|
Sudden Cardiac Death (SCD) is estimated at 200 000-400 000
cases/year in the United States1. The exact number is difficult to determine precisely, because of the
difficulty in determining cause and circumstances of death in the out-of-hospital setting. There is little
doubt that these estimates include other non-cardiac causes of relatively sudden death, such as massive
pulmonary embolism, CVA, ruptured aortic aneurysm, etc. that went unrecognized because of the circumstances
at the time of death. Nonetheless, Sudden Cardiac Arrest (SCA) represents a significant health issue.
The majority (60-70%) of these individuals will be found to have structural heart disease (most often
coronary artery disease) at autopsy, or in the fortunate few, at time of hospital evaluation.
The best hope for survival from SCA is quick re-establishment of a perfusing cardiac rhythm, and
spontaneous circulation. Unfortunately, for patients who are asystolic, or in Electromechanical Dissociation
(EMD) at the time of discovery, the chance of resuscitation is dismal. Similarly, for those whose arrest is
triggered by a catastrophic cardiac mechanical failure such as massive myocardial infarction, free-wall rupture,
etc. even normalization of cardiac rhythm is unlikely to be adequate for survival even to hospital admission.
For the majority, however, the key to survival is early defibrillation. In patients whose rhythm is determined
within 4 minutes of arrest, 95% will be in ventricular fibrillation. After 12 minutes, only 71% will be found in VF.
If elapsed time is unknown (and presumably longer), only 40% will still be in VF with 40% asystolic, the
unfortunate circumstance in the majority of cases2. In Seattle, Washington, a city with a relatively prompt
EMS system and an average time to medic arrival of 6 minutes, only 40% of patients are found in VF. This
observation likely reflects the typical course of events in SCD in the majority of cases of SCD, i.e. VF progressing
to asystole and electromechanical dissociation. Electrical therapy for VF (defibrillation) is relatively straightforward
and efficacious in restoring sinus rhythm with proper equipment, with success rates approaching 95%. In
contrast, asystole of more than brief duration and certainly EMD, are associated with a dismal chance of
restoration of sinus rhythm. It is no surprise therefore, that survival to hospital discharge for patients
presenting in VF approaches 25% under ideal circumstances in comparison with 6% for EMD and 1%
for asystole3. Interestingly, even in studies using data from relatively prompt resuscitation (30 seconds
to 3 minutes), VT was a very uncommon presenting arrhythmia in out-of-hospital cardiac arrest, accounting
for only 1.2-8 % of cases4. It is unlikely that any adjuvant therapy will significantly improve outcome under
these circumstances. A recent study demonstrated the value of i.v. amiodarone in the resuscitation of
shock-refractory VF in out-of-hospital arrest. In victims failing the first 3 external shocks, amiodarone
improved survival to hospital admission to 44 from 34% with placebo, without any improvement in
survival to hospital discharge5. While promising for the patients studied, this addresses only those
who survived to arrival of trained emergency personnel.
It should be evident therefore, that the key to improving survival of SCA is bystander CPR and minimizing
the time to defibrillation. Under ideal circumstances, defibrillation would be done by trained medical personnel
capable of also delivering advanced life support, but from a practical standpoint, this is not viable. As noted above,
in Seattle, Washington, with an EMS system carefully “tweaked” over the past 15 years, arrival of
“first-responders” i.e. police or fire personnel equipped with Automatic External Defibrillators (AEDs) takes
on average 2.6 minutes with advanced medical personnel arriving in 6 minutes after the patient is found
and 911 is called. With defibrillation by the first-responders, 41% of SCA victims have return of spontaneous
circulation. If defibrillation is delayed until the arrival of medics, this falls to 28%. In a single-center study
in a relatively small community, resuscitation with AEDs by first-responders was 49%, but this could not been
reproduced in larger communities6,7. Even then, only 17-25% (10% of all SCA victims) will survive to
hospital discharge neurologically intact presumably because of the relative delay involved. In other US
cities such as Chicago, Illinois, this figure is only 2.6%. Minorities and socioeconomically disadvantaged
populations fare particularly poorly due to a combination of factors including underlying health status,
lower rates of bystander CPR, and education about procedures for getting emergency help. If all SCA
victims received CPR within 3 minutes, and could be defibrillated within 6 minutes, resuscitation could
theoretically approach 70%1. The relatively recent availability of portable, easy-to-use AEDs now provides
the potential for improvement by allowing lay-persons to participate not only in conventional CPR, but
also early defibrillation. This would involve placement of AEDs, like fire-extinguishers, in public places such
as shopping areas, stadiums, movie theaters, commercial aircraft, etc. Case reports of defibrillation by
flight attendants and casino personnel have demonstrated the feasibility of this concept8. A recent report
has demonstrated that the current generation of AEDs can be quickly and successfully operated even by
elementary school age children9. The placement of AEDs in public places addresses only part of the problem
however. In a recent study from Sweden, 65.8% of SCA arrest occurred in the home10. While eventually
AEDs may be inexpensive enough to be in homes as smoke-detectors and fire-extinguishers are now,
the first strategy should be to place AEDs in the homes of persons who are known to be at higher risk
of SCA e.g. following large MI and others not meeting criteria for ICD implantation. Widespread training
in bystander CPR, and access to rapid, lay-person delivered defibrillation hold the greatest promise for
improving survival from SCA. The results of randomized trials both planned and in-progress will hopefully
provide the data needed to validate these concepts.
|
