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Discussion

Morphine

I always knew Morphine reduced the oxygen demand on the heart, but I never knew through what mechanism. An instructor said that it is done by decreasing the pain so the patient isn't breathing as hard. However, Morphine releases histamine so does that mean it also vasodilates? I know it can affect your hemodynamic state by decreasing your bp and o2 but is that how? I love patho so I am constantly trying to put all the pieces together instead of just knowing the results. Thanks in advance.

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    I remember that it binds to Mu receptors, among others. I think the Mu receptors were the primary ones, and that is also one involved in respiratory depression. Did you Google? :D

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    Pain also stimulates the sympathetic nervous system which increases heart rate and contractility both driving up myocardial oxygen demand. Decreasing pain decreases the sympathetic stimulation. Along with vasodilation both preload and afterload are decreased. All these factors work together to decrease myocardial oxygen demand.

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    Since it's the pathophysiology that you're interested in, recognize that the respiratory rate doesn't affect the oxygen demand of the heart. The heart needs what it needs whether the RR=3 (e.g. opioid overdose) or RR=30 (e.g. septic patient).

    Morphine *does* reduce air hunger but *not* the metabolic need for oxygen.

    As you've learned, its benefit is pain control and mild vasodilation.

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