Pain management and regional anaesthesia
Patient controlled analgesia (PCA)
Mechanism of action
1. Different modes of analgesic administration can be employed:
a) patient controlled on-demand bolus administration (PCA)
b) continuous background infusion and patient controlled bolus administration.
2. The initial programming of the pump must be tailored for the individual patient. The mode of administration, the amount of analgesic administered per bolus, the ‘lock-out’ 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.
3. 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.
4. 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.
5. The devices have memory capabilities so they retain their programming during syringe changing.
6. Tamper-resistant features are included.
7. 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.
8. PCA devices operate on mains or battery.
9. Different routes of administration can be used for PCA, e.g. intravenous, intramuscular, subcutaneous or epidural routes.
10. Alarms are included for malfunction, occlusion and disconnection.
11. Ambulatory PCA pumps are available allowing patient’s mobilization during use (Fig. 12.2).
Fig. 12.2 The CADD Legacy portable PCA.
Problems in practice and safety features
1. The ability of the patient to co-operate and understand is essential.
2. Availability of trained staff to programme the device and monitor the patient is vital.
3. In the PCA mode, the patient may awaken in severe pain because no boluses were administered during sleep.
4. Some PCA devices require special giving sets and syringes.
Syringe pumps
These are programmable pumps that can be adjusted to give variable rates of infusion and also bolus administration (Fig. 12.3). They are used to maintain continuous infusions of analgesics (or other drugs). The type of flow is pulsatile continuous delivery and their accuracy is within ±2–5%. Some designs can accept a variety of different size syringes. The power source can be battery and/or mains.
Fig. 12.3 The Graseby 2000 syringe pump.
Volumetric pumps
These are programmable pumps designed to be used with specific giving set tubing (Fig. 12.4). They are more suitable for infusions where accuracy of total volume is more important than precise flow rate. Their accuracy is generally within ±5–10%. Volumetric pump accuracy is sensitive to the internal diameter of the giving set tubing. Various mechanisms of action exist. Peristaltic, cassette and reservoir systems are commonly used.
Fig. 12.4 The Graseby volumetric pump.
Elastomeric pumps
These recently designed light, portable and disposable pumps allow continous infusions of local anaesthetic solutions. Continuous incisional infiltration or nerve blocks can be used so allowing the delivery of continuous analgesia (Fig. 12.5).
Mechanism of action
1. 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.
2. Catheters are designed with multiple orifices allowing the infusion of local anaesthetic solution over a large area.
3. An extra on-demand bolus facility is available in some designs. This allows boluses of 5 mL solution with a lock-out time of 60 min.
4. Some designs allow the simultaneous infusion of two surgical sites.
5. Silver-coated dressing for anti-microbial effect is provided.
Problems in practice and safety features
1. Some of the local anaesthetic may get absorbed into the balloon.
2. 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 under filled. The infusion rates tend to decrease over the infusion period.
3. 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.
Epidural needles
Epidural needles are used to identify and cannulate the epidural space. The Tuohy needle is widely used in the UK (Fig. 12.6).
Components
1. The needle is 10 cm in length with a shaft of 8 cm (with 1-cm markings). A 15-cm version exists for obese patients.
2. The needle wall is thin in order to allow a catheter to be inserted through it.
3. The needle is provided with a stylet introducer to prevent occlusion of the lumen by a core of tissue as the needle is inserted.
4. The bevel (called a Huber point) is designed to be slightly oblique at 20° to the shaft, with a rather blunt leading edge.
5. Some designs allow the wings at the hub to be added or removed.
Mechanism of action
1. The markings on the needle enable the anaesthetist to determine the distance between the skin and the epidural space. Hence the length of the catheter left inside the epidural space can be estimated.
2. The shape and design of the bevel (Fig 12.7) enable the anaesthetist to direct the catheter within the epidural space (either in a cephalic or caudal direction).
3. The bluntness of the bevel also minimizes the risk of accidental dural puncture.
4. Some anaesthetists prefer winged epidural needles for better control and handling of the needle during insertion.
5. A paediatric 19-G, 5-cm long Tuohy needle (with 0.5-cm markings), allowing the passage of a 21-G nylon catheter, is available.
6. A combined spinal–epidural technique is possible using a 26-G spinal needle of about 12 cm length with a standard 16-G Tuohy needle. The Tuohy needle is first positioned in the epidural space then the spinal needle is introduced through it into the subarachnoid space (Fig. 12.7). A relatively high pressure is required to inject through the spinal needle because of its small bore. This might lead to accidental displacement of the tip of the needle from the subarachnoid space leading to a failed or partial block. To prevent this happening, in some designs, the spinal needle is ‘anchored’ to the epidural needle to prevent displacement (Fig. 12.8).
Problems in practice and safety features
1. During insertion of the catheter through the needle, if it is necessary to withdraw the catheter, the needle must be withdrawn simultaneously. This is because of the risk of the catheter being transected by the oblique bevel.
2. In accidental dural puncture, there is a high incidence of postdural headache due to the epidural needle’s large bore (e.g. 16 G or 18 G).
3. Wrong route errors: in order to avoid administering drugs that were intended for intravenous administration, all epidural bolus doses are performed using syringes, needles and other devices with safer connectors that cannot connect with intravenous Luer connectors.
Epidural catheter, filter and loss of resistance device (Fig. 12.9)
Components
1. 90-cm transparent, malleable tube made of either nylon or Teflon and biologically inert. The 16-G version has an external diameter of about 1 mm and an internal diameter of 0.55 mm.
2. The distal end has two or three side ports with a closed and rounded tip in order to reduce the risk of vascular or dural puncture (see Fig. 12.7). Paediatric designs, 18 G or 19 G, have closer distal side ports.
3. Some designs have an open end.
4. The distal end of the catheter is marked clearly at 5-cm intervals, with additional 1-cm markings between 5 and 15 cm (Fig. 12.10).
5. The proximal end of the catheter is connected to a Luer lock and a filter (Fig. 12.10).
6. In order to prevent kinking, some designs incorporate a coil-reinforced catheter.
7. Some designs are radio-opaque. These catheters tend to be more rigid than the normal design. They can be used in patients with chronic pain to ensure correct placement of the catheter.
Mechanism of action
1. The catheters are designed to pass easily through their matched gauge epidural needles.
2. The markings enable the anaesthetist to place the desired length of catheter within the epidural space (usually 3–5 cm).
3. There are catheters with a single port at the distal tip. These offer a rather sharp point and increase the incidence of catheter-induced vascular or dural puncture.
4. An epidural fixing device can be used to prevent the catheter falling out. The device clips on the catheter. It has an adhesive flange that secures it to the skin. The device does not occlude the catheter and does not increase the resistance to injection (Fig. 12.11).
