Revisiting Transcutaneous Cardiac Pacing

“Transcutaneous cardiac pacing is an effective procedure for patients experiencing unstable bradycardia.”

Or is it?

False Capture

False Capture

False Capture

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”:

ACLS's idea of 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?

Rhythm Generator Without Capture

One spike, no complex. Easy as pie, we need more milliamps. We’ve got this, right?

Rhythm Generator Obtains Capture

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:

Unstable bradycardia 04

Unstable bradycardia 05

Unstable bradycardia 06

Unstable bradycardia 07

Unstable bradycardia 08

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:

Uh Please Standby - Rhythm Defibrillation

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:

  1. Phantom Impulses
  2. Pacemaker dissociation
  3. Pseudo-fusion
  4. True Capture

What do 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.

Phantom Pacing Complexes

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.

False Capture

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.”

Unstable bradycardia 05

Pacemaker Dissociation

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.

TCP Pseudo Fusion

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.

True Capture

True Capture

You should also compare the ECG tracing in multiple leads to the pulse oximetry waveform, or better yet, confirm with increasing end-tidal CO2′s.

If you’ve got a strip of successful or unsuccessful transcutaneous cardiac pacing, we’d love to see it! Send it our way at ems12lead@gmail.com or post it to our Facebook page.

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:

Post-Arrest True Capture

 

9 Comments

  • PO Jansson says:

    Are there any evidence at al that shows any benefit on survival with external pacing. My experimence is that it often works but the patient doesn’t survive. Are there any randomisied controlled studies om different types of patients showing any inproved survival?

    • Christopher Watford says:

      PO Jansson,

      Good question. I think the question “does TCP improve mortality” is not useful in general, as the causes of bradycardia are far too varied.

      When applied to “all comers”, research has shown clinical equipose with other treatments, such as epinephrine or dopamine infusions.

      Do I feel this is because TCP is inferior to these other methods? Not really. Seems more likely that really sick patients are really sick and temporary changes in heart rate make little difference statistically.

  • Jared Thompson says:

    Great post. I’ll defiantly use this in the field and classroom.

  • NICELY done Christopher! Excellent points that are well illustrated – which I know that you and Tom Bouthillet have espoused for a long time.

  • Lance Lynch says:

    Great post, Christopher… I always learn something relevant, practical and applicable when reading them. Muchas Gracias!

  • Ben says:

    Absolutely brilliant post. This is the best summary of TCP capture I’ve seen. It’s not just paramedics that have trouble with this. I’ve seen emergency physicians, intensivists and cardiologists get this wrong.

    Ultrasound is a nice way to confirm mechanical capture if you have it available.

  • Mike says:

    Worth pointing out that a radial or brachial pulse is incredibly hard to palpate reliably when the patient is leaping about the bed. Please use the femoral artery. Paramedic brought me a patient recently whose arm was doing 70bpm, but the pulse only really about 10. I believe he genuinely thought he was feeling a pulse.

  • Nathan Gilbert says:

    I have had similar experiences as the first poster. Many of the pts I pace seem to end up dying. While they are obviously sick with poor perfusion and unstable bradycardia to begin with I can’t shake the feeling sometimes that the pacing is playing some role in their demise. Perhaps I’m not obtaining proper capture so thanks for this article. When ACLS basically said dopamine infusion maybe as good a treatment as pacing in the most recent changes I wish AHA would elaborate so I can make a better decision.

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EMS 12-Lead

Cardiac Rhythm Analysis, 12-Lead ECG Interpretation, Resuscitation

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Comments
Stuart
Anterior T wave inversions and PE.
Studies (can't remember them off the top of my head) have shown that TWI in V1-4 + III is 85% likely to be RV strain, 15% ischemia. If I see anterior TWI, the very next lead I look to is III
2014-11-23 18:00:51
Arlene R
The Trouble with Sinus Tachycardia
It has been very insightful for me as i read this post. Thanks to the may people who commented. Like many nurses, I was also taught to differentiate svt from st by rate and now I stand corrected. I have a Telemetry test coming up soon, I wont have the patient in front of me…
2014-11-20 19:59:33
Nick
100 yof CC: Rib pain and intermittent spasms
Can't be a potassium imbalance. The TW's wouldn't change and then change back. If it was coronary spasm, I would expect some ST segment elevation. The TW'S are also not hyperacute (peaked). Does she wear some sort of electronic stimulator?
2014-11-19 01:05:43
Anterior T wave inversions and PE. | EMS 12 Lead
Not just S1Q3T3: Look at the other 10 leads!
[…] Last week, I described the case of a middle-aged male with a vague history of heart failure who had been having progressive shortness of breath for 4-5 days. On the day he called 911, he had been walking a short distance when he syncoped. EMS obtained an ECG: […]
2014-11-18 18:33:47
Christine
100 yof CC: Rib pain and intermittent spasms
I believe this may be coronary artery vasospasm.
2014-11-18 11:02:45

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