J R Soc Med 2005;98:3-6
doi:10.1258/jrsm.98.1.3
© 2005 Royal Society of Medicine
How good is your defibrillation technique?
Daniel M Sado BM BSc 1
Charles D Deakin MD FRCA 2
1 Department of Oncology, Poole General Hospital, Poole BH15 2JB
2 Department of Cardiothoracic Anaesthesia, Southampton General Hospital,
Southampton SO16 6YD, UK
Correspondence to: Dr Dan Sado E-mail:
dan_sado{at}yahoo.com
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INTRODUCTION
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In 1956, a patient in ventricular fibrillation (VF) was successfully
treated
via externally applied
electricity.
1 The
history of human defibrillation,
which is still the only effective means of
cardioverting VF
back into sinus rhythm, began here. By 1962, electric shock
had
also been found effective in atrial fibrillation and atrial
flutter.
2
In August 2000 the International Liaison Committee on Resuscitation (ILCOR)
published evidence-based guidelines for adult and paediatric life
support.3 All the UK
training courses on advanced life support are now based on the ILCOR
guidelines; yet, in this document of over 400 pages, the defibrillation
section is only 4 pages long. This is partly because the topic had been
under-researched. New information has emerged since 2000. Our own group has
taken a special interest in the defibrillation technique used by hospital
practitioners and how it could be improved.
Survival to discharge from in-hospital cardiac arrest in the UK is still,
at best, only 17% in a monitored area of the hospital such as a coronary care
unit.4 It is
considerably worse on general hospital wards. The most important factor in
determining whether defibrillation will be successful is the time that elapses
before delivery of the first
shock.5 For every
minute post-arrest, the chances of cardioversion decrease by as much as
10%.3 Many
defibrillators in hospitals now have an automated facility, so that a nurse
suspecting a cardiac arrest can connect the patient to the machine via two
pads;6 the device
analyses the cardiac rhythm and delivers a shock if appropriate. A study in
hospital inpatients has shown that use of the automated facility on new
biphasic defibrillators can yield a 2.6-fold increase in survival
post-arrest.7 All
nurses should therefore be trained to use an automated defibrillator if one is
available on their
ward.8
Recently, there have been two major developments in defibrillator
technology. One is the emergence of pads made of flexible conductive
material.9 Our group
found that the transthoracic impedance (TTI) to current flow created by use of
these pads is the same as that with
paddles.10 At
present there is uncertainty as to which method is preferable. The second
advance concerns the waveform used for defibrillation. Originally, the
waveform created by defibrillators was monophasic but over the last ten years
the biphasic waveform has taken over and nearly all new models use this
technology11
(Figure 1). The relative merits
of these two waveforms are beyond the scope of this article. In summary, a
biphasic shock of 150 joules is far more likely than a monophasic shock of
either 200 or 360 joules to convert VF; yet, so far, no study has shown a
survival advantage for the biphasic
method.12
Successful defibrillation depends on delivery of the shock to a critical
mass of
myocardium13 and
this in turn depends on the transthoracic current flow (TCF) achieved. If the
TCF is too low, defibrillation will fail; if it is too high the result may be
myocardial cell damage and even necrosis. On the evidence of post-mortem
studies and troponin measurement after defibrillation, necrosis is not a
common result of multiple shock
delivery.14,15
The most likely reason for failure is a TCF that is too low.
TCF is determined by the energy selected and the
TTI:16
Clearly a low TTI will result in a high TCF. Numerous
studies have shown that
a high TTI will decrease the chances
of delivering a successful shock to the
patient;
17-19
therefore,
during defibrillation, TTI has to be reduced as far as possible.
Some
contributory factors, such as thoracic size, are intrinsic and
cannot be
altered; but extrinsic factors include paddle force,
presence of chest hair,
use of an electrical coupling agent,
20 the size of the
paddles (larger electrodes have a lower impedance
but when excessively large
can result in less
TCF
18), the number
of
shocks
delivered
21 and
paddle position. This article focuses
on techniques for minimizing TTI.
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PAD/PADDLE PLACEMENT
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According to the ILCOR guidelines, the sternal paddle should
be placed
'just to the right of the upper sternal border below
the clavicle'
and the apical paddle 'to the left of the nipple
with the centre of the
electrode in the mid-axillary
line'.
3 In a
survey of 101 doctors we found that only 22% placed the
apical paddle in this
position with the majority placing it
too
superomedially;
22 a
Finnish group made similar observations
in 136 healthcare
professionals.
23
Whether these deviations
from guidelines decrease the success rates is
uncertain. No
group has yet elicited the pathways taken by defibrillator
current
through the human body. In theory, a paddle position that is
too
superomedial means that less current will traverse the myocardium.
During the gel pad placement study we noticed that about 50% of doctors
placed the rectangular apical paddle vertically upwards, pointing towards the
left armpit (Figure 2). The
other 50% placed it in a horizontal position across the chest
(Figure 3). The present ILCOR
guidelines do not specify which orientation should be used for defibrillation.
We hypothesized that, with the paddle method for defibrillation, it would be
more difficult to get good skin contact across the curved chest wall with the
horizontal orientation, and in a small study this proved to be the
case.24 When 60 N
(the median force used by defibrillator operators in clinical
practice25) is
applied to both paddles, the resulting TTI is 5% greater with the horizontal
orientation. Thus, if paddles are used, we recommend a vertical orientation.
This question has not been addressed in relation to pads. We would expect
their flexibility to allow better electrode/skin contact across the curved
chest wall; however, in the absence of any evidence to the contrary, we advise
vertical orientation for this method as well.
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PADDLE FORCE
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As increasing force is applied to defibrillator paddles, the
better
paddle/skin contact and expulsion of air from the lungs
should decrease the
TTI.
26 In a study
of forces used in clinical
practice
25
operators
applied a median of 60 N to each paddle. This is equivalent
to the
force created by placing six one-litre bags of water
on the chest. The 1992
European Resuscitation Council (ERC)
guidelines recommended 120 N applied to
each paddle.
27 We
asked
50 operators to press with 120 N on two defibrillator paddles
placed on
a resuscitation
mannequin;
28 only 7
were physically
able to attain this pressure. The more recent ILCOR guidelines
simply
specify 'firm
force'.
3
Neither of these recommendations was
evidence-based. Studying the relation
between paddle force and
TTI,
29 we found
that 95% of the decline from 5 N to 120 N could be
achieved with 80 N
(
Figure 4). In a subsequent
investigation,
30
19%
of this decrease in TTI proved to be due to expulsion of air
from the
lungs and 81% probably due to better contact at the
paddle/skin interface.
This observation has implications for
the use of pads: since no force is
applied to these, the extra
19% drop in TTI caused by expulsion of air will
not be attained.
One group has shown, in pigs, that application of force to
pads
via non-conductive material does further decrease
TTI.
31
View larger version (11K):
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Figure 4. Relation of paddle force to transthoracic impedance (TTI) [from Ref
29 by courtesy of the
American Journal of Cardiology]
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Our own investigations indicated that 80% of operators were physically
capable of pressing with 80 N but only 14% at 120
N.28 At present
over 50% of operators are not pressing hard enough on the paddles when
defibrillating and 80 N seems a reasonable force to aim
for.25
It would help if training rooms were equipped with paddles instrumented to
measure force. For those who do not know what constitutes 80 N we recommend
pressing down as hard as possible.
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CHEST HAIR
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In 2000 Bissing
et
al.
31 examined
the effect of chest hair on
the TTI when self-adhesive pads were used. Removal
of chest
hair from hirsute patients decreased the TTI by 35%, so they
recommended
chest shaving in such patients before defibrillation. We conducted
a
similar study looking at the effect of chest hair with the gel
pad/paddle
method.
31 Might an
increase in paddle force in itself
be sufficient to overcome the additional
TTI? The answer was
that it only partly did. At 60 N chest hair caused a 10.4%
increase
in TTI whereas at 120 N it caused only a 7.1% increase. We also
found
that the more hirsute the chest, the greater the effect
on TTI produced by
shaving.
From these studies, it seems that the effect of chest hair on TTI will be
much greater with pads than with paddles. There was, however, one confounding
factor in the self-adhesive pad study. The apical gel pad was placed directly
over the cardiac apex and not in the mid-axillary line as specified by the
ILCOR guidelines. In our experience, patients who are hairy over their cardiac
apex are usually hairless in the mid-axillary line. Thus incorrect pad
position would exaggerate the effect of hair on TTI since hair would have been
present under both pads rather than just one.
Whatever method of defibrillation is used, chest hair increases TTI and
decreases the chance of successful shock delivery. In addition, very high
impedance at the paddle/ skin interface puts the patient at risk of skin
burns. The ILCOR guidelines therefore recommend shaving of the hirsute chest
before defibrillation. The counter-argument is that shaving takes time and the
main determinant of shock success is time to first shock. In hirsute patients
who have had a cardiac arrest or are at high risk, we recommend chest shaving
in the area where the defibrillation pads or paddles would be placed; in case
of an arrest, no time would then be lost in shaving.
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CONCLUSIONS
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At present, there is no research showing how much of an impact
each ohm
decrease in TTI will have on patient outcome. We do
know, however, that a high
TTI decreases the chances of successful
shock delivery and we suspect that
many defibrillation failures
are due to insufficient transthoracic current
flow.
15-20
Most
healthcare workers are not achieving optimal TTI during defibrillation.
There
is now good evidence that use of a coupling agent, chest hair
removal,
placement of the apical paddle in a vertical orientation
lateral to the nipple
in the mid-axillary line and application
of at least 80 N of force are all
measures that help minimize
the
TTI.
21,24,29,33
Whether self-adhesive pads or defibrillator
paddles should be used is still an
area of debate. In the hands
of skilled operators the paddle method is
preferable because
force can be applied. This is of particular benefit in
hirsute
men.
30,31
In
non-hirsute persons, the TTI produced by the two methods does
not differ
importantly.
9
Self-adhesive pads are preferable for
use by less experienced operators and
are ideal for the automated
devices used in the community. They also allow the
automated
mode to be used by ward nurses before the cardiac arrest team
arrives.
Box 1 summarizes the main points of this article according to
our
ASAP mnemonic.
| Box 1 The ASAP for defibrillation
Axilla pad/paddle position — lateral to nipple in mid-axillary line
in vertical orientation
Shave — a hirsute chest before defibrillation
Apply correct paddle force — at least 80 N
Punctuality saves lives
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INTRODUCTION
PAD/PADDLE PLACEMENT
