Sudden cardiac arrest (SCA) is a serious and often lethal medical condition, but its exact definition, the statistics behind its occurrence, and recorded survival rates are not always in consensus. In order to better understand why SCA has these discrepancies in data, this article will further explore its range of definitions, challenges in reporting and data gathering, and the differences between in-hospital and out-of-hospital occurrence outcomes.
Many of the statistics set forth in this compilation are derived from the American Heart Association’s 2017 update on Heart & Stroke Statistics. Even within this authoritative and lengthy document, the sources of the statistics gathered vary widely, and one set of data may be from as recently as 2016, while others reach back as far as 2009. What is important to remember is most of this information are estimates with varying margins of error.
Definitions of SCA
The general definition of sudden cardiac arrest appears to be fairly consistent, with most sources agreeing that SCA is the stopping of the heart due to a disruption of the heart’s electrical impulses, which results in inadequate oxygenated blood flow to the brain and vital organs, which causes the victim to lose consciousness.
The causes of SCA can be difficult to determine. To further complicate the matter, there are both cardiac and non-cardiac causes, which may not be readily evident when the incident occurs. Cardiac causes are attributed to the structure or function of the heart directly; non-cardiac causes are attributed to outside factors such as a blow to the chest, choking, or electrocution.
Another term which tends to cloud the results is sudden cardiac death, as opposed to sudden cardiac arrest. Make no mistake, someone in sudden cardiac arrest is technically dead. Some argue whether or not survivors of SCA are included within the broader category of SCD. SCD generally requires victims to have died from sudden cardiac issues, but despite its name, death does not seem to be a requirement for classification as an SCD in every case.
Let’s review some of the definitions from various authorities on this topic.
Sudden Cardiac Arrest:
American Heart Association
In the American Heart Association’s (AHA) 2017 report, cardiac arrest is defined as “the cessation of cardiac mechanical activity, as confirmed by the absence of signs of circulation.”
National Institutes of Health SCA Definition
The National Institutes of Health’s National Heart, Lung, and Blood Institute (NHLBI) defines sudden cardiac arrest as “a condition in which the heart suddenly and unexpectedly stops beating. If this happens, blood stops flowing to the brain and other vital organs. SCA usually causes death if it’s not treated within minutes.”
Sudden Cardiac Death:
American Heart Association
In the same report as above, the AHA defines sudden cardiac death (SCD) as “unexpected death without an obvious non-cardiac cause which occurs within one hour of symptom onset (witnessed) or within 24 hours of last being observed in normal health (unwitnessed).” With this in mind, the AHA is also quick to point out that the definitions of “sudden” and “unexpected” death are difficult to define medically, and that this definition of SCD is difficult to use in realistic settings.
Putting SCA and SCD in context
Drs. Deo and Albert in AHA Journal
In their report on Sudden Cardiac Death (SCD) published in the AHA Journal Circulation, Dr. Deo and Dr. Albert define SCD as “an unexpected death from a cardiovascular cause in a person with or without pre-existing heart disease.” They further elaborate that the definition may depend upon whether the event was witnessed or not, but generally studies include witnessed collapses, death within one hour of acute changes in clinical status, or unexpected deaths occurring within a previous 24- hour period.
The doctors define SCA as a subset of SCD cases in which resuscitation was recorded or as nullified SCD cases for instances in which the victim survived the SCA. Please notice the use of the words “may” and “generally”. This is the crux of definition difficulties leading to statistical anomalies.
Discrepancies Identified in Comparing SCA and SCD Studies
In Dr. Kong et al.’s 2011 article “Systematic Review of the Incidence of Sudden Cardiac Death in the United States,” the doctors used six different studies carried out in time spans ranging from 1980 to 2007, which reported estimates for the number of SCA and SCD cases in the U.S. They found criteria for arriving at the statistics varied enough to influence numbers significantly.
- Three of the six studies included time restrictions within their definition of SCD.
- Four of the six studies used location of the event as part of the definition.
- Two of the studies limited SCD to deaths caused by ischemic or coronary heart disease, while the other four studies used a wider definition including deaths from cardiac or cardiovascular etiologies.
- One study included survivors of SCA in their definition of SCD.
- The final study focused on out-of-hospital cardiac arrest and included both survivors and deaths.
SCA Event and Survival Data
As the most consistent gatherer of information on sudden cardiac arrest numbers, the American Heart Association is the recognized expert when it comes to knowing the numbers. According to their 2017 report:
- Roughly 209,000 patients are treated for in-hospital cardiac arrests every year with 25.5% surviving to discharge
- Approximately 356,500 people in the US experienced an out-of-hospital cardiac arrest from January 1, 2015, to June 30, 2015.
- Of the EMS patients who experienced non-traumatic cardiac arrest outside a hospital, and did not have bystander intervention, only 11.4% survived until hospital discharge
- The survival rate for out-of-hospital cardiac arrest victims more than tripled for those witnessed by bystanders to 37.4%.
- Between June 1, 2014, and May 31, 2015, over 6,000 children under the age of 18 suffered from sudden cardiac arrest
Data is reported and recorded separately for in-hospital versus out-of-hospital cardiac arrests, and for cardiac arrests in children versus adults. These variances within the data sets can make it difficult to view cardiac arrests as a whole. To add to the difficulty of obtaining reliable statistics, there are also no current national standards for tracking and monitoring the rate and outcomes of cardiac arrest.
Interpreting Out-of-Hospital SCA Statistics
Cardiac arrests which are assessed or treated by EMS personnel may be subject to regional or cultural differences related to emergency care access and treatment decisions. The causes of these variations have been attributed to varying definitions of cardiac arrest, different methods of collecting cardiac arrest data, and even variations in patient treatment after the start of cardiac arrest.
For example, while one EMS department may employ hypothermia techniques to assist in a patient’s recovery, another may not; or where one ambulance is able to reach the patient within three minutes, it may take another much longer in a larger city with more traffic. Where the statistics speak loudest is the difference in survival rates when a bystander steps in and takes the initiative to step in and start CPR and/or find a defibrillator versus those who call 911 and simply wait for EMS to arrive.
Out-of-Hospital Youth SCA Statistics
According to the AHA, approximately 7,000 children, age 18 or younger, experience an out-of-hospital cardiac arrest every year. The majority of sudden deaths in young athletes are caused by hypertrophic cardiomyopathy. A young athlete dies every 72 hours from SCA. According to a chart in the AHA’s 2017 report (Chart 18-1, Pg. 330):
In children ages 0-2: the causes of sudden cardiac arrest are overwhelmingly congenital at 84%
Children ages 3-13 who suffer sudden cardiac arrest have a close range of causes:
- Congenital defects at 21%
- Hypertrophic cardiomyopathy at 18%
- “Other” (any cause other than those listed) at 18%
- Long QT at 14%
- Myocarditis and Primary Arrhythmia both at 11%
- Mitral Valve prolapse at 7%.
For youth and young adults ages 14-24, the causes are once again varied:
- “Other” causes at 26%
- Congenital defects and primary arrhythmia both coming in at 23%
- Dilated cardiomyopathy at 14%
- Long QT at 8%
- Myocarditis at 4%
- Hypertrophic cardiomyopathy at 2%
Further Breakdown of Out-of-Hospital SCA Data
The AHA estimates over 350,000 people experience an out-of-hospital cardiac arrest every year.
- The median age is 66 years old.
- Approximately 60% are treated by EMS responders.
- 25% of individuals experiencing an out-of-hospital cardiac arrest had no previous symptoms.
- Around 23% of those individuals initially have ventricular fibrillation (VF) or ventricular tachycardia (VT), either of which can both be shocked by an automated external defibrillator.
- If ventricular fibrillation is witnessed by a bystander, the survival rate increases to 31.4% for patients of any age.
- If a bystander witnesses an irregular rhythm which can be treated with shock, the survival rate slightly increases to 33%.
- For out-of-hospital cases, cardiac arrest is witnessed by bystanders 36.9% of the time
- 12.1% of the time, cardiac arrest is witnessed by EMS providers.
- 51.1% of the time, the cardiac arrest is unwitnessed.
- 10.6% of EMS-treated cardiac arrest for patients of any age survive and 8.6% of them leave the hospital with good neurological function.
- 63.3% of patients die in the hospital after the out-of-hospital treatment of a cardiac arrest, as of 2015.
Out-of-Hospital Regional Differences
Further studies have been conducted on the potential for regional differences in out-of-hospital cardiac arrests. In April of 2016, a study was published which included 132 counties across the US and included 96,662 patients. Of these cases:
- Only 9.6% of patients survived to hospital discharge, and of those, 7.4% retained neurological functional.
- At a county level, there was marked variation in rates of survival to discharge. (3.4% to 22.0%) and survival with functional recovery (0.8% to 21.0%).
- County-level rates of bystander CPR and AED use were positively correlated with both outcomes.
- Patient demographic and cardiac arrest characteristics explained 4.8% and 27.7% of the county-level variation in survival, respectively.
- Additional adjustment of bystander CPR and AED explained 41% of the survival variation, and this increased to 50.4% after adjustment for county-level socio-demographic factors. Similar findings were noted in analyses of survival with functional recovery.
Conclusions: Although out-of-hospital cardiac arrest survival varies significantly across U.S. counties, a substantial proportion of the variation is due to differences in bystander response across communities.
Finding consistent data for in-hospital sudden cardiac arrest is challenging since regional and cultural differences in the access to healthcare, the treatment methods used, and definitions of conditions and treatments may vary.
The survival rates for children, age 18 or younger, who experienced an in-hospital cardiac arrest are not available due to small sample sizes. According to a report by the American Heart Association, out of a sample size of just over 1,000 children in 12 different hospitals from 2001 to 2009, there was asystole (no heartbeat at all) and pulseless electrical activity in 84.4% of children identified as having suffered SCA, and ventricular fibrillation and pulseless ventricular tachycardia in 15.2% of hospitalized children experiencing an initial cardiac arrest. The risk-adjusted rate of survival to discharge was 43.4% in 2009. This rate was up from just 14.3% in 2000.
The AHA estimates that there are approximately 209,000 in-hospital cardiac arrests every year based on the reported statistics and the total number of hospitalized patients within the United States. The report goes on to say that adults who experienced a cardiac arrest in-hospital had a 23.8% survival to discharge rate and of those, 86.5% had good neurological function.
No matter what statistical source you look at, the reality is the majority of people who experience a sudden cardiac arrest (SCA) will not survive. Since these deaths usually occur without warning, and their survival rates drastically decrease if treatment isn’t received within minutes of the onset of the cardiac arrest, the likelihood of an SCA victim leaving the hospital with all neurological function intact is slim.
An automated external defibrillator (AED) is critical in saving victims of SCA. AEDs are often located in public places to increase their accessibility. Bystander use of an AED in conjunction with quality CPR has statistically shown an increase in survival rate over patients who must wait for EMS help to arrive. Those who receive immediate help are generally reported as having a survival rate up to three times higher.
The increasing availability of AEDs would lead the average person to guess there would be a substantial increase in SCA survival rates overall. So why aren’t we seeing these drastic changes? According to the 2015 Cardiac Arrest Registry to Enhance Survival (CARES) National Summary Report, 70% of SCA cases happen when people are either alone in their home, or witnesses do not have access to an AED. Only 20% occur in a public setting where there may or may not be an AED.