Manual Defibrillators,
Automatic External Defibrillators,
Cardioversion,
and External Pacing
© D. J. McMahon 2014
rev 141001
Very early defibrillators –
Use of electricity to stop fibrillation first discovered by
Prevost & Batelli in 1899. Used AC, not DC.
joule:
Unit of energy or work:
1 Newton through a distance of 1 meter
or
James P Joule 1 amp through 1 ohm
Mathematically, the joule can be defined both ways,
but the convention is to use joules for energy, and
newton-meters for torque.
One joule = one watt-second
= ~ 0.24 calorie
= ~ 0.74 foot-pound
Essential monophasic defibrillator
The Lown waveform:
Monophasic, damped
Basic Lown waveform creation
Energy in the capacitor :
U = ½ CVp2
Specific Lown waveform creation
Biphasic defibrillation:
> 1990’s improvement in defib circuitry.
> Achieves defibrillation in fewer attempts,
using lower energies in each attempt.
> Less time delay in salvaging a cardiac
crisis, and less damage to the heart.
Current & Time at
Various Patient Impedances
Test loads are 50Ω
Insulated Gate Bipolar Transistor (IGBT):
-
-
Combines an isolated gate FET for the input, and a bipolar power
transistor as a switch, in a single device.
Has the gate-drive characteristics of a MOSFET and the high-current
and low-saturation-voltage capability of a bipolar transistor.
Used in medium- to high-power applications.
Some manufacturers of biphasic defibs
provide a maximum energy of 200 joules,
while many provide a maximum of 360 joules.
Defib voltages are very brief, but
very high and potentially lethal !
Pads have replaced paddles in most applications of
defibrillators. Pads assure a better contact with
the patient’s skin, so resistive losses are minimized.
They are also safer for the user.
Paddles, internal as well as external, are still used in
specific clinical areas (O.R., PACU, etc.)
Correct placement of the pads
or paddles is important…
Internal paddles for use in open-heart surgery:
Placed directly on the heart to re-start the beat.
Defibrillator limits energy to them at 50 joules.
2010 AHA Guidelines for defibrillating pediatrics:
“With a manual defibrillator …, use a dose of 2 J/kg
for the first attempt, and 4 J/kg for subsequent
attempts.“
ECG Input Protection
from Defibrillator Pulses:
HP “Codemaster”: monophasic (~ 1990)
Things to watch for -
Junction of cable pulled away from a defibrillator connector
Conductive gel left on paddles from a previous use.
Automatic External Defibrillators
(AED’s)
> Semi-automated defibs that analyze cardiac status and
deliver therapeutic shock as indicated, using a pre-set
algorithm and voice commands.
> Intended for non-clinical settings, for use by non-clinicians.
> Typical biomed maintenance consists of a functional
check and battery check.
Typical block diagram for an AED
Defibs for Cardioversion –
> The use of a small energy pulse from a defibrillator to
convert an non-lethal arrhythmia (eg atrial flutter) to a
normal sinus rhythm
> Delivers a pulse about 30ms after the peak of the
R-wave, synchronized by the defibrillator
> Modern units can discriminate the R-wave from the
T-wave or noise spikes
Timing of cardioversion pulse
Do NOT cardiovert across the T-wave !
Defibs as External Pacemakers –
Defibrillators can be used as temporary external
pacemakers for patients with bradycardia or other
issues.
Pacing is either ‘synchronous’ or ‘asynchronous’:
In Synchronous pacing (“Demand” mode): pacer fires
only when no complex is sensed for a predetermined
amount of time.
In Asynchronous pacing: pacer fires at a fixed rate at
preset intervals, independent of cardiac activity.
Typical external pacemakers deliver a 40-millisecond
impulse for each beat.
The average current necessary for external pacing is
from 45-100 mA, depending on the cardiac status.
In pacing, the same defibrillator electrodes are used.
Automatic Internal
Cardiac Defibrillators
(AICD’s)
Small implanted defibrillators that sense the
ECG, and provide an appropriate pulse to the
myocardium if and when they sense a
dangerous ventricular arrhythmia such as
V-fibrillation, V-tachycardia, SVT, etc.
> These are not the same as a pacemaker
> Not a biomed concern
Zoll’s ‘LifeVest’ wearable defibrillator with ECG monitoring
Major Makers of full-function
Defibrillators and AEDs:
October, 2014
Test equipment: Defibrillator Analyzers
BC Biomedical
Fluke Biomedical
http://testequipmentandtools.com/acatalog/BC_OnLine_Store_Defibrillator_Analyzers_5.html
http://assets.fluke.com/demos/biomedical/impulse/3035532_Impulse_Demo.html
Beware of the Capacitor !