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Introduction – Did you know there is an automated external defibrillator (AED) shortage in the United States? While many believe that AEDs can be found “everywhere,” the truth is that many more AEDs are needed to ensure one is reasonably close to a large number of people having sudden cardiac arrest (SCA). How significant is the shortage? There are approximately 3.2 million AEDs currently found in public places but over 30 million AEDs are needed to provide sufficient coverage to meaningfully improve the national SCA survival rate that currently stands at less than 8 percent. Read on to see how the AED shortage was calculated.
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AED Response Time – Time is the most critical factor. Indeed, the time it takes to deliver the first AED shock has a direct effect on the chances an SCA victim will survive. Longer times to defibrillation equal lower survival rates. In non-medical settings like health clubs, shopping malls, retail stores, government and office buildings, schools, and similar locations, it is reasonable to expect that AEDs should be used within 4-5 minutes from the time SCA is first recognized. This concept of “AED response time” refers to the time it takes to recognize SCA, start CPR, tell people nearby to call 911 and retrieve an AED (assuming one is available), retrieve an AED and use an AED.
AED Retrieval Time – Because of time considerations, AEDs can only cover a limited area. Given the known time constraints associated with treating SCA, we can calculate the area a single AED can cover within defined AED response time goals. Factors impacting an AED’s coverage area include time, distance and obstacles. Generally, we allocate two minutes of the AED response time to AED retrieval (AED retrieval time). On average, adults walking rapidly can travel approximately 300 feet per minute (1). At 300 feet per minute, it will take up to one minute to reach the AED and another minute to return to the person having SCA. So, we use a 300 foot radius to calculate the maximum potential AED coverage area that will allow an AED retrieval time of up to two minutes.
AED Coverage Area – Going back to high school math, we use the formula Pi*r2 where r is a radius of 300 feet to calculate that the maximum theoretical area an AED can cover is 283,000 square feet (equal to about 5 football fields). But, because AEDs are most often not placed in open areas, we also have to account for obstacles like people, doors, hallways, stairs, elevators, escalators, physical barriers and obstructions, AED storage methods, and similar constraints that reduce the effective area an AED can cover within the two-minute AED retrieval time. For our purposes, let’s assume that obstacles reduce the coverage area radius by half to 150 feet. This means, on average, an AED can cover about 70,650 square feet.
AEDs/Square Mile – With the basic math out of the way, we can now move to the bigger picture and more data and assumptions. There are 497 urbanized cities in the U.S. covering a land area of 88,000 square miles (there are about 28 million square feet per square mile so we are talking big numbers here) and containing a population of 223 million (70 percent of the current U.S. population) (2). Crunching these numbers tells us that it takes 395 AEDs to cover one square mile (27.9 million sq. ft. per mile/70,650 sq. ft. of AED coverage area).
AEDs in Non-Residential, Urbanized Areas of the U.S. – Okay, assuming 395 AEDs per square mile and 88,000 square miles, it looks like we need about 35 million AEDs to cover the urbanized U.S. But wait, we need to consider some other factors. For one, the focus here is on placing AEDs in non-medical and non-residential settings. I haven’t found any data telling us what percentage of the urbanized U.S. is non-residential. Let’s assume 40 percent for purposes of this analysis suggesting approximately 14 million AEDs are needed to cover the non-residential, urbanized areas of the U.S..
Other Factors Impacting AED Coverage Area – But two other factors (at least) must also be considered. First, this underlying calculation is of horizontal area only. Many buildings, however, are multi-story and we need horizontal and vertical AED coverage throughout these buildings. Second, many (perhaps most) AEDs are placed in locations that limit their effective coverage range. For example, they are placed in locations smaller than 70,000 square feet and/or they are not permitted to be removed from the AED owner’s location even if an SCA event is nearby at a different location.
Total Number of Estimated AEDs Needed – Taking these additional factors into account, it seems reasonable to add back in the residential deduction leading to an estimate that over 30 million AEDs are needed to cover the non-residential, urbanized areas of the U.S. And millions more are needed to cover the non-urbanized areas and remaining 30 percent of the population.
If we need over 30 million AEDs to cover 70 percent of the population, how many are out there now? We estimate there are approximately 3.2 million AEDs now found in public settings throughout the U.S. This means we have only about 10 percent (perhaps less) of the total number of AEDs required if rapid defibrillation is going to be available to a large number of people experiencing SCA. This may help explain why, in public settings, AEDs are used by bystanders a miniscule 2.1% (5) to 3.7% (6) of the time.
Why is there an AED deployment shortage?
There are many reasons for the AED shortage. With few exceptions (a small number of legislatively enacted AED placement mandates), organizations buy AEDs on a voluntary basis with no insurance reimbursement or other subsidy. While SCA is a widespread public health problem (we don’t know who, where or when SCA will strike), we have thus far largely relied on the private sector to finance public access defibrillation programs. There are lots of good reasons for organizations to have AEDs but many more will need to join the effort if we are going to dramatically increase SCA survival rates.
Another major contributor to the AED deployment shortage is the current state of U.S. AED laws. Existing state and local laws are complicated and confusing, impose burdensome and unhelpful requirements on AED programs and don’t offer comprehensive Good Samaritan legal protections to all AED program constituents. Until laws across the country are fixed, it will remain challenging to convince more organizations to buy and place AEDs.
Sudden cardiac arrest remains a vexing public health challenge. By quantifying the AED shortage in numerical terms, I hope we can persuade more organizations to become part of the solution.
(1) Manual on Uniform Traffic Control Devices (2009); Walking data
(2) U.S. Census Bureau; List of United States urban areas
(3) Survival After Application of Automatic External Defibrillators Before Arrival of Emergency Medical Services
(4) Out-of-Hospital Cardiac Arrest Surveillance — Cardiac Arrest Registry to Enhance Survival (CARES), United States, October 1, 2005–December 31, 2010