There can be a whole range of reasons for an external pacemaker to fail in this way.I would first see if the electrode has not dislodged. Temporary electrodes have no fixation to the myocardium that would prevent dislodgement. It could even be that the electrode has perforated the myocardium and is now located on the epicardial side (can easily be seen by the reversed repolarisation on the electrogram.

If the electrode is repositioned and it still fails in the same way, it could be polarisation around the tip of the elctrode. There are some extreme cases of polarisation effects. Although, you would hardly expect these to occur with modern Pt-Ir electrodes. If it does happen, you could exchange the electrode with one that has different metal electrodes.

There is a lot to find about this topic (Under: "Polarization" AND "exit block". Among others, there are these:

Dictionary of Cardiac Pacing, Defibrillation, Resynchronization, and ...

David L. Hayes, MD, ‎Samuel J. Asirvatham, MD - 2007 - ‎Medical

evoked response sensitivity — exit block evoked response sensitivity. ... tials as distinct from the polarization poten- tials, autocapture cannot be turned on.

Cardiac Pacing and ICDs

Kenneth A. Ellenbogen, ‎Karoly Kaszala - 2014 - ‎Medical

impedance and electrode polarization impairing myocardial EGM sensing. ... Corticosteriods have dramatically reduced the risk of exit block for pacing leads, ...

Pacing Concepts

www.drheart.net/korean/pps/PacemakerImplant.ppt

... Largely as a result of polarization resistance, lead impedance varies directly ... Presence of tricuspid valve prosthesis; Repeated development of exit block of ...

Thank you very much hans!!

Sorry for the lack of information. Im an HEMS, and the electrodes we used are transcutaneus (pacing pads). I will modify the question!

http://www.physio-control.com/product-detail.aspx?id=546

http://www.skintact.com/286.0.html

Not many articles out there, one is Bull Acad Natl Med. 1990 Dec;174(9):1361-70; discussion 1370-2. (French) ,

Also local burns have been described, potentially worsening contact. Of course contac of the patches gets worse with time, though 17min points rather towards something changing in the physiology of the patient (electrolytes, sedation, heart function) What has been the reason for the AV Block III?

Thanks for your answer Dr. Pflaumer.

We do not know.

In summary The patient was in a swimming pool (we do not know how long it would take in the pool) when the pool lifeguard realized I was floating unconscious. Outside the pool, they checked the pulse and the frequency was ~ 30 lpm. After a while, the first EMS arrived, they Realized AVB III, and applied the noninvasive pacemaker requiring  200mA to achieve mechanical capture (rising ETCO2, and pulse oximetry). We arrived 15 to 20 minutes after, keeping the treatment (there was no ventricular escape rhythm), and after 17 minutes, the patient worsened and we reevaluated inflight realizing there was no mechanical capture, but electrical capture.

The first 12 lead ECG was not conclusive (for coronary patology).

It has not authorized an autopsy

I found an article, "Failure of transcutaneous pacing in a head-injured patient", where the authors stated that pulmonary oedema maybe reduced the transtoracic impedance and increased conductivity of lung tissues short circuiting the torax...

The article you mention is quite interesting. Frankly, I don't see why a reduction in the thoracic impedance would reduce the chance for cardiac pacing. What would be of influence is the current density in the myocardial tissue. If the current per mm2 reaches a certain level, the muscle depolarises. if the thorax becomes more conductive, the current density in the heart muscle would even be higher, so chances of stimulation would improve.On top of that, you didn't mention thoracic oedema in your patient.

In your patient the electrodes were applied to the skin of the patient when he had been floating in a swimming pool (with chemicals such as chlorine) maybe this has reacted with the chemicals in the electrodes and eventually made the electrodes less effective. In your initial question you mention that the device was set to 200mA and was ineffective. It may have been that the device was set to 200 mA, but was there really 200 mA going through the patient? Probably not, the pacemaker was applying its maximum output voltage to reach the current of 200 mA, but if the electrodes were not conductive enough (for whatever reason) or the skin-resistance was too high (because of being too long in the water) it will not have reached 200mA.

So, what may have happened: when you apply the patches everything is OK and there is enough current to depolarize the heart. Then when the electrodes are starting to be influenced by the chemicals from the pool, they become less and less conductive and in the end there is no more pacing.

After all, unfortunately, you can't reproduce it anymore,. One question remains: when after the 17 minutes when the pacing failed, did you consider replacing the electrodes by new ones?

Hans, thanks for your response, and sorry for the delay!!

We changed the pads by new ones after 5 minutes, unffortunateely too late!

The new pads didn't capture as well.

I think that we made 2 mistakes. First one: not realize the failure of mechanical capture sooner. Second one: not placing the pacemaker pads anteroposterior.

My hipothesis is that the wet skin with the chemicals you said, may short-circuit the external pacemaker pads, the electric impulse goes using the lower impedance circuit, so the amount of energy that arrives the miocardio is not enough.

We discard the failure of the pacemaker itself or the plug, or the wires or the pads. We are going to have a meeting with the safety squad of our institution to make a debriefing.