Pumps, pain management and regional anaesthesia

It has a pump with an accuracy of at least ±5% of the programmed dose ( Fig. 12.1 ).

The remote demand button is connected to the pump and activated by the patient.

It has an antisiphon and backflow valve.

Different modes of analgesic administration can be employed:

• a.

  patient-controlled, on-demand bolus administration

• b.

  continuous background infusion and patient-controlled bolus administration.

The initial programming of the pump must be for the individual patient. The mode of administration, the amount of analgesic administered per bolus, the ‘lock-ouť time (i.e. the time period during which the patient is prevented from receiving another bolus despite activating the demand button), the duration of the administration of the bolus and the maximum amount of analgesic permitted per unit time are all variable settings on a PCA device.

Some designs have the capability to be used as a PCA pump for a particular variable duration then switching automatically to a continuous infusion as programmed or vice versa.

The history of the drug administration including the total dose of the analgesic, the number of boluses and the number of successful and failed attempts can be displayed.

The devices have memory capabilities so they retain their programming during syringe changing.

Tamper-resistant features are included.

Some designs have a safety measure where an accidental triggering of the device is usually prevented by the need for the patient to make two successive presses on the hand control within 1 second.

PCA devices operate on mains power sources or battery.

Different routes of administration can be used for PCA, e.g. intravenous, intramuscular, subcutaneous or epidural routes.

Alarms are included for malfunction, occlusion and disconnection.

Ambulatory PCA pumps are available, allowing patienťs mobilization during use ( Fig. 12.2 ).

The ability of the patient to cooperate and understand is essential.

Availability of trained staff to programme the device and monitor the patient is vital.

In the PCA mode, the patient may awaken in severe pain because no boluses were administered during sleep.

Some PCA devices require special giving sets and syringes.

Technical errors can be fatal due to the potential for opiate overdose.

It is important to ensure the infusion is unidirectional with no free flow. Antisiphon valves are usually used to achieve this. An alarm is activated if there is a crack in the syringe, allowing air to enter.

The syringe should be securely clamped to the pump. Inadvertent free flow can occur if the syringe barrel or plunger is not engaged firmly in the pump mechanism.

Syringe drivers should not be positioned above the level of the patient. If the pump is more than 100 cm above the patient, a gravitational pressure can be generated that overcomes the friction between a nonsecured plunger and barrel.

Some pumps have a ‘back-off’ function that prevents the pump from administering a bolus following an obstruction due to increased pressure in the system.

An antireflux valve should be inserted in any other line that is connected to the infusion line. Antireflux valves prevent backflow up the secondary (and usually lower pressure) line, should a distal occlusion occur. They would then avoid a subsequent inadvertent bolus.

Newer, smart infusion pumps are designed to alert the user when there is a risk of an adverse drug interaction or when the user sets the pump’s parameters outside of specified safety limits. This is done via programmable dose and infusion rate limits.

In order to change the rate or to give a bolus, there is a need for two distinct or simultaneous actions.

A small balloon-like pump is filled with local anaesthetic. Variable volumes of 100–600 mL are available.

Specially designed catheters have lengths of 7–30 cm and of different gauges.

It has a bacterial filter and a flow restrictor.

The balloon deflates slowly and spontaneously, delivering a set amount of local anaesthetic solution per hour. Rates of 2–14 mL/h can be programmed.

Catheters are designed with multiple orifices, allowing the infusion of local anaesthetic solution over a large area.

An extra on-demand bolus facility is available in some designs. This allows boluses of 5 mL of solution with a lockout time of 60 minutes.

Some designs allow the simultaneous infusion of two surgical sites.

A silver-coated antimicrobial dressing is provided.

Some of the local anaesthetic may get absorbed into the balloon.

The infusion rate profile can vary throughout the infusion. It is thought that the initial rate is higher than expected initially, especially if the pump is underfilled. The infusion rates tend to decrease over the infusion period. To reduce such potential risks, the Pajunk FuserPump, with a 350-mL capacity, has a hard shell, a flow rate selector and a locking key. Rates of 3, 5 and 8 mL/h can be delivered ( Fig. 12.6 ).

It is important to follow the manufacturer’s instructions regarding positioning of the device in relation to the body and ambient temperature. Changes in temperature can affect the flow rate. A change of 10°C in the temperature of water-based fluids results in altered viscosity, which causes a 20%–30% change in flow rate.

A cylindrical tube with the patient’s mouthpiece.

A polypropylene wick within the tube to accommodate 3 mL of methoxyflurane.

An inlet chamber that allows the addition of methoxyflurane and fresh air.

An activated charcoal chamber.

The wick gradually vaporizes the 3 mL of methoxyflurane.

Patient inhales and exhales through the mouthpiece.

During inhalation, the patient gets vapour through the mouthpiece.

During exhalation, the expired methoxyflurane is captured by the activated charcoal, so reducing pollution.

3-mL methoxyflurane provides 25–30 minutes of analgesia with continuous use. Longer durations can be achieved with intermittent use.

If stronger analgesia is needed, the inlet chamber is covered with a finger, allowing less dilution and higher concentration of methoxyflurane.