The Automated External Defibrillator (also
see Chapter 35
)
The most frequent cardiac rhythm responsible for witnessed cardiac
arrest in adults is ventricular fibrillation. CPR prolongs the duration of VF but
cannot convert the arrhythmia to an organized rhythm. Successful termination of
this arrhythmia requires prompt electrical defibrillation, not medications. The
most recent AHA guidelines include early defibrillation with an automated external
defibrillator (AED) as part of BLS training and the concept of public-access defibrillation,
which endorses the policy of making defibrillation available to victims of cardiac
arrest through nonconventional providers (e.g., police, security guards, and others).
[8]
The AED, when applied to a patient, is capable
of analyzing cardiac rhythm and detecting VF. A trained rescuer's role is to apply
the defibrillator pads to the patient's chest, activate the AED, and if the device
indicates that a defibrillatory shock is indicated, manually deliver the shock through
the push of a button when prompted to do so by the AED. A series of up to three
shocks will be provided in short succession based on the detected cardiac rhythm.
To prevent the potential for incorrect rhythm analysis, chest compressions must
be suspended and the patient cannot be moved during this sequence of rhythm analysis
and shocks. VF and rapid ventricular tachycardias (VTs) are the only rhythms recognized
by the AED. Because supportive measures such as CPR will also be necessary in circumstances
in which the AED is applied, AED use must be reserved for rescuers trained in BLS.
The first AED was introduced in 1979.[85]
In 1986, the first microprocessor-based AED was introduced; this AED used three
successive, nonescalating, 180-J monophasic damped waveform shocks. Advances in
defibrillator technology, including conversion from monophasic to biphasic defibrillator
waveforms, resulted in smaller, lighter units with more accurate rhythm analysis,
which allowed inclusion of defibrillation with the AED as part of
BLS interventions. The same technologic advances in AEDs have been incorporated
into manually controlled defibrillators. Impedance-compensated, nonescalating low-energy
biphasic waveforms (150 J) were first introduced in 1996 and have been shown to achieve
success in terminating VF equivalent to that of traditional, escalating-energy monophasic
waveforms (200 to 300 J) in the controlled setting of an electrophysiology laboratory.
[86]
[87]
[88]
Biphasic waveforms deliver energy first in one direction and then reversed for the
duration of the impulse. Impedance compensation allows the defibrillator to change
waveform morphology based on the impedance measured during defibrillation. It is
believed that these components allow effective defibrillation at lower energies than
with monophasic defibrillators. Experience in the out-of-hospital cardiac arrest
environment also demonstrates that his biphasic waveform is at least as effective
as escalating monophasic defibrillation waveforms[89]
[90]
[91]
when defibrillation
success was defined as termination of VF into an organized rhythm or asystole 5 seconds
after shock, regardless of the hemodynamic response.[92]
Additional discussion related to cardioversion and defibrillation will be included
in the treatment algorithms for specific arrhythmias presented later.
