Morphine can also cause histamine release which causes itching

							24                                                                                            Update in Anaesthesia

MORPHINE
Dr I Kestin, Consultant Anaesthetist, Derriford Hospital, Plymouth.


Structure.                                                  Morphine can also cause histamine release, which
Morphine is obtained from opium, the juice secreted         causes itching of the skin and nose and a mild
by the seeds of the poppy. It works on several              flushing of the skin. Morphine has little direct
types of receptors, widely found in nervous tissue.         effect on the heart or blood pressure. However
"Opioids" is a term used for all drugs that act on          the blood pressure may fall slightly following the
these receptors. These other opioids may be natural         pain relief produced by morphine and also with
occurring substances, such as morphine, or made             the sedation which may be produced. Significant
in the laboratory such as pethidine.                        hypotension following morphine is usually due to
Morphine works on receptors in the cell membranes.          other causes such as hypovolaemia. Occasionally
These are protein-lipid molecules that alter their          patients with severe asthma may become wheezy.
shape when stimulated to effect molecular changes           An unimportant, but early noticeable effect is that
within cells. There are thought to be three types of        the pupils become small. This helps to differentiate
receptors that respond to opioids, mu, kappa and            respiratory depression caused by morphine from
lambda. Morphine acts mainly on mu receptors to             other causes.
cause a wide variety of effects. The most important         Precautions.
of these are relief of pain and respiratory depression.     All patients who have been given morphine must
In anaesthesia morphine is used to relieve pain. This       be carefully observed for evidence of respiratory
is an effect of its action on the spinal cord to decrease   depression. This can be detected as a slow respiratory
the transmission of painful stimuli from body to            rate and a very sleepy patient with pin point pupils.
brain, and its action within the brain itself.              Oxygen should be given by face mask, and positive
Side Effects.                                               pressure ventilation of the lungs started if necessary.
Morphine has many side effects. The most                    Naloxone, 100-400 micrograms may be given
dangerous is respiratory depression. Minor degrees          intravenously if available.
of respiratory depression may be detected following         Dose and Uses
standard doses of morphine, but this is not clinically      Morphine can be given orally, rectally, by
important. With higher doses or in frail patients,          intramuscular or intravenous injection, sub-
the respiratory rate decreases, the patient becomes         cutaneously, sublingually, or injection into the
increasingly sedated, and the pupils very small.            epidural or subarachnoid space. The dose for
Common side effects are nausea and vomiting due             analgesia by IM injection is 100-150mcg/kg,
to a central action of morphine stimulating one of          repeated 2 hourly as required. The dose used
the centres in the brain concerned with vomiting            intravenously during anaesthesia depends on the
called the chemotactic trigger zone. Other central          nature and duration of the surgery. The usual dose
nervous system side effects of morphine are cough           given at the start of surgery when intermittent
suppression, sedation, and dependence leading               positive pressure ventilation is used is 100-200mcg/
to addiction. Addiction is not a problem when               kg, followed by required additional doses of 1-2mg
morphine is used to treat acute pain after surgery;         intravenously when required. After the patient has
sufficient morphine should always be given to               woken up further doses of 1-2mg may be given
relieve the pain.                                           until the patient is free of pain. Much higher doses
Morphine also has an effect on the muscle of the            are sometimes used during specialist surgery such
bowel and urinary tract, causing the sphincter to           as cardiac or neurosurgery.
contract and reduce the peristalsis (the wavelike           Morphine can be given to relieve chronic pain
movements of the bowel muscle that propel its               especially from cancer. Much higher doses are
contents forwards). This results in a delayed               needed orally than by injection. The oral dose is
emptying of the stomach, constipation, and may              very variable for individual patients; sufficient
also lead to urinary retention.                             should be given to relieve the pain.
Update in Anaesthesia




Morphine can be given into the epidural or              initial injection because of the slow circulation of
subarachnoid space. This is to deliver the morphine     cerebral spinal fluid which carries the morphine
close to one of its sites of action, the spinal cord.   up to the brain to act on the respiratory centre. It
It is thought that pain relief of long duration can     is vital that all those caring for a patient who has
be obtained with very low doses of morphine. A          had this technique are aware of the potential side
single dose of epidural morphine may relieve pain       effects and watch out for them. This technique can
for 12-24 hours. The usual adult dose for epidural      only be used in specialist hospitals with intensive
morphine is 3-7 milligrams, and of subarachnoid         care facilities. In all other aspects the side effects
morphine between 250mcg and 1mg. The main               of epidural and subarachnoid morphine are similar
risk of this technique is that severe respiratory       to morphine given by any other route.
depression may occur up to 18 hours after the