Reader Question
Hi all
Got this readers question. It’s one of those things not taught often!
Are you a cardiac physicology guru. Can you answer in 100 words?
Leave your explanation on the comments please
Casey
Here’s the Q-
: Hi, I am confused by the Units we use when Defibrillating and Trans-cutaneous Pacing.
When we Defibrilate we dial to 200J (Not Amps). (Cardiovert=100J)
When we Pace Transcutaneously we dial using Amps (Not Joules).
Yet both deliver a ‘shock’ from the same device which causes Skeletal Muscle Contraction.
Why dont they both use the same Unit(Joule or Amp)?
What Amp roughly does 200J deliver?
How many Joules roughly in a 80Amp pacing ‘shock’?
And in both cases, what is the voltage and time length of discharge? Fixed or Variable?
To refresh anyone who is more than 1 day out of year 12 Physics: Voltage =Amps(Current ie Vol/time) x Resistance
Watts (Power) =Joules/1second=VoltagexAmps
1 Joule=1Voltx1Ampx1Second
(Out of interest does anyone therefore know the Units a Implantable Cardiac device uses given it delivers directly to the Myocardium of interest?)
Thanks!
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About The Author
Casey Parker
I am a GP working in Broome, NW of Western Australia. I work as a hospital DMO (District Med Officer) doing Emergency, Anaesthestics, some Obstetrics and a lot of miscellaneous primary care. Also on the web as @broomedocs | + Casey Parker | Contact
Well, to be fair during defibrillation you also deliver amps. Just you select Joules to indicate the total energy you’d like imparted on the myocardium. The device will then run a test impulse (in mA) to determine the impedance, and then uses banks of capacitors to deliver the flow (amps) over time through the chest wall. Devices vary a number of parameters of the electrical waveform over time as well.
200 J “selected” for 25 Ohms of resistance, per one device, says it will deliver a peak of 30A over ~12.5 ms. Except that amperage is not constant and will vary over time. Each device manufacturer has their own strategy (rectilinear, truncated exponential, etc).
To defibrillate we need to impart the flow over a critical mass of myocardium. This requires far higher energies than simply depolarizing a small patch of tissue during pacing.
Transcutaneous pacers typically feature a simple rectangular pulse (fixed output over 10-40 ms) which does not vary with time. Amperage is what you set, because it is the only variable.
Defibrillation’s goal is to impart energy into a critical mass of myocardium to polarize enough of the tissue to interrupt the macro and microreentry circuits during a dysrhythmia (cardioversion merely changes the timing). Pacing’s goal is simply to impart enough electrical stimulation to a small patch to start the chain reaction of depolarization.
I believe internal devices use the same units as their external counterparts, but I could be mistaken.
Does this completely answer the question? Not sure!
Thanks Chris
Very interesting question indeed Casey.
ON
>>>>How many Joules roughly in a 80Amp pacing ‘shock’?<<<<<<
"The average current necessary for external pacing ranges from about 50 to 100 milliamperes (mA); 100 mAs applied to an average chest with 50-ohm (Ω) resistance for 20 msec delivers 0.1 Joules (J). "
Source:
http://emedicine.medscape.com/article/780639-overview#aw2aab6b3
When they started experimenting with pacing the hearts, Zoll found out that tweaking the duration of delivery of the shock would change the current required to produce an effect
ON THE ISSUE OF
>>>>time length of discharge <<<< (and I am mentioning this in reference to external pacing only):
"""An important feature of modern transcutaneous external pacemakers is the use of a long electrical pulse duration. Early devices used short (1-2 msec) impulses, more closely resembling the action potential duration of skeletal muscle rather than cardiac muscle (20-40 msec). This preferentially stimulated skeletal muscle with resultant patient discomfort.
Zoll found that increasing the duration from 1 msec to 4 msec caused a 3-fold reduction in threshold (the current required for stimulation). Increasing the pulse duration from 4 msec to 40 msec further halves the threshold, but longer durations produce no additional advantage. Current transcutaneous external pacemakers deliver 40-msec pulses (Zoll) or 20-msec pulses (all others). """
Source:
http://emedicine.medscape.com/article/780639-overview#aw2aab6b3
On:
>>>>Why dont they both use the same Unit(Joule or Amp)?<>>>”””Types of defibrillators
•
Most defibrillators are energy-based, meaning that the device charges a capacitor to a se‐
lected voltage and then delivers a prespecified amount of energy in joules. The amount of
energy which arrives at the myocardium is dependent on the selected voltage and the
transthoracic impedance (which varies by patient).<<<<<"""
Source for PDF:
http://www.heartrhythmcharity.org.uk/www/media/files/InTech-Principles_of_external_defibrillators.pdf
ICD units are joules.
energies delivered depend on whethere the ICD is “cardioverting” or is “defibrillating”:
>>>Although ICDs can be programmed to deliver synchronized shocks at a range of energies up to the maximum output of the device (usually 30 to 35 joules), synchronized cardioversion can often terminate VT with relatively low energy (eg, 10 joules or less). “Low energy cardioversion” is defined as a synchronized shock of less than two joules.<<<<>>>Defibrillation can be delivered across a range of energies. Initial shocks are often programmed for lower energies in order to reduce capacitor charge time and expedite therapy (although all shocks should be at least 10 joules above the defibrillation threshold). Subsequent shocks are usually delivered at higher energies, often at the maximum output of the ICD (eg, 30 to 35 joules), in order to optimize efficacy.<<<<<<
Source: Uptodate online.
In the end, its the “current” (milli Amperes) that “defibrillates” and not the energy (joules).
PS:
To Casey and other readers:
————————————–
Sorry to post answer in multiple comments but the intent was to break down the answer(s) into separate parts for ease of understanding. Not sure if that was a success or a failure. Or does it depend on the reader?
Please feel free to make suggestions.
Thanks.
Thanks Umer and Chris for this very useful explanation. I have been contemplating this questions for a while now and never really understood until now. Regards, Zaf