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
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
(Out of interest does anyone therefore know the Units a Implantable Cardiac device uses given it delivers directly to the Myocardium of interest?)
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!