Page 666 - Cardiac Nursing

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642 PA R T I V / Pathophysiology and Management of Heart Disease
DISPLAY 27-3 2005 AHA Classification Guidelines Using Level of Evidence Criteria
Class I Class IIA Class IIb Class III Indeterminate
Benefit risk Benefit risk Benefit risk Risk benefit Risk/benefit unknown
Procedure, treatment, It is reasonable to Procedure, treatment Procedure, treatment Research just starting or
or test should be perform procedure, or test may be or test should not ongoing. Further
performed or treatment/test. The considered. Evidence be performed or research needed
administered. weight of evidence documents short- administered. before recomm-
Supported by high supports action or term benefit or May be harmful endations
level prospective therapy. It is positive results established
studies considered with lower levels of
acceptable and useful evidence
Adapted from 2005 American Heart Association Guidelines for CPR and ECG.
for patients with hemodynamically compromised bradycardias AEDs because they are easier to handle, less expensive, and more
(Chapter 28). convenient.
Rapid defibrillation can be performed with manual, auto-
Early Defibrillation matic, or semiautomatic external defibrillators. Well-trained per-
VT and VF are the most common arrhythmias during cardiac ar- sonnel, often ACLS responders, who are able to interpret cardiac
rest, although the incidence of VF seems to be declining as re- rhythms on a rhythm strip or monitor, must operate manual de-
ported by two studies from European cities, and from analysis of fibrillators. Automatic advisory or semiautomatic external defib-
cardiac arrest events in Seattle, Washington from 1980 to rillators have been developed for use by first responders. AEDs
2000. 33,34 Defibrillation is the definitive therapy for cardiac arrest are accurate and easy to use and, unlike standard defibrillators,
caused by VF. Rapid, early defibrillation is a key step and the most have detection systems that analyze the rhythm and advise the
important intervention likely to save lives. Survival rates are best operator to shock when VF/VT characteristics are determined.
when immediate bystander CPR is provided and defibrillation oc- Thus, successful defibrillation can be achieved without requiring
curs within 3 to 5 minutes. 35,36 A major obstacle to rapid, early the operator to have rhythm recognition skills. AEDs are at-
defibrillation is that most cardiac arrests occur outside of the hos- tached to the patient with the use of adhesive sternal and apex
pital, indicating a need for public health initiatives to improve pads that are connected to a cable, allowing for “hands-free de-
1
early recognition of heart attack signs and symptoms. The wide- fibrillation.” AEDs were shown to help emergency personnel de-
spread use of automated external defibrillators (AEDs) assists in liver the first shock on an average of 1 minute sooner than per-
making early defibrillation a reality by expanding the number of sonnel using conventional defibrillators. 40 Early defibrillation
rescuers available to treat SCD. The AHA integrates the use of with an AED has been shown to significantly increase survival in
AEDs with basic life support skills because VF is the most com- both out-of-hospital and in-hospital cardiopulmonary ar-
mon rhythm found in adults with witnessed, nontraumatic rests. 32,41
SCA. 32,33
Transthoracic Impedance. The ability to defibrillate re-
Defibrillators. Defibrillators are the power source used to quires the passage of sufficient electric current through the heart.
deliver the electrical therapy. Defibrillators typically include a ca- Current flow is determined by transthoracic impedance (TTI), or
pacitor charger, a capacitor to store energy, a charge switch, and resistance to current flow, and the selected energy (Joules). If TTI
discharge switch to complete the circuit from the capacitor to the is high, a low-energy shock may fail to produce sufficient current
electrodes. The capacitor charger converts power from a low-volt- to defibrillate. The factors that determine TTI include energy set-
age source, such as direct current, to a voltage level sufficient for a ting, electrode size and composition, electrode–skin interface,
shock. Portable defibrillators derive their power from a battery, number of and time between previous electrical discharges, elec-
which must be kept charged. Electrical output of defibrillators is trode pressure, ventilation phase, and electrode placement. 42,43
quantified in terms of Joules (J), or watt-seconds, of energy. 33 Resistance between the electrode and the chest wall must be
Defibrillators deliver energy to the electrode in either a bipha- minimized. Bare electrodes produce high resistance to electrical
sic or a monophasic waveform. Biphasic waveforms deliver cur- flow. Defibrillation electrode gel or paste, made specifically for de-
rent in a positive direction for a specific duration, and then reverse fibrillation, will help to decrease impedance. Self-adhesive moni-
the current to a negative direction for the remaining discharge. A tor or defibrillator pads are also available and effective. The adhe-
monophasic waveform delivers the current in one polarity or di- sive defibrillator pads are thought to be more convenient and
rection. Studies show that biphasic waveforms achieve shock suc- safer, as they reduce the possibility of electrical arcing. AHA rec-
cess rates at lower energies, 150 J compared to 200 J, and produce ommends the use of adhesive pads. The pads can be placed as the
less ST-segment change than shocks delivered with monophasic patients condition deteriorates, allowing for monitoring of the pa-
waveforms. 37–39 Lower energy requirements reduce the size and tients heart rhythm and providing the ability to deliver a shock
weight of the defibrillator, which in turn increases public access to rapidly if necessary. 32,33