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Potential impact of public access defibrillators on survival after out of hospital cardiopulmonary arrest: retrospective cohort study

Jill P Pell, honorary clinical senior lecturer ^a, Jane M Sirel, research assistant ^a, Andrew K Marsden, medical director ^b, Ian Ford, professor of statistics ^c, Nicola L Walker, senior house officer ^a, Stuart M Cobbe, Walton professor of medical cardiology ^a. 

^a Department of Medical Cardiology, University of Glasgow, G31 2ER, ^b Scottish Ambulance Service Headquarters, Edinburgh EH10 5UU, ^c Robertson Centre for Biostatistics, University of Glasgow G12 8QQ

Correspondence to: Stuart M Cobbe stuart.cobbe{at}clinmed.gla.ac.uk

	Abstract

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Objective: To estimate the potential impact of public access defibrillators on overall survival after out of hospital cardiac arrest.
Design: Retrospective cohort study using data from an electronic register. A statistical model was used to estimate the effect on survival of placing public access defibrillators at suitable or possibly suitable sites.
Setting: Scottish Ambulance Service.
Subjects: Records of all out of hospital cardiac arrests due to heart disease in Scotland in 1991-8.
Main outcome measures: Observed and predicted survival to discharge from hospital.
Results: Of 15 189 arrests, 12 004 (79.0%) occurred in sites not suitable for the location of public access defibrillators, 453 (3.0%) in sites where they may be suitable, and 2732 (18.0%) in suitable sites. Defibrillation was given in 67.9% of arrests that occurred in possibly suitable sites for locating defibrillators and in 72.9% of arrests that occurred in suitable sites. Compared with an actual overall survival of 744 (5.0%), the predicted survival with public access defibrillators ranged from 942 (6.3%) to 959 (6.5%), depending on the assumptions made regarding defibrillator coverage.
Conclusions: The predicted increase in survival from targeted provision of public access defibrillators is less than the increase achievable through expansion of first responder defibrillation to non-ambulance personnel, such as police or firefighters, or of bystander cardiopulmonary resuscitation. Additional resources for wide scale coverage of public access defibrillators are probably not justified by the marginal improvement in survival.

What is already known on this topic Three quarters of all deaths from acute coronary events occur before the patient reaches a hospital Defibrillation is an independent predictor of survival from out of hospital cardiac arrest The probability of a rhythm being amenable to defibrillation declines with time Interest in providing public access defibrillators to reduce the time to defibrillation has been growing, but their potential impact on overall survival is unknown What this study adds Most arrests occur in sites unsuitable for locating public access defibrillators Arrests that occur in sites suitable for locating defibrillators already have the best profile in terms of ambulance response time, use of defibrillation, and survival of the patient Public access defibrillators are less likely to increase survival than expansion of first responder defibrillation or bystander cardiopulmonary resuscitation

	Introduction

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Defibrillation is an independent predictor of survival after cardiac arrests that take place out of hospital. Within 2 minutes of arrest, two thirds of patients have electrocardiographic evidence of ventricular fibrillation or tachycardia,¹ but the probability of a rhythm being amenable to defibrillation declines over time.² Even when an ambulance meets its target response time, some delay between the emergency telephone call and attendance is inevitable. Therefore, bystander interventions must be considered together with efforts to minimise ambulance response times. To shorten time to defibrillation, locating automated external defibrillators in public places, for use by bystanders before the arrival of the ambulance, has been considered. The Department of Health is committed to providing 700 public access defibrillators in 72 sites across England and Wales.³ The present study aimed to estimate the potential impact of public access defibrillators on overall survival after out of hospital cardiopulmonary arrest.

	Methods

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Setting
The Scottish Ambulance Service is the sole provider of emergency pre-hospital ambulance care in Scotland. In the period of study no public access defibrillators were deployed in Scotland. Pre-hospital defibrillations were undertaken by ambulance personnel (98.9%) or general practitioners (1.1%).

Data
Ambulance crews collect data on all resuscitation attempts after out of hospital cardiopulmonary arrests. The forms include the time from the emergency telephone call to the arrival of the ambulance crew at the arrest scene, location of the arrest, and whether defibrillation was performed. Forms completed by hospital staff document whether patients admitted to hospital after cardiac arrest survive to discharge.

Cohort study
Our cohort comprised all out of hospital cardiopulmonary arrests over a seven year period from May 1991 that fulfilled criteria for having a cardiac cause,⁴ were not witnessed by the ambulance crew, and did not occur in an ambulance, ambulance helicopter, general practice, dental surgery, or non-acute hospital. We reached a consensus on individual arrest sites that were suitable for locating public access defibrillators, possibly suitable (if defibrillators were to be very widely distributed), or unsuitable (see also bmj.com). Actual survival to discharge was calculated for each of the three types of site.

Statistical model
We assumed that public access defibrillators could improve survival, at best, to that obtained by early ambulance attendance. We calculated predicted survival in the suitable and possibly suitable sites by applying the survival rate among patients attended in 3 minutes to the number of patients who experienced a delay of >3 minutes. We assumed that future arrests would have the same distribution of arrest sites and attendance times as the cohort data. We calculated two predicted overall survival rates, according to location of public access defibrillators only in suitable sites or in both suitable and possibly suitable sites.

	Results

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Cohort study
Over the seven year study period, 21 481 cardiopulmonary arrests were attended by the Scottish Ambulance Service, and 15 189 fulfilled the inclusion criteria. Of these, 12 004 (79.0%) occurred in sites not suitable for the location of public access defibrillators, such as the person's home or a friend's home, 453 (3.0%) occurred in possibly suitable sites, such as buses and multistorey car parks, and 2732 (18.0%) occurred in suitable sites, such as shops, places of business, and sports centres (see bmj.com).

In all types of site, patients who received defibrillation were significantly more likely to survive (8.0% of patients (95% confidence interval 7.4% to 8.6%) versus 0.6% (0.5% to 0.9%)). Among all patients, people who had an arrest in sites suitable for public access defibrillators had a higher baseline survival rate (8.7% (7.7% to 9.9%)) than people who had an arrest in possibly suitable sites (4.5% (2.7% to 6.8%)) or in sites that were not suitable (4.2% (3.8% to 4.6%)).

Overall, 70.1% of patients who were attended in 3 minutes received defibrillation, compared with 58.3% in patients who experienced longer delays (table). Among the 14 850 patients with complete data on ambulance response time and defibrillation, 744 (5.0% (95% confidence interval 4.7% to 5.4%)) survived to discharge from hospital.

Statistical model
The model predicted that locating public access defibrillators only in suitable sites would increase the number of survivors from 744 to 942, a survival rate of 6.3% (5.6% to 7.1%). If defibrillators were located in suitable and possibly suitable sites, the predicted number of survivors increased only slightly further to 959, a survival rate of 6.5% (5.7% to 7.2%) (table). In Scotland, with a population of 5.1 million, these rates equate to an average number of additional lives saved a year of 28 and 31, respectively.

A response time cut-off of 2 minutes rather than 3 minutes produced a predicted number of survivors of 1008 (6.8% (5.6% to 7.9%)) for suitable sites only and 1028 (6.9% (5.7% to 8.1%)) for suitable and possibly suitable sites. A cut-off of 4 minutes produced figures of 892 (6.0% (5.7% to 6.3%)) and 903 (6.1% (5.5% to 6.7%)).

	Discussion

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Cost effectiveness
Making public access defibrillators as widely available as fire extinguishers would greatly increase early access but would incur considerable costs and would be problematic in terms of maintenance and avoiding misuse and vandalism. Nichol et al modelled the incremental cost effectiveness of expanding access to automated external defibrillators beyond the emergency medical services in the United States.⁵ Their model excluded the 25% of arrests in the United States that occur in non-urban areas. They reported a baseline survival of 7.9%, which increased to a predicted 8.7% with lay responder defibrillation and to 11.8% with the police as additional first responders. The incremental costs per patient treated were estimated as $7100 (£4590; 7148) and $9200, respectively, and the cost per additional quality adjusted life year (QALY) was $44 000 and $27 200. The authors concluded that these costs were similar to those of a number of existing medical interventions. However, their lay responder model used costs from a police responder pilot. It is likely that the cost per QALY of a public access scheme will be much higher, owing to the large number of defibrillators needed to provide adequate coverage. Unlike automated external defibrillators used by a police service, public access defibrillators are fixed to a location, and the location of future arrests cannot be predicted precisely from the location of previous arrests.

Conclusions
Targeted placement of defibrillators in public places such as airports and shopping centres, frequented by a large number of susceptible people, could, at best, increase overall survival from 5.0% to 6.3%. Further expansion of the coverage of public access defibrillators to enable lay use in all potentially suitable sites would require much greater resources and would produce little additional improvement in survival. Public access defibrillators should not be provided in preference to the expansion of defibrillation given by first responders or increased cardiopulmonary resuscitation by bystanders.

	Acknowledgments

Contributors: See bmj.com

	Footnotes

Editorial by Engdahl

Funding: British Heart Foundation.

Competing interests: None declared.

The full version of this article appears on bmj.com

	References

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5.	Nichol G, Hallstrom AP, Ornato JP, Riegel B, Stiell IG, Valenzuela T, et al. Potential cost-effectiveness of public access defibrillation in the United States. Circulation 1998; 97: 1315-1320[Abstract/Free Full Text].

(Accepted 2 April 2002)