Epidural: The Principles and Benefits of Epidural Analgesia

The discovery of opioid receptors in the spinal cord in 1976 [1] raised interesting possibilities. The intraoperative use of regional analgesia could be extended into the postoperative period. The side effects of local anaesthetics alone in the epidural space (hypotension and excess motor block) could be reduced by the use of epidural opioids alone or mixtures of lower concentration local anaesthetics and opioids. At the time it was predicted that the advantages of opioid use were:

• Selective analgesia with no sensory or motor loss, no sympathetic block and the fact that a specific antagonist (naloxone) existed.
• The extent of analgesia could be controlled and determined by the spinal level of administration.
• It was also hoped that the opioid-related side effects of nausea, vomiting, pruritus, sedation and respiratory depression could be reduced. However, all of these side effects can occur as well as two additional ones, urinary retention and late onset respiratory depression.

The specific benefits for epidural analgesia are particularly attractive and relevant in some subgroups of patients such as the elderly, the obese or those with chronic respiratory disease.   An effective epidural will enable a patient to deep breathe, cough and move with ease. This in turn facilitates a more speedy recovery with a reduction in co-morbidities such as chest infections and deep

Results of studies are inconsistent and must be interpreted cautiously; it is difficult to show a benefit in low-risk patients. At least one study has shown a shorter hospital stay and reduced morbidity in morbidly obese patients who received epidural opioids postoperatively [2]. The effects on the metabolic stress response (increase of cortisol, insulin, glucagon) to surgery are not consistent and seem to depend on the site of surgery. This response can only be blocked with an epidural infusion in patients who undergo lower abdominal surgery [3]. A significant reduction in the incidence of deep vein thrombosis and coronary artery thrombosis has been reported in patients with epidural local anaesthetics during and following surgery ([4], [5]) This may result from improved hyperkinetic lower limb flow and effects on fibrinolysis and platelet aggregation [6]. Paralytic ileus is a frequent problem and may be caused by surgery, by postoperative pain or by the opioids used to treat the pain. The duration of the ileus can be reduced when epidural infusion analgesia is used with both opioids or local anaesthetics compared with other routes of administration of parenteral opioids ([7], [8]).

It may be useful to refer to the Masters Study – Peyton et al [9] and associated responses. This study examined perioperative epidural analgesia and outcome after surgery in high risk patients and its findings have been highly influential in decreasing the use of epidural analgesia in Australia.

Pain can impair pulmonary function, deep inspiration and coughing. Incisions close to the diaphragm are a particular problem. The reduction in functional residual capacity after surgery may lead to hypoxia. Atelactasis will follow if the functional residual capacity falls below the closing volume. Studies have demonstrated improved functional residual capacity, vital capacity and forced expiratory volume or peak expiratory flow in patients after the administration of epidural infusion analgesia ([10], [11]). However, these values did not return to normal. Patients with pre-existing pulmonary disease particularly benefit from epidural analgesia. Local anaesthetics can impair coughing if profound motor block results. The use of epidural analgesia to abolish therapy-resistant cardiac ischaemic pain is well known [12]. Cardiac complications can be reduced in high-risk patients by reduction in heart rate, preload, afterload and an increase in diameter of epicardial arteries [13]. In addition to these anti-ischaemic properties, effects on platelet aggregation may reduce the risk of coronary artery thrombosis.

For additional information please refer to epidural analgesia section 5 of the acute pain guidelines reproduced courtesy of Cardiff and Vale University Health Board. 

References