As mentioned at the beginning of this chapter, complete analgesia of the operation wound may not be enough to make the patient completely comfortable. Other parenteral analgesics, particularly opioids, will usually be required and should not be withheld. Apart from dealing with pain or discomfort outwith the blocked area, they will often provide much needed sedation. Conventional doses given by -the intramuscular route, e.g. 10 mg morphine, will have particularly good analgesic effect when the main postoperative pain has been relieved by the epidural block.
It is well established that opioids injected into the epidural space can cross the dura mater and gain access to the dorsal horns of the spinal cord. By attaching themselves to//-receptors in the substantia gelatinosa of the dorsal horns, they produce excellent analgesia without obvious neural blockade. The doses required are generally less than is used intramuscularly, but there is a wide individual variation in regard to effectiveness and duration.
The oil/water partition coefficient of the various opioids is very important. Low lipid solubility increases the time required for the drug to reach the spinal cord, but increases the duration of effect as the drug will leave the cord only slowly. However, the relatively high water solubility of such drugs will lead to high concentrations in the cerebrospinal fluid and the possibility of cephalad migration with coriseL quent respiratory depression. Morphine, which is the most commonly used drug for this purpose, has a low partition coefficient. Its onset time is long, but its duration is also long, hence its popularity. However, it has been responsible for the great majority of respiratory depressions that have been reported, the incidence of cases requiring active treatment of the depression being of the order of 1 in 200.
High lipid solubility on the other hand leads to a shorter onset and a low incidence of respiratory depression, but the duration is relatively short. Drugs such as fentanyl (and its derivatives), pethidine, methadone and diamorphine all have high lipid solubility and if used alone required frequent injections. Lipid soluble drugs should be injected at or near the appropriate spinal segments subserving the pain, as
they will occupy receptors in the nearest part of the spinal cord and be unable to migrate any distance.
Most opioids are given epidurally in doses 20-50% of that usually used IM.
The main side effects are:
1. Respiratory depression. This unfortunately is not easy to monitor though the respiratory rate may be helpful. Particular care is necessary if IM opioid has also been given.
2. Pruritis. Usually this is tolerable but can be troublesome.
3. Urinary retention.
4. Nausea and vomiting.
Naloxone is effective in reversing respiratory depression but it may have to be continued for many hours. It will also reverse excessive pruri-tis without affecting the analgesia.
Combination of local anaesthetic drugs and opioids
Several studies have now shown that the best results in treating postoperative pain come from epidural injections of a mixture of a local anaesthetic and an opioid, particularly when given as an infusion.
Because the two drugs have a different site of action, the dose of each can be kept to a minimum with a consequent reduction in side-effects.
Because the duration of effect of a drug is of little consequence when given by continuous infusion, it is possible to use an opioid of high lipid solubility with little risk of respiratory depression, e.g. fentanyl, sufentanyl or diamorphine, given at 5-10% of their normal IM dose per hour, mixed with 0.125% bupivacaine at 15 ml/h. Morphine at a rate of 0.5 mg/h is also effective mixed with 0.125% bupivacaine. The avoidance of bolus doses of opioid may also protect against respiratory depression.
The local anaesthetic drug will retain most of the advantages of continuous nerve block mentioned above.
See Tables 220:1 and 220:2.
Operation Catheter Initial bolus Infusion rate site position dose Bupi- Bupivacaine _________________ vacaineO.5% 0.125%
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