Chapter 19 Infusion equipment and intravenous anaesthesia

Principles of infusion devices

Target-controlled infusion (TCI)

Patient-controlled analgesia (PCA)

Autotransfusion

Related equipment

Microprocessor-controlled infusion devices are now so ubiquitous that outside the operating theatre it is rare to see intravenous infusions administered without these. Such devices can be commanded in any number of units from ml h⁻¹ to mass units of drug per unit patient weight per unit time. Within operating theatres the ease of use of a target-controlled infusion (TCI) as a mode of drug delivery has dramatically increased the use of intravenously maintained anaesthesia. From the viewpoint of the infusion device, TCI is, of course, simply another layer of calculations for its microprocessor. ‘Open Label TCI’ (where the device does not require the manufacturer’s specially presented drug product to allow administration by TCI), has become a reality; and for more than one drug. TCI appears no longer to be seen by regulatory authorities as a peculiar licensing issue linking infusion device and drug manufacturer.

Increasing functionality obviously carries increased risk of drug maladministration and in mitigation the use of ‘drug libraries’ loaded as additional software onto infusion devices has become commonplace. These together with various other ‘error traps’ aim to reduce the opportunities for common errors.

Software revisions, access codes, data logging, alarms and the additional desire by manufacturers for miniaturization and for producing singular machines that can be made to behave effectively as different devices (e.g. TCI pump, critical care unit infusion pump, patient-controlled analgesia (PCA) device), render these machines complex, occasionally prone to software failure and with often cumbersome user interfaces. In this light, the popularity of simple elastomeric infusion devices is not difficult to comprehend.

Evolution of infusion systems

It was once common to see doctors and nurses with watch in hand, converting infusion rates from ‘duration of infusion’ to drops per minute and adjusting the roller clamp on an intravenous ‘drip’ set. Infusates would run through too fast or too slow, because the plastic tubing altered shape or the downstream resistance in the intravenous cannula altered for a number of reasons. Often it was simply the calculations that were wrong.

This was superseded first by electronic drip counters, which were subsequently made to control the adjustable clamp on a gravity-fed giving set (infusion controllers) and, ultimately, by microprocessor-controlled infusion pumps (Fig. 19.1), able to generate flow irrespective of the effects of gravity, and incorporating many features such as sensing infusion line pressure and the facility for being programmed in a variety of units and even languages to give stepped infusions based on patient weight. The first devices to not rely on a drip counter used a syringe type cassette to reliably control the volumes delivered and were hence known as ‘volumetric pumps’ to differentiate them (Fig. 19.2). This design is less common now, and most devices rely on a peristaltic mechanism to push fluid along. The term should now probably be made obsolete to be replaced by ‘infusion pump’.

Figure 19.1 Evolution of infusion pumps, (MP = microprocessor).

Figure 19.2 Principle of a cassette type (‘volumetric’) infusion pump.

Syringe drivers (also called syringe pumps) have traditionally been used for more accurate control of smaller volumes of infusion. The very first were simple clockwork devices designed to drive the plunger of a syringe at an even rate usually over 24 h. Syringe pumps now use the same technology as other infusion pumps and all these devices are considered together here.

1996 saw further progress with the commercial introduction of the Diprifusor system. This, a proprietary ‘chip’ added into a microprocessor-controlled syringe driver, allows the delivery of specially packaged Diprivan (propofol) by target-controlled infusion (TCI), and is discussed in greater detail below and also again later in the chapter. Licensing issues initially hampered the arrival of ‘open label TCI’ (see below), but at least four manufacturers now produce devices with open label pharmacokinetic algorithms capable of infusing at least the drugs propofol and remifentanil by TCI. Although it would be logical to use such systems for any drug (anaesthetic or otherwise) with a short half-life that needs to be given by continuous infusion, the development costs and licensing concerns for this route of administration for any given drug mean that it is only rarely commercially viable.

Microprocessor controlled/software driven

Modern electrically powered infusion devices use a stepper motor (see below). This is controlled by a microprocessor which ultimately simply varies the flow rate of the device from between 0 to 999 ml/h (infusion pump) or 1200 ml/h (for a syringe driver, depending on syringe size) at any given time.

Depending on the intended purpose of the infusion device the processor can be made to accept commands from the user for controlling the infusion in many different ways:

• simple ml/h request giving rise to the simplest infusion pump, say for ward use

• unit of drug per unit patient weight per unit time with variable units used for each; for example mg/kg/h or µg/kg/min. Clearly when dosing drug in this manner, the drug concentration in use and the patient weight must be input to the pump to allow automatic calculation of the flow rate. Such pumps may be used in the intensive care unit setting, but are more often termed ‘anaesthesia pumps’ as liberal discretion for choice of units is now usually only entrusted to anaesthetists

• simple infusion rate with an additional bolus volume when commanded by a separately attached control handle, thus constituting a PCA pump

• the in-built processor (or an additional piggybacked processor) can contain algorithms for the pharmacokinetics of particular drugs, allowing automated drug delivery based on achieving any desired theoretical patient plasma or effect site concentration of that drug. Such a configuration is termed a ‘target-controlled infusion (TCI) pump’. These systems are currently only commercially available for use in anaesthesia and are principally limited to four drugs: propofol, remifentanil, sufentanil and alfentanil. Patient variables such as age, sex, weight and height may need to be inputted to complete the algorithm. The algorithm, using this information together with the known drug concentration in the syringe, thereafter allows the anaesthetist to simply select a target patient concentration of drug. The pump automatically alters the infusion rate to most rapidly attain and maintain the calculated drug concentration in the patient at the set target. Such systems are very simple to use and obviate the need for complex calculations and detailed knowledge of pharmacokinetics by the user. They are largely responsible for a revolution in the administration of anaesthesia in a trend away from volatile agent maintenance towards total intravenous anaesthesia (TIVA).

Depending on the design of the chip these functionalities may be imbedded as hardware or firmware or added as software instructions. It can be seen how one basic infusion pump chassis can thus be made to give rise to effectively many different devices according to the microprocessor configuration. Each configuration or mode of use has its own attendant hazards in addition to those general to infusion pumps. Although all functionalities may be available in the one device, for risk management they may be selectively disabled through a restricted access menu such that only facilities that are needed in a clinical area are available to the user.

Modern devices are also able to communicate with centralized automated data archiving systems to automatically record the administration rate of a drug alongside the output of patient physiological monitors.

Simple infusion systems

In the operating theatre where there is closer observation of the patient’s hydration and circulating volume as necessitated by their rapidly changing status, most intravenous fluids are still administered under gravity from flexible plastic containers using single-use fluid administration sets. These are of several types:

• simple fluid administration, no filter, droplet size approximately 15 drops/ml (0.07 ml)

• blood and fluid administration with clot filter at about 200 μm mesh size. These giving sets use a larger-bore tubing and a double drip chamber containing a float: by squeezing the bottom chamber, the float jams the inlet and acts as a one-way ball valve allowing fluid to be pumped. Droplet size is usually approximately 15 drops/ml

• burette, 100–150 ml in volume, droplet size either 15 drops/ml or, for paediatric usage, 60 drops/ml. A flap or ball valve at the bottom of the burette prevents air entering the drip chamber when the burette is empty

• platelet giving sets, designed to reduce the risk of aggregation in the giving set as would occur with conventional blood giving sets.

Some giving sets are now also designed so as to also be compatible with volumetric infusion pumps. This necessitates a narrower-bore tube made from softer plastic to function with the peristaltic pumps (see below). Flow rates are, therefore, lower and the tubing is less kink-resistant. These sets usually have a 15 μm filter at the base of the drip chamber and are not suitable for infusing blood or for use in adult resuscitation.

The rate of infusion in a simple gravity fed system depends on:

• the height of the fluid container above the infusion site

• the resistance to flow caused by the infusion set

• occlusion of the tubing from a rate controlling device

• the physical properties (i.e. viscosity) of the fluid to be administered

• the bore of the intravenous cannula

• the hydrostatic pressure in the veins of the patient.

The manufacturers of giving sets quote the size of drops as number of drops per millilitre, usually between 10 and 60, but it must be remembered that the actual volume of the drops depends on the physical properties of the fluid being administered.

Rapid infusion

When rapid infusion of blood or other fluids is required, the rate of administration may be increased by the use of an inflatable pressure bag (Fig. 19.3), which may even be contained in a rigid box to increase the speed at which pressure may be applied.

Figure 19.3 Pressure infusion device.

When fluids are administered at a rapid rate, provision should be made for warming them to body temperature, otherwise significant cooling of the body may ensue (see Chapter 30).

Principles of infusion devices

Pumped infusion systems overcome the variation in infusion rates caused by changes in back-pressure, tubing resistance and the vertical height of the fluid container above the patient. Hence, they have to be powered by some form of motor, which must be coupled to a mechanism for driving the fluid

The stepper motor

The driving force in the majority of infusion pumps and electronic syringe drivers is provided by an electronic stepper motor, which is directly controlled from a digital microprocessor system. The speed of a conventional electric motor driven from either an AC supply or a DC supply may vary with mechanical load, the voltage or the frequency of the supply. It is, therefore, difficult, without electronic feedback, to control such a motor accurately. The stepper motor (Fig. 19.4) is designed so that a series of pulses applied to the stator windings of the motor cause the shaft to rotate by a fixed amount for each pulse, typically 1.8°, 2.5°, 3.75°or 7.5°, irrespective (within certain limits) of the load. Infusion systems are designed so that a pulse generator, whose output frequency is varied by the microprocessor, can produce accurate control of an infusion by varying the speed of the stepper motor.

Figure 19.4 The stepper motor and lead screw assembly of an Alaris PK syringe pump during manufacture.

Originally, the most accurate (and expensive) infusion pumps used syringe type cassettes (see Fig. 19.2) and were referred to as volumetric infusion pumps. This, like the newer peristaltic pumps, is driven by a stepper motor controlled directly by a microprocessor. The volume of the cassette is typically about 5 ml, with the dedicated disposable ‘syringe cassette’ for each manufacturer’s pump being supplied separately as a sterile product. A valve operating in harmony with the piston directs flow from the infusate bag to the reservoir or from there to the patient. Fluid is drawn rapidly from the reservoir bag into the cassette in less than 1 s. The valve is then actuated such that on the piston upstroke the contents are expelled at the required rate into the patient, and the cycle is repeated. Although effectively this produces an intermittent flow, it also gives overall extremely accurate infusion rates, with only infrequent 1 s interruptions.

These are seen much less frequently now owing to the expense of the disposables and the feasibility of getting good accuracy using simple intravenous giving sets in modern peristaltic pumps.

Peristaltic pumps

The principle of the peristaltic infusion pump is shown in Fig. 19.5. The tubing of a giving set is compressed by a series of rotating rollers or by a wave of mechanical ‘fingers’ or cam followers. The section of tubing in the peristaltic mechanism must be hard wearing, of known and consistent internal volume, and have no memory after compression so that it easily fills on being released. Depending on the manufacturer, specific proprietary tubing with dedicated fittings may be needed to allow it to be loaded into the pumping mechanism. Precision silicone tubing is often used in this section of the giving set.

Figure 19.5 A. Linear, and B. rotary peristaltic mechanisms.

Rotary peristaltic pumps are now more often seen in use for the less demanding requirements of enteral feeding rather than intravenous administration.

Linear peristaltic mechanisms allow much easier loading of the infusion tubing and are now by far the commonest design of infusion pump. The driving force is again a stepper motor. The rotary motion from the motor is translated into a linear peristalsis by the use of cams and cam followers as shown in Fig. 19.6.

Figure 19.6 The rotary motion from the motor is translated into a linear peristalsis by the use of cams and cam followers.

Because such infusion pumps have the theoretical capacity to inject limitless quantities of air into a patient should air ingress occur upstream of the pump (for example due to an empty infusion bottle), these devices incorporate sophisticated ultrasonics (Fig. 19.7) or optics-based ‘air in line’ detection systems capable of sensing air bubbles as small as 0.1 ml volume. These are usually placed downstream of the pump mechanism. Further protection is conferred by setting target delivery volumes smaller than the volume in the bag of infusate.

Figure 19.7 Braun Infusomat Space infusion pump with front cover opened to show (1) line pressure sensor, (2) ultrasonic air sensor, (3) peristaltic mechanism with precision silicone tubing loaded.

To detect obstructed or extravasated catheters, electrically powered infusion devices must have some measure of the pressure generated in the infusion line beyond the device. Peristaltic intravenous pumps, therefore, use a sensing piston pressing on the infusion line immediately downstream of the pumping chamber. This is calibrated to indirectly measure line pressure and can be programmed to alarm for occlusion at different pre-set levels.

Syringe drivers

There continues to be a range of small simple battery-operated syringe drivers (Fig. 19.8). The driving mechanism is a miniature DC motor that is switched on and off intermittently and drives a screw-threaded rod (lead screw), which is linked to the syringe plunger, causing its advancement. They may have a variable rate that is altered by adjusting a recessed control using a small screwdriver. These pumps are small and light enough to be worn in a holster by an ambulant patient and are now used chiefly for narcotic infusions for the relief of cancer pain. Great care must be taken in calculating drug dilutions and to ascertain that the correct units are used for setting the infusion rates, as the pumps are available in different models with rates set either as mm per 24 h or mm per h of plunger movement (Fig. 19.9).