Revisiting Transcutaneous Cardiac Pacing
Updated: Aug 14, 2021
Transcutaneous cardiac pacing is an effective procedure for patients experiencing unstable bradycardia.
Or is it?
If you’ve read Tom’s introduction to the subject of false capture you’re already ahead of the game.
We’ve shown case after case of unstable bradycardia patients receiving ineffective transcutaneous pacing due to a lack of capture. In each case, phantom pacing impulses are interpreted by the paramedic as electrical capture. Typically, this is confirmed by an improvement in other vital signs, such as mental status or blood pressure.
If I was being shocked 70 to 80 times a minute, you would likely notice an increase in my blood pressure too!
So why do we fail to recognize true electrical capture?
Are paramedics just that bad at ECG interpretation? Absolutely not.
This is a failure on the educational side.
We’ve conditioned paramedics to fail by showing them unrealistic ECG strips time and time again. We tell them to start at unrealistically low outputs and ask that they gingerly increase the output. We scare them that high outputs are painful. Check out the ACLS standard for electrical capture:
That looks pretty simple to me! 60 mA? Awesome. Another SAVE!
How about what a rhythm generator shows to a real live cardiac monitor during training?
One spike, no complex. Easy as pie, we need more milliamps. We’ve got this, right?
That is really, really easy to see.
Except that’s not what the progression of false capture to true capture looks like at all!
Check out this great progression, used by permission, from the amazing Float Nurse Mike blog:
Can you find all of the conducted and non-conducted complexes?
What about fusion of false capture and underlying beats?
This looks a lot harder than the ACLS strips or the rhythm generator! Why don’t these strips feature the same phantom complexes?
The phantom complexes are due to the interaction of the electrical stimulation applied during pacing and the recording electrodes. The energy being delivered polarizes the electrodes as they receive the massive stimulus in comparison to the electrical impulses of the heart.
It takes time for that polarization to bleed away. A great example of this is the exaggerated deflections after you defibrillate somebody, such as in the conclusion to our last case:
It jumps off the screen before returning to the baseline, in fact, the cardiac monitor stopped recording for a full second, and yet it still sees the effects of polarization.
In our pacing example, we’re polarizing the recording electrodes 70 to 80 times a minute, which they must recover from. Because TCP uses much less energy than defibrillation (less than 1 J), the recovery time of the electrodes is much faster. However, it isn’t instantaneous!
Rhythm generators and textbook examples do not feature these phantom impulses because the recording electrodes do not see the electrical pacing stimulus and thus do not become polarized!
In order to succeed at TCP, we must expect all of the electrocardiographic findings of transcutaneous pacing:
What does each of these look like?
Phantom Impulses are easy to spot once you know what they look like. Starting with the pacemaker impulse, you’ll see a sharp deflection that rapidly returns to the baseline without a true T-wave. There may be a pseudo-T-wave, but it won’t be real-looking. Remember, when we’re pacing a patient we’re activating the ventricles without using the normal conduction system. The complex will be broad, slurred, and bizarre. It will not be sharp and pointy. Sharp and pointy means speedy conduction, which cell-to-cell ventricular depolarization is not.
The Physio-Control clinical note on transcutaneous pacing artifact has a great example of phantom complexes (pdf), and they are the only manufacturer which acknowledges this problem in their literature! Kudos to Physio-Control.
Pacemaker Dissociation is when the pacemaker impulses are dissociated from the underlying rhythm. With phantom impulses, this will look like two competing rhythms. You’ll notice physiologically impossible R-R intervals as well, i.e. beats during the absolutely refractory period.
Pseudo-fusion of the pacemaker and underlying rhythm may be present, lending to the possibility that the impulse is real. However, do not be fooled by these imposters! They may look appropriately wide, with discordant T-waves, but they are not truly paced impulses.
True capture looks like any other paced rhythm. Broad, slurred, QRS complex with a discordant T-wave. There should be no evidence of pacemaker dissociation, pseudo-fusion, or physiologically impossible complexes! Better yet, you should see a realistic capture threshold. If you’re using anterolateral pad positioning, which is common in EMS patients, be very suspicious of capture less than 90-100 mA.
You should also compare the ECG tracing in multiple leads to the pulse oximetry waveform, or better yet, confirm with increasing end-tidal CO2.
May the 4th Be With You Update: one of the crews who read this encountered an asystolic arrest, obtained ROSC, and paced the patient during transport. Great example of true capture here: