EMS in the field and physicians in the ED are faced with rapidly assigning some degree of stability to the patients they treat. What exactly are the shades of stability, and what considerations are there for each degree?

In my mind, there are three levels of “stability”:

• Unstable – this one is easy to figure out. The patient has obvious physiologic compromise, which may be objective (low blood pressure, low GCS or poor neuro exam, etc) or subjective (just plain looks bad). 
  EMS: These patients need transport to an appropriate level trauma center (I or II) immediately. If they need airway control or IV access that can’t be obtained in the field, stop at the nearest Level III or IV for assist, then continue on your way FAST. 
  ED: These patient must be a trauma activation. If not activated as your top-tier trauma, activate or upgrade now! These patients must be seen by a trauma surgeon immediately, and can only go to the OR. No diagnostics outside the resuscitation room are allowed unless they can be converted into one of the two stability levels below.
• Stable – this one is usually easy to figure out, too. These patients look good, have good vitals, and a low to moderate energy mechanism for their trauma. Look out for those few patients that may be hiding something like moderate bleeding into some body cavity.
  EMS: Follow your usual transport protocols to select the closest, appropriate hospital.
  ED: Follow your standard protocols for trauma activation if needed. Transport for standard imaging is fine.
•  Metastable – this is a term I invented. It describes patients who have evidence of ongoing volume loss that can be controlled with infusion of crystalloid and/or blood products. It is possible to maintain a certainly level of stability using higher than normal volume infusions. This allows physicians to consider diagnostics or interventions outside of an OR.
  EMS: Ensure adequate IV access and give fluids and/or blood per your local protocols. Transport to a Level I or II trauma center as quickly as possible.
  ED: Activate or upgrade to your highest level of trauma activation. The trauma surgeon needs to be present to help direct diagnostics or interventions. These patients may go to CT, IR or other appropriate areas with nurse and physician accompaniment to diagnose and possibly treat bleeding. If the patient changes to unstable at any point, they must immediately be taken to the OR.