Another Defib. question

[mikie]

So I never noticed this before...in the rectangular box where it says 'Impedance' - is that the actual amount of joules delivered the patient?

(This reminded me of my recent question about the 200-300-360 on biphasic monitors; this was biphasic defibrillation at 200J; second was at 300J but I don't have the rest of the strip).

BTW- ROSC was achieved with ~40 minutes of downtime. Don't know neurological outcome.

Thanks!

Edit: downtime = CPR in progress

 

[MSDeltaFlt]

Impedance = resistance. Since you dialed in 200J and it said 116, I'm guessing that was the amount delivered due to "impedance".

 

[Christopher]

Impedance = resistance. Since you dialed in 200J and it said 116, I'm guessing that was the amount delivered due to "impedance".

Close, when talking about DC circuits impedance is the same as resistance (in AC circuits it includes the phase as well as the magnitude/resistance). The number given is the amount of impedance in Ohms most likely (it may be scaled, but I don't believe so).

Modern biphasic defibrillators send a test pulse through the body at a really low amperage to measure the impedance between the two electrode pads. The defibrillator then adjusts the amount of current delivered, using banks of internal resistors, in order to "normalize" the waveform to the specific situation encountered.

Older monophasic defibrillators did not measure impedance and delivered a fixed energy level through the body. This resulted in varying current levels measured at the heart (in the 20-30A range).

Interestingly enough, impedance has not been found to correlate to body mass or age but largely depends on electrode size and placement. The bigger the electrode the smaller the impedance.

 

[mikie]

Close, when talking about DC circuits impedance is the same as resistance (in AC circuits it includes the phase as well as the magnitude/resistance). The number given is the amount of impedance in Ohms most likely (it may be scaled, but I don't believe so).

Modern biphasic defibrillators send a test pulse through the body at a really low amperage to measure the impedance between the two electrode pads. The defibrillator then adjusts the amount of current delivered, using banks of internal resistors, in order to "normalize" the waveform to the specific situation encountered.

Older monophasic defibrillators did not measure impedance and delivered a fixed energy level through the body. This resulted in varying current levels measured at the heart (in the 20-30A range).

Interestingly enough, impedance has not been found to correlate to body mass or age but largely depends on electrode size and placement. The bigger the electrode the smaller the impedance.

I think I understand the concept but what is the 'value'/importance of the impedance being displayed? Is there any clinical 'correlation'? I assume you would want the lowest amount of impedance possible? Is there a normal?

On an unrelated question- how is the HR calculated if it's VF? (See HR of 151?)

 

[Christopher]

I think I understand the concept but what is the 'value'/importance of the impedance being displayed? Is there any clinical 'correlation'? I assume you would want the lowest amount of impedance possible? Is there a normal?

Normal? The range I've always seen referenced is 50 to 150 ohms, however, every person will be different. The amount of air you have in your chest cavity can affect the impedance. The biggest predictor of impedance is the surface area of the electrode pads and their distance between them.

You would prefer a lower impedance as that means less energy will be delivered through the skin. This is why adult pads on a pediatric patient are actually a good thing if they fit!

On an unrelated question- how is the HR calculated if it's VF? (See HR of 151?)

The same way in any other rhythm: mostly a crap shoot. *rimshot*

In all honesty the way heart rate algorithms work will differ device to device, but a naive implementation would look for rapid positive or negative deflections of a certain amplitude and duration (based on derived norms) and count them.

Last night I'd brought in a patient whos axis made lead II's T-waves at least as large as the R-waves. My LP12 got confused so I switched to Lead I, which had a definitive QRS complex and T-wave relationship, and got an HR that matched the SpO2. Upon arrival at the hospital the nurse asked me if my patient had been tachycardic, which got my attention a he was running in the 60's for me. Their Philips monitor was also using Lead II and was double counting the QRS and T-waves! Switching their monitor to Lead I fixed the problem.

 

[Physio Control]

So I never noticed this before...in the rectangular box where it says 'Impedance' - is that the actual amount of joules delivered the patient?

(This reminded me of my recent question about the 200-300-360 on biphasic monitors; this was biphasic defibrillation at 200J; second was at 300J but I don't have the rest of the strip).

BTW- ROSC was achieved with ~40 minutes of downtime. Don't know neurological outcome.

Thanks!

Edit: downtime = CPR in progress

According to one of Physio-Control's principal scientists, yes, the impedance shown on the strip chart is the thoracic impedance in Ohms. Most defibrillators measure the resistive part of the impedance by sending a high-frequency (e.g., 20 kHz) signal at very low (safe) current between the pads. The resulting voltage is proportional to the impedance between the pads (Ohm's law). The impedance can be affected not only by electrode size, but also by fluid content in the thorax. For example, decompensation due to heart failure causes buildup of fluid in the lungs, substantially reducing thoracic impedance.

The impedance measurement is commonly used in two ways. First, a shock will not be delivered if the measured impedance exceeds an upper limit because that suggests that the pads are not in good contact with the patient. Second, most biphasic defibrillators optimize the shock parameters based on the measured impedance. For example, the shock voltage can be increased for low-impedance patients to keep the current above some minimum level.

 

[Christopher]

Thanks for the verification!