Why altered LOC/multiple trauma morphine contraindications?

[Jim Squire]

Contraindications for morphine use include (among others): head injury, decreased mental status, multiple trauma.  Why?

I understand why it's respiratory depression effect would make contraindicated for patients with COPD and asthmatic attacks, and why its effect on blood pressure would make it a bad choice for a hypotensive pt.  But why would multiple trauma and altered LOC/head injuries make it so?  I'd think head injuries would make rise of ICP an issue, one in which morphine would, if anything, help.

[rock_shoes]

http://www.vch.ca/about-us/news/news-releases/vgh-leads-the-way-in-traumatic-brain-treatment

 

Very long story short, look up the monro-kellie doctrine, and principles of cerebral perfusion pressure.

[Jim Squire]

Rock_shoes,

First, thanks for  the response.  I appreciate the time people with experience like you take to answer questions from people like me who are in the process of starting out.  I have no idea why I find all this so fascinating, but I do.  

Unfortunately, I'm not sure I understand your point - I'm still just a student without a single ride to my name yet searching for a class that's open during COVID without much success.  Can you check my analysis below and tell me if I'm right?

The Monro-Kellie Doctrine (which was a new one for me; thanks!) says that if any one of the 3 volumes of brain, blood, or CSF increases then another volume must decrease and ICP will rise.  That makes sense intuitively to me, and is why an intracranial bleed 2/2 head trauma would cause an rise in ICP.  But why is morphine contraindicated here?  I would think morphine would decrease BP, therefore decreasing cranial blood volume proportional to the brain's arterial compliance, therefore decreasing ICP, and therefore improving things.  So from that, morphine is good.

Cerebral perfusion pressure (yet another thing I hadn't heard of before; thanks!) says the greater the differential between the MAP and ICP, the greater perfusion.  Also makes sense intuitively.  From that I see that the drop in BP from morphine combined with the increase in ICP if there is a brain bleed or post-traumatic swelling would be bad; it would decrease the pressure gradient and therefore decrease neural cellular respiration. So from that morphine is bad.

Combining those two things, the takeaway is that, in practice, the damage that morphine does from decreasing cerebral perfusion is worse than the improvement it does by reducing ICP, so don't use it.  

Is that right?

[rock_shoes]

On 4/27/2020 at 11:21 AM, Jim Squire said:

Rock_shoes,

First, thanks for  the response.  I appreciate the time people with experience like you take to answer questions from people like me who are in the process of starting out.  I have no idea why I find all this so fascinating, but I do.  

Unfortunately, I'm not sure I understand your point - I'm still just a student without a single ride to my name yet searching for a class that's open during COVID without much success.  Can you check my analysis below and tell me if I'm right?

The Monro-Kellie Doctrine (which was a new one for me; thanks!) says that if any one of the 3 volumes of brain, blood, or CSF increases then another volume must decrease and ICP will rise.  That makes sense intuitively to me, and is why an intracranial bleed 2/2 head trauma would cause an rise in ICP.  But why is morphine contraindicated here?  I would think morphine would decrease BP, therefore decreasing cranial blood volume proportional to the brain's arterial compliance, therefore decreasing ICP, and therefore improving things.  So from that, morphine is good.

Cerebral perfusion pressure (yet another thing I hadn't heard of before; thanks!) says the greater the differential between the MAP and ICP, the greater perfusion.  Also makes sense intuitively.  From that I see that the drop in BP from morphine combined with the increase in ICP if there is a brain bleed or post-traumatic swelling would be bad; it would decrease the pressure gradient and therefore decrease neural cellular respiration. So from that morphine is bad.

Combining those two things, the takeaway is that, in practice, the damage that morphine does from decreasing cerebral perfusion is worse than the improvement it does by reducing ICP, so don't use it.  

Is that right?

I love it when a plan comes together. The heart of the concept is minimizing cerebral oxygen demand while maintaining a sufficient cerebral perfusion pressure and flow for tissue oxygenation. Assuming an ICP of 20mmHg, it would take a MAP of 80mmHg to maintain a CPP of 60mmHg (I bet MAP guidelines for the management of TBI are suddenly making more sense). Some sedative/analgesic medications balance those considerations better than others. This brings in the concept of flow metabolic coupling (Propofol is particularly good at this as sedative agents go). Agent's with good flow metabolic coupling such as Propofol reduce cerebral oxygen demand in balance with the amount they reduce cerebral blood flow. Agents such as Morphine or Midazolam do a poor job balancing the two considerations and reduce cerebral blood flow relatively more than they reduce cerebral oxygen demand.