Transcutaneous Pacing: "Put It Up To Eleven!"
Updated: Aug 14
What. The. HECK?!
But there is a short answer -- and it's actually just a number. And as the title suggests, the answer to this case is (both literally and figuratively) 11.
But what are the questions?
EMS brought in a middle-aged male with altered mental status and an ill appearance. Over the past 3-4 days he had become increasingly weak and now seemed “out of it” to his family.
He had a recent admission to the hospital for atrial fibrillation and was discharged on an ACE inhibitor, as well as both a beta-blocker and a calcium-channel blocker. (No digoxin.)
The paramedic found his heart rate to be in the 20s.
Here is the ECG:
I’m unclear what the BP was, but transcutaneous pacing (TCP) was started. Here is the sequence as the medic raised the mA:
Pitfalls in Pacing
Now, TCP is full of pitfalls, of which the second-largest is failure to capture. The most-largest is, of course, failure to recognize the failure of capture! It’s easy to get wrong, especially since the topic is not well understood, even by many physicians.
Did the medics get “true” capture? Let’s examine the sequence as the amperage is turned up.
First, look at each QRS marked with an inverted triangle. These native beats are circled with a red circle. They look the same at 10 mA and at 100 mA: a small QRS followed by a long QT interval.
Next, note the paced complexes (e.g. the complexes with the green arrows) in the 70 mA strip have a narrow QRS, and no discernible T wave. These are phantom impulses, and they do not indicate capture. They only indicate that the pacer has fired and that it has sensed its own energy.
As we go up in energy, we see at 90 mA there is pacemaker dissociation (indicated by the pink arrows). The native beats (red circle) come too soon after a paced beat, occurring during what should be the refractory period. (Remember that the uncorrected QT interval is almost 750 ms.)
This is further evidence that the pacer complexes we see at 90 mA are not capturing electrically, let alone mechanically.
However, at 100 mA, we see (bigeminal) true capture. The QRS in these complexes (Solid blue arrows) is wide, and the T wave is large and discordant like you would see with a BBB or PVC.
Verification in the ED
So, easy for me to say, but where is the proof? Well, in the ED we were able to “look” directly at the heart with the echocardiogram.
When the patient arrived, we obtained our own ECG:
We then turned the pacing back up! Once we got up to 110 mA, we felt a pulse, and we then repeated the ECG:
While useless for diagnosing a STEMI, this ECG strongly suggests successful TCP. The QRS complexes are wide, and the T waves are large and discordant.
But pulses can be difficult to discern from muscular twitching, so we looked at the heart to verify. Don’t be intimidated by echo, you’ll get the idea!
The heart without pacing:
The heart with pacing:
Which one looked faster?
What were the questions? The answers are "11."
The first question was “How high should you turn the mA when trying to perform TCP?” Eleven is the figurative answer.
In other words, you need to bring up the amperage a bit farther than you may have been taught. EMS and the ED didn’t get capture until 100 mA. As Tom says, the pacer goes up to 200 mA for a reason!
The second question was “What was the patient’s potassium?”
Eleven was the literal answer -- in this case (110 mA). The patient was in severe renal failure and received multiple doses of calcium gluconate, as well as insulin, bicarb, and fluids. He recovered after a few sessions of hemodialysis and is doing well.