Morphine

Morphine is the standard opioid with which others are compared and remains a valuable drug for the treatment of acute, severe pain.Peak effect after IV bolus is 15 min. Duration of action is between 2 and 3 hours. Both liver and kidney function are responsible for morphine elimination. The liver mainly metabolises it. One of the principal metabolites, morphine 6-glucuronide (M6G), is also a potent opioid agonist and may accumulate in renal failure.

USES

• Relief of severe pain
• To facilitate mechanical ventilation
• Acute left ventricular failure – by relieving anxiety and producing vasodilatation

CONTRAINDICATION

• Airway obstruction
• Pain caused by biliary colic

ADMINISTRATION

• IV bolus: 2.5 mg every 15 min PRN
• IV infusion rate: 1–5 mg/h

Dilute in glucose 5% or sodium chloride 0.9% 
Stop or reduce infusion each day and restart when first signs of discomfort appear. Failure to assess daily will result in overdosage and difficulty in weaning patient from ventilation

• If the patient is conscious the best method is to give an infusion pump they can control (PCAS): 50 mg made up to 50 ml with sodium chloride 0.9%; IV bolus: 1 mg; lockout: 3–10 min

How not to use morphine

In combination with an opioid partial agonist, e.g. buprenorphine (antagonises opioid effects)

ADVERSE EFFECTS

• Respiratory depression and apnoea
• Hypotension and tachycardia
• Nausea and vomiting
• Delayed gastric emptying
• Reduced intestinal mobility
• Biliary spasm
• Constipation
• Urinary retention
• Histamine release
• Tolerance
• Pulmonary oedema

CAUTIONS

Enhanced and prolonged effect when used in patients with renal failure, the elderly and in patients with hypovolaemia and hypothermia.
Enhanced sedative and respiratory depression from interaction with:
• benzodiazepines
• antidepressants
• anti-psychotics

Organ failure

• CNS: sedative effects increased
• Respiratory: respiratory depression
• Hepatic: can precipitate coma
• Renal: increased cerebral sensitivity. M6G accumulates

Renal replacement therapy

CVVH dialysed dose as in CC 10–20 ml/min, i.e. use smaller than usual dose, e.g. 2.5–5 mg. HD dialysed dose as in CC 10 ml/min, i.e. use smaller doses, e.g. 1.25–2.5 mg and extended dosing intervals. PD not dialysable, dose as per HD. Active metabolite M6G accumulates in renal failure.Titrate to response, such as pain/sedation scores.