Alimentary tract. Morphine activates receptors on the smooth muscle of the stomach (antrum) and of both large and small bowel, causing it to contract. Peristalsis (propulsion) is reduced and segmentation increased. Thus, although morphine 'stimulates' smooth muscle, delayed gastric emptying and constipation occur, with gut muscle in a state of tonic contraction. Delay in the passage of the intestinal contents results in greater absorption of water and increased viscosity of faeces, which contribute to the constipation. The management of such opioid-
induced constipation is an important aspect of palliative care.
Morphine increases pressure in the sigmoid colon and colonic diverticula may become obstructed and fail to drain into the colon. Pethidine neither produces these high pressures nor prevents drainage, and so is preferable if the pain of acute diverticulitis is severe enough to demand a narcotic analgesic. Morphine may also endanger anastomoses of the bowel immediately postoperatively and it should not be given in intestinal obstruction (excepting in palliative care).
Intrabiliary pressure may rise substantially after morphine (as much as 10 times in 10 minutes), due to spasm of the sphincter of Oddi. Sometimes biliary colic is made worse by morphine, presumably in a patient in whom the dose happens to be adequate to increase intrabiliary pressure, but insufficient to produce more than slight analgesia. In patients who have had a cholecystectomy this can produce a syndrome sufficiently like a myocardial infarction to cause diagnostic confusion. Naloxone may give dramatic symptomatic relief, as may glyceryl trinitrate. Another result of this action of morphine is to dam back the pancreatic juice and so cause a rise in the serum amylase concentration. Morphine is therefore best avoided in pancreatitis; but buprenorphine has less of this effect.
Bronchial muscle is constricted, partly due to histamine release, but so slightly as to be of no importance, except in asthmatics in whom morphine should be avoided anyway because of its respiratory depressant effect.
Urinary tract. Any contraction of the ureters is probably clinically unimportant. Retention of urine may occur (particularly in prostatic hypertrophy) due to a mix of spasm of the bladder sphincter and to the central sedation causing the patient to ignore afferent messages from a full bladder.
In general, when morphine is used and the smooth muscle effects are objectionable, atropine may be given simultaneously to antagonise spasm. Unfortunately this does not always effectively oppose the rise of pressure induced in the biliary system, nor does it restore bowel peristalsis. Glyceryl trinitrate will relax morphine-induced spasm.
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Did you ever think feeling angry and irritable could be a symptom of constipation? A horrible fullness and pressing sharp pains against the bladders can’t help but affect your mood. Sometimes you just want everyone to leave you alone and sleep to escape the pain. It is virtually impossible to be constipated and keep a sunny disposition. Follow the steps in this guide to alleviate constipation and lead a happier healthy life.