⁴ ); thus the flow rate is most dependent on the radius of the tubing.

$Q = \frac{Δ P}{R} = \frac{(Δ P) π r^{4}}{8 η L}$

Considerations

Advantages

Typically used for outpatient regional anesthesia (e.g., On-Q Pain Relief System [Avanos, Irvine, CA]), elastomeric reservoirs are highly portable and do not depend on head height which allows more flexibility in where the pump is carried. In addition, they can be hidden in bags or purses to afford discreet portability.

Disadvantages

Although these pumps are portable and discreet, they have several disadvantages. They primarily work only on microbore tubing, which is inherently less accurate than the tubing in controller-style pumps. In addition, the flow rate is not easily adjustable without changing the size of the microbore tubing, and the infusion continues until it is completed. These devices are typically filled at a pharmacy or by a third-party vendor because of the high pressures required.

Spring-Powered Passive Syringe Pumps

Mechanism of Operation

Spring-powered or passive syringe pumps are very similar to electromechanical syringe pumps, but they use a constant-force spring to apply pressure ( Fig. 16.4 ). The constant-force spring applies pressure to the syringe plunger and forces the fluid out. The flow is then restricted by microbore tubing, which limits the fluid flow to a known rate because of Poiseuille’s law, as previously demonstrated.

Fig. 16.4, Sigma Multidoser spring-powered syringe pump (Baxter, Deerfield, IL) without a syringe installed.

Considerations

Advantages

Spring-powered syringe pumps have many advantages; the biggest is probably their low cost. These inexpensive pumps are excellent for the delivery of antibiotics, and they allow an inexpensive but constant means of infusion where high levels of accuracy and precision are not important. Spring-powered syringe pumps can accept most syringes and have a high level of interoperability.

Disadvantages

The main disadvantage of spring-powered syringe pumps is lower accuracy compared with controller-style pumps. Also, the flow rate is limited to the flow rate set by the microbore tubing, such that the administration set must be changed to change the flow rate.

Electromechanical Pumps

Electromechanical pumps are the most common pumps used in a hospital. They have historically been classified into large volume pumps (peristaltic and volumetric pumps) and microinfusion pumps (syringe pumps). Large volume pumps operate at a wide range of infusion rates, generally 0.1 mL/h to 1200 mL/h with ±5% accuracy. Large volume pumps are also used for many purposes other than drug delivery, such as rapid infusions of fluid and enteric and parenteral feeding.

Peristaltic Linear And Rotary Pumps

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