Probability of a shockable presenting rhythm as a function of EMS response time.

Introduction: Survival from cardiac arrest is associated with having a shockable presenting rhythm (VF/pulseless VT) upon EMS arrival. A concern is that several studies have reported a decline in the incidence of VF/PVT over the past few decades. One plausible explanation is that contemporary cardiovascular therapies, such as increased use of statin and beta blocker drugs, may shorten the duration of VF/PVT after arrest. As a result, EMS response time would become an increasingly important factor in the likelihood of a shockable presenting rhythm, and consequently, cardiac arrest survival.

Objective: To develop a model describing the likelihood of shockable presenting rhythm as a function of EMS response time.

Methods: We conducted a retrospective observational study of cardiac arrest using the North Carolina Prehospital Care Reporting System (PreMIS). Inclusionary criteria consisted of adult patients suffering nontraumatic cardiac arrests witnessed by a layperson between January 1 and June 30, 2012. Patients defibrillated prior to EMS arrival were excluded. Chi-square and t-tests were used to analyze the relationship between shockable presenting rhythm and patient age, gender, and race; response time measured as elapsed minutes between 9-1-1 call receipt and scene arrival; and bystander CPR. Logistic regression was used to calculate the adjusted odds ratio (OR) of shockable presenting rhythm as a function of response time while controlling for statistically significant covariates.

Results: A total of 599 patients met inclusion criteria. Overall, VF/PVT was observed in 159 patients (26.5%). VF/PVT was less likely with increasing EMS response time (OR 0.92, 95% CI = 0.87-0.97, p < 0.01) and age (OR 0.98, 95% CI = 0.97-0.99, p < 0.01), while males (OR 1.98, 95% CI = 1.29-3.03, p < 0.01) and Caucasians (OR 1.86, 95% CI = 1.17-2.95, p < 0.01) were more likely to have shockable presenting rhythm. Bystander CPR was not associated with shockable presenting rhythm, although EMS response time was longer among patients with bystander CPR compared to those without (9.83 vs. 8.83 minutes, p < 0.01).

Conclusions: We found that for every one minute of added ambulance response time, the odds of shockable presenting rhythm declined by 8%. This information could prove useful for EMS managers tasked with developing EMS system response strategies for cardiac arrest management.