Morphine is the most generally useful high-efficacy opioid analgesic; it eliminates pain and also allows subjects to tolerate pain, i.e. the sensation is felt but is no longer unpleasant. It both stimulates and depresses the central nervous system. It induces a state of relaxation, tranquillity, detachment and well-being (euphoria), or occasionally of unpleasantness (dysphoria), and causes sleepiness, inability to concentrate and lethargy, always supposing that this pleasant state is not destroyed by nausea and vomiting—more common if the patient is ambulant. Excitement can occur but is unusual. Morphine excites cats and horses, though it is illegal to put this to practical use. Generally, morphine has useful hypnotic and tranquillising actions and there should be no hesitation in using it in full dose in appropriate circumstances, e.g. acute pain and fear, as in myocardial infarction or road traffic accidents.
Morphine depresses respiration, principally by reducing sensitivity of the respiratory centre to increases in blood PaC02. With therapeutic doses there is a reduced minute volume first due to diminished rate and then tidal volume. With higher doses carbon dioxide narcosis may develop. In overdose the patient may present with a respiratory rate as low as 2/min.
Morphine is dangerous when the respiratory drive is impaired by disease, including C02 retention from any cause, e.g. chronic obstructive lung disease, asthma or raised intracranial pressure.
In asthmatics, in addition to the effect on the respiratory centre, it may increase viscosity of bronchial secretions, which, with depression of cough and bronchospasm (see below) will increase small airways resistance.
Morphine also suppresses cough by a central action. It stimulates the third nerve nucleus causing miosis (pin-point pupils are characteristic of poisoning, acute or chronic; at therapeutic doses the pupil is merely smaller).
The chemoreceptor trigger zone of the vomiting centre is stimulated, causing nausea (10%) and vomiting (15%), an effect which, in addition to being unpleasant, can be dangerous to patients soon after abdominal operations or cataract surgery. A preparation of morphine plus an antiemetic, e.g. cyclizine (Cyclimorph) reduces this liability. Some spinal cord reflexes are also stimulated, causing myoclonus and so morphine is unsuitable for use in tetanus and convulsant poisoning; indeed, morphine can itself cause convulsions.
Morphine causes antidiuresis by releasing antidiuretic hormone, and this can be clinically important.
Peripheral nervous system. The discovery of opioid receptors is sensory nerves and their inhibiting effect on inflammatory mediators may lead to advances in pain control.
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