Without evidence of benefit, an intervention should not be presumed to be beneficial or safe.

- Rogue Medic

A Conversation on Mechanism of Injury

I was talking with one of the long time, weekend, night shift nurses. The people I count on to do what is right for the patient, because the administrators are not around.

Well, I mentioned in passing – I thought it was going to be just in passing – about the recent comment kerfuffle about MOI (Mechanism Of Injury criteria for trauma triage, or just mechanism). This is the assessment skill substitute for assessment that people use as a justification for flying uninjured patients in helicopters.

As if that is safe.

All of a sudden, the nurse started a little tirade about a medic who brought in a patient to this non-trauma center ED (Emergency Department) because he did not bother to report on the MOI when calling for medical command destination decision.

I do not remember what the mechanism was, but it was something vehicular and must have sounded bad, because that’s what MOI means –

The 911 call sounds bad!


That dent looks like it is going to cost a lot to repair!.

That has nothing to do with the patient, except that the mechanism suggests things to be more careful in assessing for.

This is all that mechanism means.

You might want to pay extra attention to these things suggested by mechanism.

Mechanism is not assessment.

Mechanism is the equivalent of stereotype, or prejudice, or bias, or racism.

Mechanism is not about understanding.

Mechanism is a shortcut that encourages ignorance.

Mechanism is just a superficial substitute for a patient assessment.

Mechanism is for those who cannot assess real patients.

Anyway, being the blunt person that I am, I interrupted the nurse’s rant, because my shift is only 12 hours long and her rant was looking like a filibuster. I didn’t even have to ask the obvious question about what a simple assessment showed, because the nurse mentioned over a dozen rib fractures and a flail chest.

Clearly, this is not a patient who should have been transported to the local ED with several trauma centers less than 20 minutes away by ground. This is a case, if reported accurately, of an incompetent medic. And not just a little bit incompetent.

Back to mechanism.

What does mechanism add to the assessment of a patient with a flail chest?

A flail chest is a portion of the ribs acting like a trap door. The ribs are broken in so many places that there is no resistance to pressure, except when the patient exhales.

Breathing is not very complicated. The diaphragm creates negative pressure. On inhalation, the diaphragm pulls away from the chest and the accessory muscles also cause the chest to expand. This sucks air in.

On exhalation, the diaphragm and accessory muscles relax and create pressure. This forces air out.

With a flail chest segment, breathing mechanics are mostly normal for everything except the flail segment. The rest of the chest is creating a pressure difference that moves the air. As long as the ribs are intact, they will all move together. When there is a flail segment (2, or more, ribs broken in 2, or more, places is the textbook definition) that broken part of the ribs will move the opposite direction from the rest of the ribs. The flail segment will move in the opposite direction from the intact part of the ribs.

When the ribs are expanding out to create negative pressure, the negative pressure is pulling air into the chest, but the negative pressure is also pulling the broken ribs inward.

When the ribs are relaxing and creating positive pressure, the positive pressure is  forcing the air out of the chest, but the positive pressure is also forcing the broken ribs outward.

This is one of those assessment findings that is hard to miss. The patient may be trying to keep you from assessing that part of the chest, because . . . well . . . it hurts. It doesn’t hurt a little bit. This isn’t just a hairline fracture that hurts a lot. This is a bunch of broken bones that are moving around – a lot – with every bit of breathing.

Not – It only hurts when I laugh.

Not – It only hurts when I move.

But – It only hurts when I breathe.

The normal response to the first two is pretty easy. If it hurts, when you do that, don’t do that.

That doesn’t work very well for breathing. Go ahead. See how long you can hold your breath. Now take a hammer and break a bunch of your ribs. Now, how long can you hold your breath? Not the same thing, at all.

The only time that a flail chest should be missed is when the ribs are not completely broken, in which case, it is not really a flail chest, except for the textbook definition of 2, or more, ribs broken in 2, or more, places. That is the textbook definition. The textbook definition should include the paradoxical movement. Paradoxical movement is what everyone is supposed to be looking for.

Paradoxical means the opposite of what we would ordinarily expect. We would ordinarily expect the ribs to all move together. With a flail chest, the flail segment is moving in the opposite direction from the rest of the ribs.

If the patient is conscious and not disoriented, the pain should be a clear clue to examine the part of the chest being protected. The patient’s arm may act as an excellent splint. Expect to use a lot of morphine/fentanyl/Dialudid. Pain will interfere with breathing more than the opioids will. Fentanyl is less likely to affect cardiac output (blood pressure), so that is my preference.

If people are missing flail chest, we need to ask Why?

We don’t need to complain that the person is ignoring mechanism.

Focusing on mechanism just ignores everything we understand about assessment.

Or do we just not understand assessment?

Mechanism Of Injury criteria for trauma triage encourage incompetence.



  1. Great write! This too is one of my pet peeves!! Beyond what you’ve mentioned consider this. Today’s autos are made to break apart, crush and do all sorts of things to absorb energy keeping it away from the driver. Many MOI “trauma guidelines” specify how much exterior damage is to be considered. When were these guidelines written and how much different were the designs then? Adhering to these “guidelines” will certainly cause over use of trauma resources.

    • Anonymous,

      Thank you.

      The improved design of modern autos is an important reason to avoid using auto damage as a triage tool.

      There is research that looks at the over-triage vs. under-triage effects of various trauma triage criteria. Mechanism Of Injury is just not a good independent predictor of injury. Why do we not expect seriously injured patients tend to have assessment findings that support triage to a trauma center?

      We do need to improve our assessments, but we don’t do that by substituting mechanism for critical judgment and thorough assessment.

  2. Even thought I worked in the automobile business for several years before I went into EMS, I didn’t get into EMS to be a insurance appraiser. MOI parameters based on 1980s auto design are pretty useless anyway. A rollover with seat belts worn is far different than a rollover with no seat belts worn, but many people in EMS (and medicine in general) don’t seem to get that.

    Missing a flail chest won’t be corrected by blind adherence to mechanism of injury. It will be corrected by doing a complete assessment.

    I think we lost a lot of assessment skills when the industry went to mnemonic based teaching instead of actually teaching people what, how, and why to assess. People tend to focus on the rote recitation of the mnemonic without understanding the thought process that goes with it.

  3. Very well done! I had a similar situation where I transported a patient from a rollover crash on a busy interstate. Of course I report my assessment findings to the trauma center instead of the mechanism of injury, we take her in a wheelchair as she is ambulatory and without deficit. I go back in for signatures and the nurse asks me if I knew the crash was a rollover and that another passenger was ejected (because they were not using the vehicle’s restraint system), I knew this, and she proceed to lecture me about the mechanism of injury. These are TRAUMA nurses, same in your case, I resisted the urge to blast her in the face with her own ignorance but decided to let it go.

    I had a student with me that day, and I properly educated her before moving on.

    I find it annoying that the nurses at that particular hospital complain about us bringing them non-critical traumas (which is a different matter entirely) or “non Level 1 traumas” (which all the criteria are physiologic based) and then talk about mechanism of injury like it’s a big deal. I’m tired of the doublespeak. The outlying hospitals feed into this. I had a 16 y/o girl roll her vehicle, self extricate and be without injury only to be turned away diverted away from a trauma center and the outlying hospitals tried to send us to the hospitals that just diverted them away. I think the problem has been fixed, largely, but there is a serious lake of education and communication among hospital personnel on what actually constitutes a serious trauma. It’s like they make up their own rules, based on what day it is.


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