Potential efficacy of public access defibrillation may be underestimated

EDITOR—Pell et al provide useful data on the appropriate location of public access defibrillators.1 Not all of their conclusions, however, seem to be based on firm evidence.

They say that arrests that occur in obvious sites for locating defibrillators already have the shortest response times, yet only 340 out of 2646 (13%) patients in a suitable site received an ambulance within 3 minutes. Response time is not defined: a detail important in allowing valid comparison with other early defibrillation strategies.

The Department of Health defines this interval as starting when a chief complaint is identified by an ambulance dispatcher and ending when an ambulance stops at the scene. The time taken for a bystander to determine that action is needed and make a 999 call, as well as the time taken by the crew to reach the patient's side and deliver a counter-shock, are excluded.

The complete interval from collapse to first counter-shock is a key variable in determining outcome from cardiac arrest.2 Public access defibrillators may reduce the delays inherent in dialling 999 and consequently the time to first shock, even when the ambulance response is under 3 minutes. This potential benefit has not been included in the model used by Pell et al. Neither do they allow for the incremental benefits of a reduction in response time at intervals of more than four minutes. Mortality increases by 4% for each minute's delay to first shock.3

The authors say that provision of automated external defibrillators to other first responders could double overall survival to discharge, referring to their previously published article.4 This paper does not provide such evidence, concluding only that responding to 90% of calls within five minutes would increase the proportion of survivors to 10-11%. It requires a very large assumption to suggest …