Transcutaneous Pacing (TCP) – The Problem of False Capture

2005 American Heart Association Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care – Part 7.3: Management of Symptomatic Bradycardia and Tachycardia:

“If bradycardia produces signs and symptoms (eg, acute altered mental status, ongoing severe ischemic chest pain, congestive heart failure, hypotension, or other signs of shock) that persist despite adequate airway and breathing, prepare to provide pacing. For symptomatic high degree (second degree or third degree) atrioventricular (AV) block, provide transcutaneous pacing without delay.”

Transcutaneous pacing (TCP) is perhaps the most underutilized and misunderstood Class I intervention in all of ACLS. Why? Simple. Because it’s impossible to simulate during training.

Be honest. In paramedic school, when you went through the bradycardia station, and you were given a scenario with a patient who was experiencing a hemodynamically unstable bradycardia, what happened when you told the instructor that you wanted to immediately attempt transcutaneous pacing?

If you’re like hundreds of other paramedic students all over the country (and for all I know, the world) you were told “the pacer is broken.” That was your cue to say, “Okay, then I’d give 0.5 mg Atropine rapid IV push.” Never mind that the rhythm might have been 3rd degree AV block with wide complexes (for which Atropine is not indicated).

Is it any wonder that so many paramedics (and to be fair, other health care providers) perform this skill poorly or not at all?

Let’s look at a case study.

This was an elderly male that the treating paramedic found supine on the floor with an altered level of consciousness. Radial pulses were present, but slow and irregular. The cardiac monitor was attached and the following ECG was obtained.


I don’t remember any other details about the history or clinical presentation, but it’s irrelevant to the central point of this case study.

The treating paramedic elected to perform immediate transcutaneous pacing (TCP). The combopads were attached and the pacer was turned on.

As you can see in the ECG strip below, the computer began tracking QRS complexes and the pacer was set for 60 PPM.


I would also like to point out that this particular LP12′s pacer had a default setting of “non-demand mode”. This is somewhat unusual, but it turns out to be the key to solving this case.

The treating paramedic increased the current to 40 mA.


60 mA.


90 mA


At this point, the paramedic reported radial pulses that corresponded to the pacer and an improved level of consciousness. The rate was changed from 60 to 70 PPM.


Does the paramedic have capture? Be honest! It looks like it, right?

Unfortunately, no. The paramedic does not have capture.

Then what in the Wide World of Sports are the QRS complexes after the pacer spikes?

The answer is that the monitor is showing “phantom” QRS complexes or “false capture”.

Don’t believe it? Let me prove it to you.

Here is the same rhythm strip. The underlying rhythm appears to be junctional at approximately 40 beats/min.


In the next strip, you can see the underlying rhythm marching through the absolute refractory period of a (presumed to be) paced QRS complex. That’s not scientifically possible!


In the next strip, you can see a (presumed to be) paced QRS complex in the absolute refractory period of a QRS complex from the underlying rhythm. That’s also impossible!


Finally, you will note that the SpO2 monitor is counting the pulse rate at 42 BPM, not 70 BPM.


Whatever these complexes are that follow the pacer spikes, they do not represent ventricular depolarization.

So what are they?

Artifact.

What kind of artifact?

Electrical artifact.

Let’s look at a side-by-side comparison of the “phantom” QRS complexes as the current was dialed up.


As you can see, the QRS complexes look essentially the same (QS complexes with an almost vertical downstroke and slightly curved upstroke back to the isoelectric line, non-distinct ST segment and a virtually absent T wave). The only difference is the size. As the current was dialed up, the complexes got larger.

As you can see in the following graph, there’s an almost linear relationship between the amount of current and the amplitude (or depth) of the “phantom” QRS complexes.


Where does this electrical artifact come from? Why didn’t anyone tell us it would be there?

Good questions!

I discussed this case at length with a Sr. Clinical Specialist from Medtronic Physio-Control. He told me that the LP12 essentially closes its eyes for approximately 40 ms (one small block) after each pacer spike (a pacer spike is nothing more than a graphic representation that an electrical current is about to be sent between the combopads).

To understand why the LP12 “closes its eyes” when it delivers an electrical impulse, you need only ask yourself one simple question. What does an ECG monitor measure?

Electrical activity!

If it didn’t “close its eyes” so to speak, the ECG recording would go right off the paper! So the idea is that the monitor closes its eyes while the current is delivered, and then “opens them” in time to see the QRS complex it creates.

Do you see where this is going?

If the monitor “opens its eyes” too soon, the electrical signal has not yet returned to baseline. The result is a “phantom” QRS complex on the ECG.

It certainly doesn’t help that the ACLS textbook has shown the exact same rhythm strips for transcutaneous pacing for as long as I’ve been a paramedic!

Let’s take a look.


The first strip shows sinus bradycardia. The second strip shows sinus bradycardia and pacer spikes without capture. The third strip shows a beautiful paced rhythm!

Ta-da!

If only it was this simple in the real world!

I know what you’re thinking. Why did the paramedic report pulses that corresponded with the pacer?

Think about it!

The patient has an underlying rhythm, so some pulse waves are going to be felt. In addition, do not underestimate the combination of skeletal muscle twitching and wishful thinking! You are being visually stimulated with every pacer spike, and it’s impressive!

Ever heard of cough-CPR? I am convinced that the contraction of pectoral muslces, intracostal muslces, and other intrathoracic structures produces some type of arterial pulse wave.

But you said the paramedic also reported an improved level of consciousness!

That’s true, but something tells me you’d be more alert, too, if someone started to shock you once a second for a couple of minutes! Your blood pressure might even go up.

Here are some clinical pearls to get you through the procedure.

The most common cause of failure with transcutaneous pacing (TCP) is poor pad placement combined with insufficient milliamperes! Remember, the pacer goes up to 200 mA! If you lose your nerve at between 70-90 mA, there’s a good chance you’re not going to achieve capture. Consider anterior/posterior pad placement to “sandwich” the left ventricle between the pads and reduce transthoracic resistance.

Look for a tall, broad T wave that is the telltale sign of true electrical capture.

Perform, but do not rely solely on a manual pulse check. Consider using an instrument like an SpO2 monitor, doppler, or bedside 2D echo (for inhospital patients) to verify mechanical capture.

Run a continuous rhythm strip that shows the transition from “false” capture to true electrical capture. Be able to document the exact milliamperes that capture is gained, and capture is lost. (Note: one of the “quirks” of the human heart is that once you gain capture it is harder to lose. In other words, you might achieve capture at 120 mA, but then you might have to dial it back down to 80 mA to lose it again). Many protocols state that you should add 10 mA as a “safety margin” once capture is achieved. In my experience this is unnecessary for the reason stated.

Finally, you can consider placing the pacer in “non-demand” mode and examine the absolute refractory periods of the underlying rhythm and the (presumed to be) paced rhythm. If the paced rhythm and the underlying rhythm are marching through each others’ absolute refractory periods, you don’t have true electrical capture.

See also:

Pacing Artifact May Masquerade as Capture (Phsyio-Control website)

Transcutaneous pacing (TCP) with a Lifepak 12

Using capnography to confirm capture with transcutaneous pacing (TCP)

58 year old male CC: Unconscious (Transcutaneous pacing failure in the setting of hyperkalemia)

Transcutaneous pacing (TCP) for asystole

16 Comments

  • SoCal Medic says:

    Tom,One of the things I have started to do is lay the display of hte monitor to show Lead II, the Pulse Ox wafevorm, and the CO2 waveform. I have read that the pulse ox waveform will aide in knowing you obtain mechanical capture, not to mention a brief increase in CO2 because of the increased perfusion. Thoughts?Chris

  • Tom B says:

    I agree 100%! I think that’s an excellent idea, and an innovative use of both pulse oximetry and waveform capnography!Tom

  • ecgblog says:

    Tom, this is a great post. Thanks for sharing. Im not a paramedic (but an ER nurse), but Ill be sure to pass this post on to my paramedical colleagues! Im sure they will find this very helpful!

  • walma says:

    I am impressed, great interpretation , you must love the ecgs…Keep it that wayMaciek

  • jim holt says:

    I am glad to find this blog, I am impressed. I would have never known. Can I use this when I explain pacing in ACLS class.

  • Tom says:

    one major reason that people don't use enough current is that in acls classes the people teaching are inevitably people who haven't paced anyone themselves. so they run a scenario and tell students that there is capture at 60mA, which in my experience is completely on the low side.so now these students go to pace real patients, and they get fat ones, hairy ones, and add the poor pads placement (squarely over the sternum and spine, groan) and poor adhesion, you have false capture at some unrealistically low current setting being interpreted as actual capture.

  • Tom B says:

    @jim holt – Please do use this when teaching ACLS class! Glad you're enjoying the blog. Sorry I missed your comment last July.@Tom – I agree with you! That's assuming that TCP is covered at all! ACLS is video tape driven now, and is completely worthless in my opinion.Tom

  • VinceD says:

    I've gone back to this post about a half dozen times in the past year, because I've seen three cases where the paramedics brought in a patient who had a decreased level of consciousness w. bradycardia, they paced, and then the patient became more alert, apparently due to the increased heart rate. Only problem was, the strips looked exactly like the ones in this article, and if you actually took a few seconds to look at the monitor, you could see the QRS complexes marching through the pacer complexes. At this point, there would be a doctor, two nurses, and the transporting medic and emt in the room, and none of them recognized the false capture or would consider the possibility. This should be required reading for any ACLS class, and I just wanted to thank you (albeit well after the initial posting), for providing these resources that increase our understanding of what should be bread and butter topics for emergency medical providers.

  • Tom B says:

    Thanks, VinceD! I attended a capnography class today and the instructor showed a case where capture was confirmed with a rise in ETCO2! Pretty cool. It was with an LP12 and the paramedics captured @ 140 mA. I’m hoping to get a copy of the strips.

    Tom

  • with proper capture, the improvement is very obvious. however, people are simply not familiar enough with TCP to do it properly, which is a shame, since it’s a very useful substitute until an internal pacer is inserted.

    quick observations:
    1. proper pad placement – sandwich the left ventricle, not over the sternum and spine. (look at any pad package for the right spots.) shave if necessary.

    2. you’re probably going to need a lot of current, usually over 100mA. don’t be shy.

    3. it will hurt. again, don’t be shy. if situation warrants, pace first and deal with pain later. this is not the time to be all sensitive about pt’s pain. it can take up to a few minutes to get IV access and meds in. you can fix pain, but it’s a lot harder to fix pulselessness.

    4. don’t be afraid to touch patients being paced.

    5. take auscultated blood pressures to verify mechanical capture. as long as you know what you’re listening to, i think it’s easier than looking for femorals in a moving rig, which is what is what’s always been taught.

    6. do not let the hospitals (especially some overeager er tech) disconnect the pacing until they’re reading with their own pacer. asystole is not unheard of when the pacer gets turned off.

  • typo:
    6. reading = ready

  • Sweden says:

    Hey.

    Greate blogg!

    What kind of med`s do you use for pain/sedation before/while pacing ?

    /EMT. Sweden.

  • Mel says:

    We have versed on the truck for most of our sedation… So, fo rus we would use versed, what does everyone else use?

  • Bonnie says:

    As a current medic student having completed my first semester that included cardiology I must say I am loving this blog. This is great information for those of us that will soon be going into the field. I will be sharing this blog with my current classmates as we are taking our ACLS class the first of January.

  • medicdad29 says:

    to add to the pulse ox waveform discussion, also note that sometimes the artifact caused by the external pacer can be overwhelming to the pulse ox and cause it not to read correctly

  • Barry says:

    Great article. Ran into same problem not long ago. Set up pacer and had the identical phantom complexes. We didn’t call it capture because the complexes just didn’t look right and no corresponding pulses. We got gun shy at about 90 mA and turned the pacer off. However like you said getting shocked once every second will make the pt more alert. Next time I will turn the juice up if needed for capture.

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Comments
know it all parapup
83 Year Old Male: Shortness of Breath
@ Kyle I would question your authority to call out people for not having a license or being a know it all parapup when your tx basically entails "call medical control." I think we can both agree that his cardiac output is not great at all. I assume your reluctance to give him any other…
2014-10-30 20:26:11
Kyle
83 Year Old Male: Shortness of Breath
Well st elevation in avr and v1 associated with anterior and lateral depression would call for possible posterior wall MI. 15 lead would be in order. Also check all the leads for appropriate placing. If v7, v8, and v9 show the elevation i would treat as a STEMI per my protocol. Asprin only until medical…
2014-10-30 18:14:05
Tim
The most awesome STEMI test on the internet!
Thanks for the app. It made me think about all that one may see in the field. The only problem was I never got a score or saw the results of how I did other than saying I had completed the test. Anyway a great way to get the old brain working.
2014-10-30 13:14:27
Brian
83 Year Old Male: Shortness of Breath
I mostly agree with dustin. I believe this is may be an isolated posterior MI. The R wave in V2 points to it being a posterior MI. otherwise it is a 1st degree av block with a LAHB. I am somewhat concerned with the concordant t segment depression noted and in fact if you were…
2014-10-30 04:22:44
Karl Brennan
Understanding Amiodarone
Great article , however in VF caused by hyperkalemia it should be avoided along with lidocaine , Since it shuts down the K channels, the eiteiology of the arrest hyper K, K channels are needed to exchange K in the cell. Calcium , Bicarbonate, dextrose and insulin should be used to decrease K levels along…
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