This is the second half of a two-part case presentation examining transcutaneous pacing. If you didn’t see yesterday’s post I highly suggest checking out Part 1 before continuing, but if you hate learning I suppose you can start here.
Yesterday we examined a series of tracings that depicted transcutaneous pacing (TCP) in all its stages: initiation, false-capture, intermittent capture, successful capture, and finally, spontaneous resolution of the bradycardia that necessitated pacing in the first place. It was a whirlwind!
As we mentioned in that post, however, there was a catch: We only discussed success in terms of electrical capture. What about mechanical capture? What good is it to achieve good electrical capture if we can’t confirm that the patient’s cardiac output has actually increased?
That is today’s topic.
Before we actually get into the topic at hand I need to touch on something that has come up several times recently. Maybe it’s just a coincidence, but of late I’ve seen several crews that I work alongside bring in patients with anterior-posterior pad placement—my preferred setup for either pacing or defibrillation—however some wires must have gotten crossed in training because the pads are set up like the patient is 7 years old.
As the image above demonstrates, sternal-spinal pad placement is only recommended for pediatrics. Proper anterior-posterior placement on adults (for pacing and defibrillation at least) requires that the pads be placed to the left of the spine and sternum.
Recall that the goal with A-P positioning is to sandwich the heart between the pads so:
- There should be as little thoracic impedance between the pads as possible.
- The current delivered is angled directly through the bulk of the ventricular myocardium.
By optimizing those two factors we hope to obtain ventricular capture at as low a current as possible—increasing the chance of success, giving us more room to escalate the “dose” as needed, and (I list this third because it is least important in the emergency setting) decreasing the amount of energy being delivered to the heart and chest.
Since surface anatomy can vary wildly from patient-to-patient I won’t set any hard rules about where the pads go—just try to sandwich as much as the ventricles as efficiently as possible—but keep the diagrams below in mind.
Okay, so we’re back on topic and discussing how to confirm mechanical capture. First up is manual pulse palpation.
This is the method all the textbooks and ACLS classes teach. First you confirm electrical capture (something that is already poorly taught and fraught with pitfalls), then you palpate a radial, femoral, or carotid artery to feel for an associated pulse. It’s so simple.
Except that it’s not.
I’ve stopped counting the number of times that I’ve seen providers of all levels and experience attest that they felt a good pulse with pacing only to see the tell-tale signs of false capture when I see the corresponding rhythm strips.
You cannot have mechanical capture without electrical capture. Now there’s a truly simple concept.
In real-time I can demonstrate, objectively, why a patient does not actually have capture, but time-and-again experienced providers will still argue that they feel a good pulse and shut out any suggestions to the contrary. I don’t doubt that they feel something, it’s just that it cannot be an effective arterial pulse.
The problem is that most of their clinical senses are telling them there should be capture. They see a monitor showing large blips that look like QRS complexes. Their fingers feel subtle movement corresponding to each discharge of the pacer. Their assessment observes a patient who is more awake and responsive, maybe with a non-invasive blood pressure that is trending upward. Sometimes they even hear the monitor beeping in-time with the pacer spikes.
They have every reason to think that the transcutaneous pacer is capturing… and they really want it to. Heck, I want there to be capture too; I’ve just been fortunate to have had some good teachers who taught me about the pitfalls associated with TCP early in my career.
I once encountered a case where an entire team of experienced, expert emergency clinicians at a regional trauma and cardiac center were absolutely convinced a patient captured at only 10 mA with TCP. I’m going to venture a guess that no adult (or even child) on Earth has ever captured with only 10 mA of current during transcutaneous pacing—yet these otherwise excellent clinicians wanted to believe in their success so strongly that they totally missed the objective signs that they were dealing with false capture. I’m sure multiple people in the resus bay felt for a pulse, yet no-one involved in the case realized that there were other, more trustworthy signs that the pacer was not capturing.
I’m not saying that pulse palpation is useless in the emergency setting—just that it’s an inferior tool and should be discarded in favor of much more objective and reliable measures when they are available. And if you are forced to rely on a manual pulse to confirm mechanical capture, approach the situation with a mindset of skepticism; looking for any indications that what you think you are feeling may not be a true pulse.
That said, there should be very few situations where you need to rely on pulse palpation because most emergency care providers already have the real device they need to confirm capture readily available.
In the preshospital setting this is my preferred method of confirming mechanical capture. Why trust your feeble human fingers to palpate a pulse when a machine can do the job better with less risk of error and no bias.
You would think that the pulse oximeter would be just as easy to fool as your fingers—they are notoriously prone to even subtle motion artifact—but you would be wrong… [most of the time]. When properly placed our often-finicky SpO2 probes actually perform quite well during TCP. Despite the apparent torso motion accompanying each pacer discharge, it’s surprisingly easy to get a clean signal unaffected by movement with the pulse-ox on a finger or toe. The latter setup is probably preferred because the feet are further from the twitching that accompanies each shock, but I’ve seen finger probes work just as well a number of times.
In fact, in every case of false-capture that I’ve encountered where the team caring for the patient obtained an SpO2 waveform, the patient’s underlying heart rate was been clearly visible on the pleth while the ECG monitor (and providers) were fooled by the pacing artifact.
Here’s an example from yesterday’s case: This is what we saw on the LP12’s rhythm monitor at the start of pacing (10 mA and 80 bpm).
At the same time the patient was also attached to a GE DASH bedside monitor with a pulse oximetry probe on his toe. Here’s what we saw on there:
As you can see, though the ECG waveform is obscured by pacer artifact and could be mistaken for capture at a rate of 80 bpm, the pulse-ox waveform is totally unphased and marches right along at about 20 bpm—the patient’s intrinsic heart rate.
What happens when we observe capture?
Recall that the patient in this case achieved only partial-capture at first. In yesterday’s post on electrical capture that allowed us to directly compare the true-capture and false-capture complexes.
We can do the same with the pulse-ox waveform!
On the strip above you can clearly see the rapid deflections corresponding to the pacer spikes that have successfully captured (note also the change in QRS morphology), while there are periods of no activity on the pleth corresponding to strings of non-conducted pacer spikes.
As the current was increased further the pulse-ox waveform became more regular.
There are still occasional dropped beats—visible on both the ECG and SpO2 waveforms—but it is clear that we are now seeing excellent electrical and mechanical capture.
If there was any doubt, check out how the pulse-ox doesn’t even falter when the patient’s intrinsic heart rate increased to the point that it usurped control from the transcutaneous pacemaker. I think it’s a thing of beauty!
Hopefully you’re buying what I’m selling here. I’ve got maybe ten other cases showing true and false-capture clearly visible on the SpO2 waveform but we’ve got to keep moving so you’ll have to look forward to seeing some of those in another post. In summary: pulse oximetry works!
I don’t have any personal experience with this modality during TCP (and didn’t use it in this case) but there are reports of folks observing an increase in cardiac output corresponding to successful mechanical capture via end-tidal waveform capnography. In fact, we’ve got one right here on this blog!
The capnographs at my facility are notoriously finicky and I personally wouldn’t be itching to use ETCO2 to confirm capture when SpO2 and the next option are so straightforward and readily available, but consider this in your own armament depending on where and how you practice. At the very least, see if you can observe an increase in cardiac output as a bump in the ETCO2 if you happen to already have the capnograph running on your patient before initiating pacing.
This one is my absolute favorite!
Though it’s not an option for most prehospital providers, if you happen to work in an emergency department or advanced transport setting where ultrasound is readily available this is the #1 method of confirming mechanical capture during transcutaneous pacing. It’s so simple:
- Slap the ultrasound probe on the patient’s heart.
- Directly observe the patient’s heart rate increase.
It couldn’t get any easier and more foolproof—assuming you’re comfortable performing a basic cardiac exam on ultrasound.
In this case, because of how rapidly the patient deteriorated on arrival, I wasn’t able to obtain a pre-pacing clip of the heart beating 20 times a minute, but once we observed electrical capture on the rhythm monitor and mechanical capture on the SpO2 I was able to confirm that the heart was indeed contracting (quite well) at 80 bpm in response to pacing:
As you can see, the heart is contracting quite vigorously in response to TCP (sorry I couldn’t avoid the PLA lung shadow with the patient on the vent). The practiced-eye will note an apparent dyssynchrony caused by the abnormal ventricular activation but it’s clear that the heart is generating a reasonable cardiac output. This was confirmed with both manual and automatic non-invasive blood pressures along with improvement in the patient’s clinical condition.
For another example demonstrating the use of echocardiography to confirm mechanical capture check out this excellent case from the Ultrasound of the Week blog. Seriously, there are some awesome clips and it’s a really short case. It’s also where I first got the idea to confirm capture with bedside USS, so show Dr. Ben Smith some love.
- Check your pad placement and sandwich the heart.
- Don’t trust a manual pulse unless you like being wrong most of the time.
- Pulse oximetry rocks for confirmation of capture and is readily available.
- ETCO2? An interesting finding but I wouldn’t use it as the “proof” I need.
- Direct visualization under ultrasound is ideal, though rarely an option outside the ED.