Unlike its use in surgical operations, when a widespread block is usually desirable, a continuous epidural block for postoperative pain should be confined to a few spinal segments related to the incision site. Thus the catheter should be placed close to the centre of the required band of analgesia and only a small volume of local anaesthetic need be used. However, the smaller the volume, and therefore the dose, the shorter the duration of effect. Thus to maintain the. analgesia, top-up injections must be frequent (e.g. hourly) or a continuous infusion must be maintained. Specially controlled syringes can inject preset volumes at preset intervals. Excellent analgesia will be maintained if the top-ups are given before the previous injection has worn off. Small volume high concentration injections will give the best results.
Continuous infusions can be made with more conventional pumps that are available in most hospitals. The best results are obtained with high volume, low concentration infusions after an initial bolus of relatively concentrated local anaesthetic. A low concentration infusion can maintain a block but not induce one. Should the sensory block become insufficient, then a further bolus dose will be required.
Site of catheter
This depends upon the site of the operation and may be:
1. Cervical, for operations on the upper limb, particularly those involving microsurgery.
2. Thoracic for thoracotomies and abdominal surgery. Upper abdominal operations should have a catheter inserted to T8, while with lower abdominals it can be inserted at Til.
3. Lumbar for operations on the hips, knees and perineum.
4. Sacral for perineal operations.
Drugs and dose
Bupivacaine is the agent of choice as it causes less motor block than other drugs. In general midthoracic blocks require less volume than the other sites of injection. 4-5 ml can block several segments with such blocks, while lower thoracic or lumbar injections may need twice this volume for the initial injections.
If an epidural block has been used during the operation, then a top up, using 0.5% bupiva-caine, should be made at the end of surgery. If repeated bolus injections are to be made, then the extent of the block Ih later should be determined and from this the volume of subsequent injections can be estimated.
If a continuous infusion is to be used, then this should be started as soon as the patient is in recovery. Dosage recommendations are given in Table 220:1. However, the extent of the block should be checked 3-4 h after the initial top-up (or if the patient complains of pain) to see if it is optimal. If not the rate of infusion can be adjusted, remembering that if there is an inadequate band of anaesthesia, a bolus injection will be required as well as an increase in the infusion rate.
The following should be monitored and recorded:
1. Heart rate and arterial pressure. Initially these should be measured fairly frequently, but once the patient is stabilised, hourly recordings are sufficient. If a bolus injection is given then recordings should be made every 5 min for 15 min.
2. The amount of drug infused every hour.
3. The height and intensity of the block. Nurses can be taught to estimate the upper extent of the sensory block (e.g. using a piece of ice) and the degree of motor block (e.g. by asking the patient to move the lower limbs).
A special form for the nursing staff indicating the variables to be monitored, the prescribed dose to be infused and instructions on the management of hypotension is particularly valuable for managing these patients. Simple measures to counteract hypotension such as raising the foot of the bed, giving 250-500 ml of IV electrolyte solution quickly or injecting ephedrine 20-30 mg intramuscularly, should be taught to the nursing staff. An anaesthetist should always be present in the hospital for consultation and action.
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