Both pethidine and morphine are suitable drugs for continuous intravenous infusions but unless a loading dose is used, a steady state is not achieved for about 24 hrs. To avoid the side effects of a large bolus, a loading dose given as an infusion is preferable, always remembering that there is drug elimination during loading. A PCA pump administers drugs, usually intravenously, when the patient pushes a button. The equipment is expensive and the technique may allow breakthrough pain on an intermittent basis because when the patient is asleep, the administration ceases. This can be overcome by a slow background infusion that is supplemented by a patient controlled additional dose.
Patient information is mandatory.
There is no sensory loss, no muscle paresis and no autonomic blockade with opioid drugs administered by these routes. There is a variation in the rate of onset of analgesia and its duration depending on whether the drug is lipophilic or hydrophilic and by transport within the cerebro-spinal fluid. When administered by the spinal routes it takes about 45 minutes to reach maximum effect and lasts for 8-12 hours. Pethidine, methadone and fentanyl being lipophilic act quickly but the duration of action is short. The dose of morphine by the epidural route is about 5-10 times that of the intrathecal route.