Transcript Transthoracic impedance

Human impedance variability
and defibrillator test protocol
Why 50 Ω loads are not enough to test modern defibrillators
Defibrillators: life saving
• Convert certain heart arrhythmias to normal
• Convert v-fib back to normal
• Have to work first time, every time.
– Batteries may not be fully charged or faulty
– Charging circuits may be faulty, may not deliver
enough energy
– Must deliver proper energy to account for differing
patient sizes/impedances
Current vs. energy
• Current—not energy—defibrillates.
• Successful defibrillation requires enough
current be delivered to the heart muscle
during the shock.
– Must transit through the chest/thorax and the
impedance it represents.
– Body mass, skin resistance, tissue type and
amount all play a part in the chest/thorax
impedance presented to the charge delivered by
the defibrillator.
Transthoracic impedance
• Transthoracic impedance =
the body’s resistance to
current flow
• Human impedance variability
has been shown to vary from
25 ohms to 180 ohms1
• Energy in respect to
impedance is the determining
factor to successful
defibrillation—not energy
alone.
Transthoracic impedance (cont.)
• Just a few known causes
of differing impedances in
human beings include:
– Body mass
– Age
– Disease
– Skin resistance
– Tissue type and amount
Transthoracic impedance (cont.)
• Successful defibrillation requires
sufficient current to the heart
muscle.
• Defibrillation current is affected by
transthoracic impedance
– Modern (biphasic) defibrillators
measure impedance and adjust
energy delivery accordingly
“Humans exhibit a
wide range of
transthoracic
impedance, and
defibrillators
compensate for this
range of impedance
in different ways.”
-- McDaniel,
Garret, Burke and
Arzbaecher2
Schematic and formula
Defibrillator
EH = I2 x RH x Time
(Watt Seconds or Joules)
How defibrillators account for
human impedance variability
• Monophasic (older)
defibrillators
– Current flows only one direction
 impedance not measured
How defibrillators account for human
impedance variability (cont.)
• Biphasic (modern) defibrillators
– Current flows first in one direction, then
reverses and flows in opposite
direction.
– Impedance measured and energy
delivery is adjusted internally based on
energy setting or (for AEDs)
arryhthmia.
Biphasic waveforms
Q: Are all biphasic
waveforms alike?
A: No. There are different
biphasic waveforms
based on different
manufacturer’s
specifications.
Rectilinear Biphasic
(ZOLL)
Smart Biphasic
(Philips)
Pulsed Biphasic
(Schiller)
Clinical impedance examples
• Low impedance (50 Ω)
– A 360-joule biphasic defibrillator delivers more current than
required, exposing patient to potentially harmful high-peak
currents
• Average impedance (75 Ω)
– A 360-joule biphasic defibrillator and a 200-joule rectilinear
defibrillator may be equally effective
• High impedance (> 100 Ω)
– A 200-joule rectilinear shock delivers a higher average
current than a 360-joule biphasic defibrillator shock,
therefore making it more effective at lower energy levels.
Impedance and defibrillation
outcomes
• Too little or too much is no good
– Too much peak current during shock can injure the heart
– It’s the peak current (not energy) that can injure the heart
 It is imperative the current delivered to the heart is
matched to the individual impedance of each patient
and not just the delivered energy.
Load testing: accounting for
impedance variability
Q: Are 50 Ω test loads enough to ensure output
conditions of modern-day defibrillators?
– Do all of your hospital’s patients have the same
input impedance? No!
A: Testing beyond the 50 Ω load is necessary to
ensure defibrillator inventory performance of
modern defibrillators
Load testing: accounting for
impedance variability (cont.)
Q: What loads are recommended?
A: Section 6.8.3 of the IEC 60601-24 standard3 and AAMI DF80
standards4 require defibrillators to
be tested on different resistance
loads of 25, 50, 75, 100, 125, 150
and 175 ohms to ensure proper
current is delivered to patients
with different impedances.
When to test with other loads
• Each hospital’s equipment history
should be considered.
• Standard protocol recommends
multiple impedance tested on
incoming inspection and after a
repair to charging circuit or applied
parts.
• May also be helpful when
troubleshooting “failed to deliver
energy” complaints.
The 7010 solution
•
•
Designed to facilitate load testing for
modern defibrillator technology
quality assurance and standards
compliance
Selectable loads
–
–
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•
25 Ω
50 Ω
75 Ω
100 Ω
125 Ω
150 Ω
175 Ω
200 Ω
The only device available today
capable of testing beyond 175 ohms
for extreme impedance conditions.
Impulse 7010 Selectable Load Accessory
References
1.
American Heart Association. Guidelines 2000 for Cardiopulmonary
Resuscitation and Emergency Care. Circulation Supplement. 2000; 102:8
2.
McDaniel W.C., Garrett, M, Burke M.C., Arzbaecher R, Comparison of the
Efficacy of Two Biphasic Waveform Defibrillators in a Model of Simulated
Higher Impedance Patients, Engineering in Medicine and Biology Soceity,
2003. Proceedings of the 25th Annual International Conference of the IEEE.
Volume I, issue 17-21, Sept. 2003 Page(s) 183-185 Vol. I
3.
IEC 60601-2-4 Medical Electrical Equipment Part 2-4: Particular
Requirements for the Safety of Cardiac Defibrillators, Section 6.8.3
4.
ANSI/AAMI DF80:2003 Medical electrical equipment—Part 2-4: Particular
Requirements for the Safety of Cardiac Defibrillators (Including Automated
External Defibrillators)
References (cont.)
Want to know more? Check these out:
• Mittal S, Ayati S, Stein KM, Knight BP, Morady F, Schwartzman D, et al.
Comparison of a novel rectilinear biphasic waveform with a damped sine wave
monophasic waveform for transthoracic ventricular defibrillation. ZOLL
Investigators. J Am Coll Cardiol 1999; 34: 1595-601.
• Schneider T, Martens PR, Paschen H, Kuisma M, Wolcke B, Gliner BE, et al.
Multicenter, randomized, controlled trial of 150-J biphasic shocks compared with
200- to 360-J monophasic shocks in the resuscitation of out-of-hospital cardiac
arrest victims. Optimized Response to Cardiac Arrest (ORCA) Investigators.
Circulation 2000; 102: 1780-7.
• Mittal S, Ayati S, Stein KM, Schwartzman D, Cavlovich D, Tchou PJ, et al.
Transthoracic cardioversion of atrial fibrillation: comparison of rectilinear biphasic
versus damped sine wave monophasic shocks. Circulation 2000; 101: 1282-7.
• Walker RG, Melnick SB, Chapman FW, Walcott GP, Schmitt PW, Ideker RE.
Comparison of six clinically used external defibrillators in swine. Resuscitation
2003; 57: 73-83.
• Niemann JT, Walker RG, Rosborough JP. Ischemically Induced Ventricular
Fibrillation (VF): A Comparison of Fixed and Escalating Energy Defibrillation.
Acad Emerg Med 2003; 10: 454.
Questions?
For more information on the Impulse 7010 solution for variable
impedance testing, contact Fluke Biomedical today.
Fluke Biomedical
Toll free: (800) 850-4608
Direct: (440) 248-9300
Fax:
(440) 349-2307
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