Circulation. 2000;102:I-90-I-94
(Circulation. 2000;102:I-90.)
© 2000 American Heart Association, Inc.
Part 6: Advanced Cardiovascular Life Support
Section 2: Defibrillation
 |
Introduction
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For a person in VF the probability of successful
defibrillation
and subsequent survival to hospital discharge is
directly and
negatively related to the time interval between onset of
VF
and delivery of the first shock.
1 2 3 Early
defibrillation
by first rescuers and trained lay responders is a
rational implementation
of the concept that the earlier defibrillation
is performed,
the better the rate of survival to hospital discharge.
Early
defibrillation has reversed VF cardiac arrest in a number of
small
case series (American Airlines, QANTAS Airlines,
4
Chicago’s
O’Hare Airport,
4A and Las Vegas
casinos
5 ). These dramatic
examples of the effectiveness of
early defibrillation by nontraditional
responders have provided a
driving force for PAD in the United
States.
6 7 8 9 10
For more than a decade the AHA has recommended that every ambulance
vehicle be equipped with a defibrillator and personnel trained in
defibrillation.11 All healthcare providers with a duty to
perform CPR should be trained, equipped, and encouraged to perform
defibrillation (Class IIa). The Guidelines 2000 Conference recommends
that early defibrillation be available throughout all hospital and
outpatient medical facilities (Class IIa). The use of defibrillation
now transcends both ACLS and BLS care. This section addresses use of
standard defibrillators, including cardioversion, for the experienced
healthcare provider in the ACLS environment. Early defibrillation,
automated external defibrillation, and PAD are also discussed in
"Part 4: The Automated External Defibrillator: Key Link in the Chain
of Survival."
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Energy Requirements for Defibrillation
|
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Defibrillation depends on the successful selection of energy
to
generate sufficient current flow through the heart (transmyocardial
current)
to achieve defibrillation while at the same time causing
minimal
electrical injury to the heart. A shock will not terminate the
arrhythmia
if the energy and current are too low. Functional
and morphological
damage may result if energy and current are too
high.
12 13 Selection of appropriate current also reduces
the number of
repetitive shocks and limits myocardial
damage.
14 There is
no definite relationship between body
size and energy requirements
for defibrillation in adults.
Transthoracic impedance does play
an important role (see
below).
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Biphasic Waveform Defibrillators
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Modern defibrillators, including AEDs, deliver energy or current
in
"waveforms." Energy levels vary with the type of device and
type
of waveform. Several types of monophasic waveforms have
been used in
modern defibrillators. Biphasic waveforms have
recently been developed
and approved for marketing and clinical
use. The body of evidence about
the efficacy and safety of devices
using biphasic waveforms has
increased dramatically in the 4
years since the first such device was
marketed. The first biphasic
AED approved for use in the United States
used a waveform set
at a lower energy (150 to 175 J) than that
recommended by the
AHA (200 J) for the first shock. This first device
also was
fixed-nonescalating, meaning the energy level of shocks could
not
be increased. In a recent review of evidence through 1997 for
nonescalating
low-energy biphasic waveforms in out-of-hospital
arrest,
15 the reviewers concluded that lower-energy
biphasic shocks, delivered
without an increase in energy, achieved
clinical outcomes equivalent
to those of monophasic shocks with
increasing energy levels.
Monophasic waveforms deliver current in one direction. Monophasic
defibrillators vary the speed and amount of waveform fall and the speed
of the return to zero voltage point. If the monophasic waveform falls
to zero gradually, the term damped sinusoidal is used. If
the waveform falls instantaneously, the term truncated
exponential is used. Biphasic waveforms, in contrast, deliver
current that flows in a positive direction for a specified duration.
The current then reverses and flows in a negative direction for the
remaining milliseconds of the electrical discharge. Biphasic waveforms
have proved superior to monophasic waveforms for defibrillation by
implantable defibrillators.15 17 (See the
Figure
.)
View larger version (12K):
[in this window]
[in a new window]
|
Figure 1. Relative efficacy of monophasic and biphasic waveforms for
transthoracic defibrillation after short and long durations
of ventricular fibrillation. Reprinted with
permission from Walcott GP, Melnick SB, Chapman FW, Jones JL,
Smith WM, Ideker RE. Relative efficacy of monophasic and biphasic
waveforms for transthoracic defibrillation after short and
long durations of ventricular fibrillation.
Circulation. 1998;98:2210–2215.
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In 1996 the Food and Drug Administration approved the first AED that
used a biphasic waveform.. This was a biphasic truncated exponential
(BTE) waveform with impedance compensation. This AED delivered only
nonescalating 150-J shocks. Studies compared this waveform with
conventional monophasic damped sinusoidal (MDS) waveform shocks at 200
and 360 J. These studies were conducted in electrophysiology
stimulation suites during implantation of automatic implantable
cardioverter-defibrillators (ICDs). In projects funded by
defibrillator manufacturers, researchers observed that 150-J BTE shocks
achieved first-shock defibrillation at the same rate as 200-J MDS
shocks. BTE 150-J shocks also produced less ST-segment change than
200-J MDS shocks.18 Researchers have collected data both
in-hospital (electrophysiological studies
and ICD testing) and out of hospital.19 This research
indicates that repetitive lower-energy biphasic waveform shocks
(repeated shocks at 
200 J) have equivalent or higher success for
eventual termination of VF than defibrillators that increase the
current (200, 300, 360 J) with successive shocks (escalating).
|
Biphasic Waveform Defibrillation
|
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The optimal energies for biphasic defibrillation have not been
determined.
Importantly, the biphasic first-shock energy level yielding
the
highest termination rate for VF is unknown. The percentage of
patients
who fail to respond to a first or successive biphasic shock
at
a constant energy of
200 J remains unknown. Do patients in
VF that is
unresponsive to multiple "lower-energy" shocks then
require
higher-energy (escalating) biphasic shocks? Or will
these patients
require only repetition of low-energy biphasic
shocks?
Research has not yet determined the optimal biphasic
waveform. The potential advantages of new biphasic waveform variants,
such as a rectilinear first pulse waveform, are also
unknown. Researchers need to study the efficacy of "impedance
compensation." Compensation for patient-to-patient differences in
impedance may be achieved by changes in duration and voltage of shocks
or by releasing the residual membrane charge (called
burping). Whether there is an optimal ratio of first-phase
to second-phase duration and leading-edge amplitude is unclear. The
threshold for the start and extent of cardiac damage from biphasic
waveforms compared with monophasic waveforms remains a mystery.
Finally, it will be important to determine whether a waveform more
effective for immediate outcomes (defibrillation) and
short-term outcomes (spontaneous circulation, admission to
the hospital) results in better long-term outcomes (survival
to hospital discharge, survival for 1 year). These are critical
questions in communities in which the interval from collapse to first
shock remains long.
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Shock Energies
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The traditional recommended energy for the first monophasic
shock
is 200 J.
20 The energy level for second and third shocks
can
be either the same (200 J) or as high as 360 J. Even a failed
shock
at one energy may be successful if simply repeated. Clinically
the
energy does not need to increase simply because the first
shock failed
to defibrillate. Any given energy has a constant
probability to achieve
defibrillation. Repeated shocks, even
at the same energy level,
add to the probability of successful
defibrillation. This is
not immediately obvious but can be clarified
with a brief example. A
waveform with a defibrillation rate
of 80% will leave 20% of the
victims in VF with each successive
shock: of 100 people, first shock=20
in VF; second shock=4 in
VF (20% of 20); third shock=1 in VF (20% of
4). Thus, this waveform,
with a 1-shock success rate of 80%, will
achieve a 99% success
rate if the 3 successive, nonescalating shocks
are considered
a single intervention.
Higher current flow will occur with subsequent shocks even at the same
energy, because transthoracic impedance declines with
repeated shocks.21 22 These arguments favor repeating the
second shock at the same energy level as the first if VF persists, but
reductions in human transthoracic impedance are only
modest.21 A more predictable increase in current occurs
when shock energy is increased. This supports second shocks of higher
energy. A compromise between these positions is the use of a
range of energies (200 to 300 J) for the second monophasic
shock.
Increase the current/voltage and deliver a third shock of 360 J
immediately if 2 monophasic shocks fail to defibrillate the
heart. If VF is initially terminated by a shock but then recurs later
in the arrest, deliver subsequent shocks at the previously successful
energy level.
We cannot make a definitive recommendation for the energy for first and
subsequent nonescalating biphasic defibrillation attempts. Current
research confirms that biphasic shock energies 200 J are safe and
effective. Even though both escalating- and nonescalating-energy
defibrillators are available, there is insufficient data to recommend
one approach over another. Any claim of superiority at this time is
unsupported. Nonescalating biphasic energies appear to have success
rates for VF termination equivalent to or better than monophasic shocks
(Class IIa) that increase in energy with each shock.
The most important determinant of survival in adult VF is rapid
defibrillation. Give shocks as soon as a defibrillator is
available.1 2 If the first 3 shocks fail to achieve
defibrillation immediately, continue CPR and follow the ACLS guidelines
for sudden VF/VT: IV access, tracheal intubation, epinephrine; shock if
still in VF; consider amiodarone or lidocaine.
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Cardioversion
|
|---|
Atrial Fibrillation
The recommended initial energy for cardioversion of atrial
fibrillation
is 100 to 200 J MDS. Atrial flutter and paroxysmal
supraventricular
tachycardia (PSVT) generally
require less energy. An initial
energy of 50 to 100 J MDS is often
sufficient, with stepwise
increases in energy if initial shocks
fail.
23 24 25 Transthoracic
cardioversion of
atrial fibrillation with a low-energy (120-J),
rectilinear, first-pulse
biphasic waveform was superior to 200
J MDS in a recent controlled
trial.
26 Cardioversion with biphasic
waveform is now
available, but more data is needed before specific
comparative
recommendations can be made.
Ventricular Tachycardia
The amount of energy required for cardioversion of VT depends on
the morphological characteristics and rate of the
arrhythmia.27 Monomorphic VT (regular form and
rate) with or without a pulse responds well to cardioversion shocks at
initial energies of 100 J MDS. Polymorphic VT (irregular morphology
and rate) responds similarly to VF. The initial shock energy should be
200 J MDS. Give stepwise increases if the first shock fails to
cardiovert.27
|
Transthoracic Impedance
|
|---|
Defibrillation is accomplished by passage of sufficient electric
current
(amperes) through the heart. The energy chosen (joules) and
the
transthoracic impedance (ohms), or resistance to current
flow,
determine the current flow. Factors that determine
transthoracic
impedance include energy selected, electrode
size, paddle-skin
coupling material, number and time interval of
previous shocks,
phase of ventilation, distance between electrodes
(size of the
chest), and paddle electrode
pressure.
21 22 28 The average
adult human impedance is
approximately 70 to 80
.
13 21 22 29 30 31 When
transthoracic impedance is too high, a low-energy
shock
will not generate enough current to achieve
defibrillation.
13 24 30 To reduce
transthoracic impedance, the defibrillator
operator should
always press firmly on handheld electrode paddles
and use a gel or
cream or saline-soaked gauze pads between handheld
electrode paddles
and the chest. Self-adhesive monitor/defibrillator
electrode pads do
not require additional pressure. Use of "bare"
handheld paddles
without a coupling material between electrodes
and the chest wall
creates high transthoracic impedance.
22 Male
patients with a hirsute chest have poor electrode-to-chest
contact plus
air trapping. This results in high impedance, with
occasional
"arcing." (Although extremely rare, in oxygen-rich
environments
such as critical care units this arcing has been
known to produce fires
if an accelerant is present.) Rapid shaving
of the area of intended pad
placement may be necessary. The
AHA textbook
Heartsaver AED for
the Lay Rescuer and First Responder contains an excellent
description of successful approaches to
removal of chest hair.
|
Current-Based Defibrillation
|
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VF and other cardiac arrhythmias can be terminated by
electric
shock when sufficient current passes through the
myocardium.
A promising alternative approach to
defibrillation is the use
of electric current (amperes) instead of
energy (joules). This
approach would prevent attempts to deliver
inappropriately low
energy levels to a patient with high impedance.
Current-based
therapy would also prevent high-energy shocks to patients
with
low impedance, which result in excessive current flow, myocardial
damage,
and failure to defibrillate.
13 24
Clinical studies using MDS waveform shocks have attempted to identify
the range of current necessary to achieve defibrillation and
cardioversion. The optimal current for ventricular
defibrillation appears to be 30 to 40 A MDS.29 30 31
Comparable information on current dosage for biphasic waveform shocks
is under investigation.
Electrode Position
Place electrodes in positions to maximize current flow through the
myocardium. The standard placement is one electrode just to
the right of the upper sternal border below the clavicle. Place the
second electrode to the left of the nipple with the center of the
electrode in the midaxillary line. An acceptable alternative is to
place the "apex" paddle anterior, over the left precordium, and
the other paddle (labeled "sternum") posterior to the heart in the
right infrascapular location.32 33 Take
care that the electrodes are well separated and that paste or gel is
not smeared on the chest between the paddles, because the current may
follow a superficial pathway along the chest wall, "missing" the
heart. Self-adhesive monitor/defibrillator electrode pads are also
effective and can be used in any of these locations.31
When performing cardioversion or defibrillation in patients with
permanent pacemakers or ICDs, do not place the electrodes near the
device generator, because defibrillation can cause malfunction. A
pacemaker or ICD also may block some current to the
myocardium during defibrillation, delivering suboptimal
energy to the heart. Finally, because some of the defibrillation
current flows down the pacemaker leads, always reevaluate the pacing
threshold after shock(s) to patients with permanent
pacemakers.34 ICD function also should be evaluated.
Electrode Size
The Association for the Advancement of Medical Instrumentation
recommends a minimum electrode size of 50 cm2 for
individual electrodes.35 The sum of the electrode areas
should be a minimum of 150 cm2. Larger electrodes
have lower impedance, but excessively large electrodes may result in
less transmyocardial current flow.36
For adult defibrillation, both handheld paddle electrodes and
self-adhesive pad electrodes 8 to 12 cm in diameter are used and
perform well.21 31 37 Even smaller pads have been found
effective38 in VF of brief duration.
|
Synchronized Cardioversion
|
|---|
Delivered energy should be
synchronized with the QRS
complex
to reduce the possibility of inducing VF, which can occur when
a
shock "hits" the relative refractory portion of the cardiac
cycle.
32 Synchronization to prevent this
complication is recommended
for hemodynamically stable
wide-complex tachycardia requiring
cardioversion,
supraventricular tachycardia, atrial
fibrillation,
and atrial flutter. VF requires
unsynchronized
defibrillation
mode. It is important to note that synchronization in VT
may
be difficult and misleading because of the wide-complex and
variable
forms of ventricular arrhythmia. The
VT patient who is pulseless,
unconscious, hypotensive, or in severe
pulmonary edema should
receive unsynchronized shocks to avoid
the delay associated
with attempts to synchronize. The healthcare
provider should
be prepared to deliver another unsynchronized shock
within seconds
if VF or pulseless VT remains or recurs.
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Blind Defibrillation
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Administration of shocks without a monitor or an ECG rhythm
diagnosis
is referred to as "blind" defibrillation. Blind
defibrillation
is rarely necessary. Handheld paddles with
"quick-look" monitoring
capabilities on modern manually operated
defibrillators are
universally available. AEDs use reliable and proven
decision
algorithms to identify VF.
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"Occult" Versus "False" Asystole
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There is no evidence that attempting to "defibrillate"
asystole
is beneficial. In rare patients, however, coarse VF can be
present
in some leads, with very small undulations seen in the
orthogonal
leads, which is called
occult VF. A flat line
that may resemble
asystole is displayed. Examine the rhythm in 2 leads
to help
differentiate this technical artifact.
39 Of
more importance,
one study noted that "false" asystole, a flat line
produced
by technical errors (eg, no power, leads unconnected, gain set
to
low, or incorrect lead selection), was far more frequent than
occult
VF.
40
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Maintaining Defibrillators in a State of Readiness
|
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User checklists have been developed to reduce equipment
malfunction
and operator errors. Failure to properly maintain the
defibrillator
or power supply is responsible for the majority of
reported
malfunctions. Checklists are useful when designed to identify
and
prevent such deficiencies. Checklists help most when (1) users
are
trained in their proper use, (2) healthcare providers who
actually use
the defibrillators perform the check, and (3) checklists
are completed
with every change in personnel.
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Footnotes
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Circulation. 2000;102(suppl I):I-90–I-94.
|
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Introduction
Energy Requirements for...
