CHAPTER 3-51: MORPHINE

INTRODUCTION

1. Pharmacology. Morphine is the principal alkaloid of opium and a potent analgesic and sedative agent. In addition, it decreases venous tone and systemic vascular resistance, resulting in reduced preload and afterload. Morphine is absorbed variably from the GI tract and usually is used parenterally. After intravenous injection, peak analgesia is attained within 20 minutes and usually lasts 3–5 hours. Morphine is eliminated by hepatic metabolism, with a serum half-life of about 3 hours; however, the clearance of morphine is slowed and the duration of effect is prolonged in patients with renal failure resulting from accumulation of the active metabolite morphine-6-glucuronide.

2. Indications

   1. Severe pain associated with black widow spider envenomation, rattlesnake envenomation, and other bites or stings.

   2. Pain caused by corrosive injury to the eyes, skin, or GI tract.

   3. Pulmonary edema resulting from congestive heart failure. Chemically induced noncardiogenic pulmonary edema is not an indication for morphine therapy.

3. Contraindications

   1. Known hypersensitivity to morphine.

   2. Respiratory or CNS depression with impending respiratory failure unless the patient is already intubated or equipment is available and trained personnel are standing by for intervention if necessary with intubation or the reversal agent naloxone.

   3. Suspected head injury. Morphine may obscure or cause exaggerated CNS depression.

4. Adverse effects

   1. Respiratory and CNS depression may result in respiratory arrest. Depressant effects may be prolonged in patients with liver disease and chronic renal failure. Risk factors or comorbidities increasing risk for morphine-induced respiratory depression include naive user lacking tolerance, hypothyroidism, morbid obesity, and sleep apnea syndrome. Note: Tidal volume may be depressed without perceptible changes in respiratory rate, and these effects are influenced by external stimuli (eg, noise, manipulation).

   2. Hypotension may occur owing to decreased systemic vascular resistance and venous tone.

   3. Nausea, vomiting, and constipation may occur.

   4. Bradycardia, wheezing, flushing, pruritus, urticaria, and other histamine-like effects may occur.

   5. Sulfite preservative in some parenteral preparations may cause hypersensitivity reactions.

   6. Use in pregnancy. FDA Category C (indeterminate). This does not preclude its acute, short-term use in a seriously symptomatic patient (Introduction).

5. Drug or laboratory interactions

   1. Additive depressant effects with other opioid agonists, ethanol and other sedative-hypnotic agents, tranquilizers, MAO inhibitors, and antidepressants.

   2. Naloxone and naltrexone will antagonize the analgesic actions of morphine and may precipitate a withdrawal syndrome in morphine-dependent patients.

   3. Morphine is physically incompatible with solutions containing a variety of drugs, including aminophylline, phenytoin, phenobarbital, and sodium bicarbonate.

6. Dosage and method of administration

   1. Morphine may be injected subcutaneously, intramuscularly, or intravenously. The oral and rectal routes produce erratic absorption and are not recommended for use in acutely ill patients.

   2. The usual initial adult dose is 2–10 mg IV (may dilute with 4–5 mL of sterile water and give slowly over 4–5 minutes as well as titrate in small increments, 1–4 mg, every 5 minutes) or 10–15 mg SC or IM, with maintenance analgesic doses of 5–20 mg every 4 hours. The usual pediatric dose is 0.05-0.1 mg/kg administered very slowly IV up to a maximum single dose of 10 mg, ...