SWAP Score - Shockable, Witness, Age, pH for OHCA

Written by Clay Smith

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Patients presenting to the ED with out-of-hospital cardiac arrest (OHCA) with a non-shockable rhythm, unwitnessed arrest, age >60, and pH≤7 likely have a 0% chance of good neurological outcome, specificity 100%.

Why does this matter?
Prior efforts, such as the modified BLS termination of resuscitation (TOR) rule (no witness of collapse, no ROSC before ED, and not shocked by AED) have been used in the prehospital arena. Having an objective way to estimate prognosis with ongoing arrest in the ED is helpful as we counsel families on what to expect should patients have return of spontaneous circulation (ROSC). If the SWAP score is 4, we need to prepare them for the worst.

SWAP score like golf - less is more
A score to determine outcome after OHCA was prospectively derived in a cohort of 852 patients with no ROSC prior to ED arrival. They found that non-shockable rhythm, unwitnessed arrest, age >60 years, and pH ≤7 were the factors that had the strongest association with unfavorable outcome (CPC score ≥3 or death - i.e. anything other than complete recovery or moderate disability). They validated the score retrospectively in a separate (but similar) cohort of 859 patients and found that patients with all 4 SWAP variables had a 0% probability of favorable neurologic outcome, specificity 100% (95% CI 99.9% to 100%), 100% PPV. It performed a little better than the modified BLS TOR rule. There are several aspects to consider before we use the score with confidence. First, it needs prospective validation in a different context. This was derived and retrospectively validated in a single center with Asian patients. Next, it should be applied only to OHCA patients with no ROSC prior to arrival in the ED. Finally, use of bedside echo to detect cardiac activity was not considered in this score and has been shown to be a useful discriminator of outcome. All these things considered, SWAP is a helpful way to generally think about prognosis after OHCA.

SWAP score summarized:

  • Shockable? Non-shockable rhythm = 1 point

  • Witnessed? Unwitnessed arrest or collapse = 1 point

  • Age? Over 60 years = 1 point

  • pH? pH ≤7 = 1 point

Source
Derivation and Validation of the SWAP Score for Very Early Prediction of Neurologic Outcome in Patients With Out-of-Hospital Cardiac Arrest. Ann Emerg Med. 2019 Feb 25. pii: S0196-0644(19)30024-1. doi: 10.1016/j.annemergmed.2019.01.017. [Epub ahead of print]

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Reviewed by Thomas Davis

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