Motivation: Last year, a man walked into my clinic, shook my hand, and said that he had survived a v-fib cardiac arrest. My heart skipped a beat. He told me that he had been unconscious for more than ten minutes. I did not know whether to believe him. What is the relation between survival and time to defibrillation in out of hospital cardiac arrest?
Paper: De Maio, V.J., Stiell, I.G., Wells, G.A. et al. "Optimal Defibrillation Response Intervals for Maximum Out-of-Hospital Cardiac Arrest Survival Rates" Ann Emerg Med. (2003) 42: 242-250.
Methods: Prospective cohort study of the Ontario Prehospital Advanced Life Supports (OPALS) study, in which 21 Ontario study communities received a basic life support level of care with defibrillation by EMS. The study assessed the relation of survival to defibrillation response time by EMS between 1991-1997.
Results:
Cohort: There were a total of 9,273 out of hospital cardiac arrests. The mean age of 68.3 with 67.7% male sex. The initial rhythm was ventricular fibrillation in 38.5%, asystole in 42.3%, and pulseless electrical activity in 19.1%. There was return of spontaneous circulation in 10.4%. Of the total 9,273 patients with cardiac arrest, 392 were discharged alive from hospital (4.2%).
Defibrillation: Of the 9,273 cardiac arrests, 4,059 received defibrillation (43.8%). The median time to defibrillation was 6 minutes. The 90th percentile for defibrillation response was 9.3%.
Survival: When survival to hospital discharge is related to response time to defibrillation:
- Response time less than 4 minute: 7.6% survival.
- Response time between 4-6 minutes: 4.0% survival
- Response time between 6-8 minutes: 2.8% survival
- Response time greater than 8 minutes: 1.6% survival
When survival is modeled continuously to time to defibrillation, the predicted survival is (90th percentile prediction):
- Defibrillation time 1 min: 28.1% survival
- Defibrillation time 2 mins: 23.1% survival
- Defibrillation time 3 mins: 18.8% survival
- Defibrillation time 4 mins: 15.1% survival
- Defibrillation time 5 mins: 12.0% survival
- Defibrillation time 6 mins: 9.5% survival
- Defibrillation time 7 mins: 7.5% survival
- Defibrillation time 8 mins: 5.9% survival
Discussion: I think that the overall lesson is that surviving an out of hospital cardiac arrest is hard! Even with good response time between 4-6 minutes, there is only a 4% survival. While defibrillation definitely appears to increase survival (by almost four fold when comparing response time less than 4 minutes to greater than 8 minutes), the really steep drop in survival comes at the very beginning of the cardiac arrest (for example, almost 20% drop in predicted survival from two minutes to three minutes). This study, however, has some flaws in accounting for the benefit of defibrillation. Most notably, there were more defibrillations performed than there were ventricular fibrillations. Consequently, some PEA and asystole arrests were also defibrillated without a clear indication. Including these in the accounting probably dilutes the effect of defibrillation.
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