How comfortable are you with transcutaneous pacing?

Written by Willy Frick

A woman in her 70s is hospitalized with undifferentiated shock after being found down at home. Her family had not heard from her and called EMS. Paramedics found her bradycardic, hypotensive, and tachypneic. She was resuscitated and admitted to ICU for presumed sepsis.

Several days into hospitalization, she continued to have occasional episodes of sinus rhythm and sinus bradycardia with periods of Mobitz I AV block and 2:1 block. Her clinical team initiated transcutaneous pacing. Here is her cardiac telemetry. (Note: these are three consecutive strips of lead V1 from top to bottom.)

Do you notice any problems?

Here is the exact same strip, with the arterial pressure waveform included:

See if you can figure out what the problem is.

Hint: What does the arterial waveform correspond to on the ECG?

The problem here is complete failure to capture. The amount of current delivered is not above the minimum threshold required to cause myocardial contraction. The patient is getting some electrical stimulation, but nothing useful is happening.

The transcutaneous pads are delivering shocks at a rate of 70 per minute. You can see the purple lines (correctly) interpreted by the telemetry software as pacing spikes. And you can see a sharp deflection in the ECG from pacing artifact. The mistake here is thinking that the sharp pacer artifact deflection is a QRS complex. In fact, the only QRS complexes here are the patient's own native rhythm!

Here is the same image with orange arrows pointing out the patient's native P waves and blue arrows pointing out the patient's native QRS complexes. (Dotted lines indicated buried waveforms).

Now it becomes apparent that the arterial tracing corresponds exactly to the native QRS complex, and has no relationship at all to the pacing spikes. This patient is getting painful electrical shocks at a rate of 70 per minute which is providing no benefit whatsoever. Meanwhile, the patient's native rhythm is sinus bradycardia with adequate perfusion.

There may be some blocked P waves in there, it is hard to be sure with all the pacer artifact. The patient did have Mobitz I AV block both before and after this.

Here is where the pacing pads were turned on. Notice that the native QRS continues on unaffected by the pacing spikes.

Fortunately, this patient did not actually need any pacing. In fact, later on the patient's native rhythm was actually faster than the pacing efforts.

Cardiology turned the transcutaneous pacer pads off and recommended observation only. It is extremely lucky for this patient that she did not actually need any pacing. If she had needed pacing, she would have died.

So, how do you confirm that there is true capture of myocardium if you need to perform transcutaneous pacing?

• If you have an arterial line already, you should see the heart beat with every pacing impulse.

• If you do not have an arterial line, use bedside ultrasound to verify myocardial contractility corresponds to pacing.
• If you don't have ultrasound (but you should), then palpate a pulse! This can be harder than it sounds. Transcutaneous pacing will also capture skeletal muscle causing twitching. Your best bet is a femoral artery. And to be clear: It is completely safe to touch a patient who is being transcutaneously paced. I have done it many times.

• Pacing can be painful even if not capturing.  

• When in doubt crank the output all the way up. The dial will usually be labeled "OUTPUT," and the units will be mA.

Learning points:

• Do not confuse pacing artifact with myocardial capture

• Confirm capture by inspection of arterial line or palpation of a pulse

• When in doubt, crank the output

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MY Comment, by KEN GRAUER, MD (12/24/2024):

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Clinical implications of the content in today's post by Dr. Frick cannot be overstated — TCP (TransCutaneous Pacing) can either be lifesaving or producing of the opposite effect if basic troubleshooting measures are not attended to.

• In an effort to reinforce these measures presented by Dr. Frick in his above discussion — I've reproduced the following Section from my ACLS-2013-ePub.

Pacing CAVEATS: Is there Capture?

Effective TCP capture can be obtained in many (not all) patients. That said — there are pitfalls inherent in the process of determining IF effective electrical and mechanical capture are occurring. Consider the 4 rhythm strips shown in Figure-1:

QUESTIONS:

• Is there effective capture for rhythm strips A,B,C and D in Figure-1?
•   — How can you tell?  Clinically — What should you do? 

Figure-1: Pacing caveats. Is there effective capture for rhythm strips A,B,C,D? (adapted from Grauer K: ACLS-2013-ePub — Section 15, KG/EKG Press).

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ANSWER to Panel A:

The underlying cardiac rhythm in Panel A is asystole. Pacer spikes are seen at a rate of 75/minute (occurring every 4th large box) — but there is no sign of capture. 

• Suggested Approach: Increase current (gradually from ~50ma up to a max of 200 ma); correct other factors (ie, acidosis, hypoxemia). Treat asystole.

 

ANSWER to Panel B:

The pacer rate is ~100/minute. There is now electrical capture of every-other-beat in Panel B (as evidenced by a wide QRS complex with broad T wave occurring after every-other pacing spike).

• Suggested Approach: Since there now is sign of ventricular capture (at least for every-other pacing spike) — increasing current further will hopefully result in capture of every spike. Increase current (up to a max of 200 ma); correct other factors (ie, acidosis, hypoxemia).

ANSWER to Panel C:

Each pacer spike in Panel C now captures the ventricles by the end of this rhythm strip (evidenced by a wide QRS complex after each spike, followed by a broad, oppositely directed T wave).

• Suggested Approach: Look for objective signs to confirm that pacing is working clinically (ie, Check for pulse and BP; pulse ox pleth wave; and ET CO2 readings that should improve if the pacemaker is truly effective).

ANSWER to Panel D:

The rhythm in Panel D shows a regularly-occurring negative deflection after each pacing spike. That said — We suspect this may be pacer artifact and not indicative of true ventricular capture because there is no broad T wave (as there should be if this was paced! ).

• Comment: Verifying that true ventricular capture has occurred with external pacing is at times easier said than done. Reasons for this include the indirect nature of external pacing and a tendency for electrical artifact to be produced by the electrical activity generated from the pacemaker.
• Electrical artifact is usually "blocked out" from the ECG monitor (by integrated pacemaker software that eliminates a 40-to-80 msec. period that occurs just after the pacer spike from the ECG recording). BUT on occasion — a portion of this electrical artifact may persist beyond the "blockout" period. When this happens – the electrical artifact that regularly appears on the ECG monitor at fixed interval immediately following each pacer spike may “masquerade” as ventricular capture. It is important not to be fooled into thinking these “phantom” QRS complexes represent true capture. 

Suggested Approach: In addition to awareness of this phenomenon — You can verify that ventricular capture has truly occurred by:

• Being SURE the QRS after each pacing spike is wide and has a tall broad T wave. (In contrast — the QRS of electrical artifact is narrow and does not have any T wave! ).
• Palpating a pulse with each paced complex (and being certain not to confuse wishful thinking from pacer-generated muscle twitching as a “pulse” ).
• Checking for other evidence of perfusion (Can you get a BP? Is there now a pulse ox pleth wave? — and — Does ET CO2 increase as it should if the pacer is truly effective?)