Infusion pumps: guidelines and pitfalls

The use of infusion pumps is increasing in the community. To ensure that a patient receives the correct dose, the appropriate infusion device should be chosen for the drug. Syringe pumps are commonly used at low rates of infusion, but may not be suitable for drugs which require constant blood levels. Usually, the longer the period of infusion, the more accurate the dose; however, errors can occur when singleuse syringes are driven at low flow rates. Rechargeable batteries need a regular cycle of charging and discharging to keep them in good condition. Infusion pumps should be protected from sources of electromagnetic interference, such as mobile telephones.

Infusion devices range from very simple mechanical devices based on elastic containers, springs and flow restrictors to sophisticated microprocessor controlled pumps. Traditionally used for the controlled delivery of drugs and fluids in hospitals, these devices will increasingly be used for home therapy. It is important to be aware of some key issues, whichever device is used.

Syringe pumps can deliver small volumes of drugs at low flow rates.

Single-use syringes are inexpensive and mass manufactured items, and are not meant to be highly accurate. When used in a syringe pump at low plunger speeds, the friction between the syringe plunger and the barrel causes a jerking effect and the fluid is delivered as a series of small boluses. The fit between the plunger and the barrel may vary from batch to batch and, consequently, the jerking effect may also vary. This problem is commonly known as 'stiction'. In general, the bigger the syringe and the lower the flow rate, the more pronounced the stiction.

The variations of physiological parameters due to the stiction problem were studied in neonates given dopamine via a syringe pump. There were swings in heart rate and blood pressure corresponding to the pulsating effect of the syringe.1

Stiction may not be a problem with drugs having a long half life or which do not require steady blood levels in the short term, such as heparin or insulin. In contrast, the delivery of powerful drugs with short half lives, like catecholamines, at rates under 5mL/hour from large syringes (>30 mL) is not recommended. Some currently available peristaltic pumps provide reasonably smooth flows at low delivery rates and should be considered as an alternative.

Occasionally, the dimensions of a particular model of disposable syringe may be changed by the manufacturer. This can be due to a change in manufacturing facilities or to the availability of moulding dies. The external packaging may look the same and the new dimensions may not be noticed by the user. Since the pump driving software will also assume that the dimensions are unchanged, and as the flow rate depends on the internal diameter of the barrel, the dose delivered will differ from the expected dose.

There is little the user can do in this particular case. Nevertheless, as a safe practice, only the syringe recommended by the manufacturer should be used with a syringe pump.

Most infusion pump manufacturers state the accuracy of the delivered dose as a percentage. A user manual may read 'accuracy: ±5%'. Is this the drive, volume or flow rate accuracy?

Ideally, this should be the flow rate accuracy, meaning that over the complete period of infusion, the flow rate (in mL/hour) will not vary beyond these limits. Such pumps should have a smooth and steady delivery. Sometimes, however, the quoted accuracy may refer to the total volume delivered by the end of the infusion period. In such cases, the final dose will be within the specified limits, but no indication is given on how constant or smooth the flow has been during infusion.

For syringe pumps which make use of single-use syringes, many manufacturers define the accuracy of the linear displacement of the plunger. This is the mechanical accuracy of the pump itself and excludes the additional error caused by the inconsistency of single-use syringes. The user should be aware that single use syringes may cause flow deviations up to 4% greater than those specified for the linear displacement.

As the maintenance of constant blood levels may be critical for some drugs, it is important to search the user manual or any accompanying literature for further references to accuracy.

Fig. 1
The trumpet curve defines, for a set flow rate, the maximum positive and negative percentage deviation from the expected dose relative to the time interval (observation window) of infusion. While for one hour, the overall deviation is -2%, over an interval of 2 minutes, deviations can reach +7, -10%.

Trumpet curve

Recently, some manufacturers have included trumpet curves in their user manuals. The graph looks like a trumpet converging to the right side with time units on the x axis and percentage on the y axis (Fig. 1). These curves represent the maximum percentage deviation from the expected dose for a given time interval, known as the 'observation window' at any time during the infusion of the drug or solution. The upper curve corresponds to positive deviations and the lower curve to negative deviations. As the accuracy also depends on the set flow rate, two or three sets of curves are usually included. The user can correlate the half life of the administered drug with the observation window interval in the graph and decide if the pump is suitable for a particular application. In general terms, the longer the time interval, the more accurate the dose.

Most administration sets for infusion pumps are, in some way, modified gravity feed sets. A few of these can also safely be used as general administration sets. An incorrectly fitted administration set with an inadvertently opened roller clamp may deliver 100 mL of solution in one hour. If the desired rate of infusion was 10 mL/hour, then this is 10 times the expected dose. Different mechanisms like fork clamps and valves have been incorporated to help prevent the risk of free flow when the administration set is released.

To avoid free flow, follow the manufacturers' recommendations and, whenever possible, count the drops in the drip chamber, to check the drip rate, before leaving the patient.

If there is an occlusion alarm, release the fluid towards the reservoir, never towards the patient.

This is a simple rule, but one worth remembering. If a pump gives an alarm and has stopped, always free the administration set or the syringe from the pumping mechanism before making any attempt to clear the patient end. If possible, any inline taps should be turned off. In this way, any built up pressure will not force a bolus into the patient.

Infusion pump manufacturers have tried various battery types. Nickelcadmium cells (known as NiCads) suffer from a strange problem called 'voltage memory'. They may appear to be fully charged when in effect they are flat. The more irregular the charging routine is, the more they deteriorate. It is important for these batteries to be fully discharged and recharged at regular intervals. This routine extends their life. Lead acid cells are similar to those in cars, but with the acidic fluids replaced with gel to make them portable. Lead acid cells, unlike NiCads, do not have the memory problem, although they may 'die' if they fully discharge. Most infusion pumps prevent this problem by giving a warning signal when a recharge is needed.

As a guide, do not ignore the 'low battery' warning. For most rechargeable batteries, maintaining a regular cycle of discharge and recharge is a healthy exercise.

Thanks to recent improvements in electronic technology and reduced power consumption, some portable pumps can be powered by disposable alkaline cells. These cells are readily available and do not require recharging.

If an infusion pump (or any other medical device) suddenly behaves in an abnormal manner, interference from a mobile telephone, a walkietalkie or another portable transmitter could be the reason.

Many past incidents with infusion pumps remained unresolved and were considered 'once only' events. Many of these incidents could have been caused by electromagnetic interference. Some medical devices used in hospitals, like electrosurgical generators or physiotherapy machines, were known as powerful sources of interference. However, these effects were understood and these machines were restricted to specific areas in hospitals. The introduction of new portable systems of communication makes the problem of interference more complex. Implanted therapeutic devices are, to some degree, protected by the surrounding body tissues which act as a screen for electromagnetic interference. Nonimplanted devices such as ambulatory insulin pumps are more exposed and receive a higher dose of interference.

Fortunately, most medical devices have been designed to 'fail safe'. In other words, no matter what the disturbance is, the device will revert to a safe standby condition and warn the user that something is wrong.

Mobile phones and similar communication devices should not be used within two metres of medical equipment. This particularly applies to critical devices like infusion pumps. Patients using ambulatory pumps outside the hospital should be given advice on the type of activities and sort of communication devices they need to avoid.

Further reading

Australian/New Zealand Standard: Guide to the safe use of infusion pumps and controllers. Joint Standard AS/NZS 3770:1993.