Morphine: Haven't Got Time for the Pain

Morphine is the prototype for opioid analgesics.  This baby is a pure mu receptor agonist, so it has some amazing pain-killing properties!

Pharmacokinetics:

Morphine can be absorbed tons of ways.  It can be given IV, SubQ, IM, PO, intrathecally, and rectally.  You can also snort it but I wouldn’t recommend suggesting that to a patient. It is distributed via the blood stream to the mu receptors in your spinal cord, brain, and small intestine.  That last part is important to remember! The liver metabolizes it and the kidneys excrete it.

Morphine is an example of a drug that does not have a ceiling.  Some drugs have a point of concentration when they actually stop working.  Adverse effects can sometimes still worsen (great, huh?) but with no more nice analgesia.  Morphine can just keep building and building, and it will relieve more and more pain.  This doesn’t mean that people can’t overdose, of course, but it does mean that although it may require more morphine to relieve the pain, more can be given and with good effect.

You may remember that opioid analgesics produce euphoria and reduce pain. They are also CNS depressants, which means we have to be VERY careful to watch patients after they are given morphine.  Why, you ask?  I’m glad you did!

Because CNS depression…

1.  Reduces the respiratory rate.  You want it between 12-20 breaths per minute, but morphine can drop that really quickly.  It also reduces air hunger (your gasping reaction when you are out of breath) so it’s difficult to see unless you are counting breaths.

2. Reduces level of consciousness.  If you patient immediately falls into a deep sleep and their respiratory rate drops, this is a problem.

The other big problem with morphine is that it makes you VERY CONSTIPATED.  You know why?  Because mu receptors are all over your gut!  Morphine slows the digestive tract and can pretty much bring it to a halt.

So when you have a patient that needs morphine, remember you always have to check their respiratory rate and level of consciousness BEFORE you give it to them (to make sure they aren’t already having issues) and AFTER (to make sure they don’t get any issues).

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