12-Lead ECG case: When is a heartbeat not a mechanical heartbeat?

Learn to distinguish and verify electrical and mechanical capture when using a transcutaneous pacemaker on a patient with symptomatic bradycardia

A 76-year-old woman calls 911 because of extreme weakness and near-syncope. The rescue crew finds her weak, pale and diaphoretic, with a pulse rate of 30 bpm. Her BP is 72/44. A look at the cardiac monitor shows a very slow sinus bradycardia without ST elevation or depression.

The crew starts an IV and attaches pacemaker electrodes. They turn the transcutaneous pacemaker on at a rate of 72 bpm, with an electrical current of 40 mA.

They determine that they have electrical capture, but the patient’s condition does not improve. They increase the rate to 80, then 90 bpm — still with no improvement in the patient’s clinical condition. 

They do not increase the electrical current (mA), because they feel they have electrical capture. They decide to give an intravenous bolus of normal saline.

When they arrive at the hospital, the patient is still pale and diaphoretic and her BP is 90/50. She complains of shortness of breath, and wants to sit up.

This is the patient’s ECG rhythm strip on arrival at the hospital.

What did the rescue crew miss?

They did not achieve capture with this pacemaker. The pacemaker is sensing Lead II, and has correctly marked the patient’s own or native beats (top arrows). 

The pacer is appropriately firing in demand mode (lower arrows). The pacer has not captured the myocardium. Transcutaneous pacemakers often show artifact after the spike. This artifact can be mistaken for a QRS complex.

What should the TCP look like on the ECG?

The pacemaker delivers a strong current to the chest wall. A normal cycle with electrical capture will begin with a spike, which is a mark placed by the device software to show when the pacemaker fired. 

Then, when the myocardium is depolarized, a wide QRS complex with a broad T wave will occur. The T wave is usually in the opposite direction of the QRS. This is called a discordant T wave, and it is normal in wide-complex rhythms.

The pacing stimulus can be picked up by the ECG electrodes and cause a deflection that may look like a QRS complex. If the pacemaker and monitor is one unit, the monitor will probably have a mechanism for avoiding this artifact. The monitor will blink, or stop showing information, for approximately 40-80 ms (1-2 small blocks) after the pacing stimulus is delivered. 

In most cases, this blanking period allows the device to avoid showing the pacing artifact on the ECG. The downside is a loss of data for 40-80 ms.

If this mechanism is not present, for instance if the monitor being used is not part of the pacemaker device, the pacer artifact will show up immediately after the spike. Occasionally, we can even see some artifact after the blanking period.

How to tell artifact from electrical capture

With pacing artifact, the wave may look like a wide QRS, or it may look bizarre. There will be no T wave following the QRS. If you increase the current, the size of the artifact will increase.

Your patient will not have a pulse that corresponds to the pacing stimuli. Remember to check the pulse peripherally, as the muscle contraction of the chest wall from the pacing makes it difficult to determine pulse at the carotid artery.

How to recognize electrical and mechanical capture

Electrical capture will result in a QRS complex with a T wave after each pacer spike. Mechanical capture will cause palpable peripheral pulses and usually a noticeable improvement in patient condition.

The pulse oximeter and ETCO2 monitor can help a lot, too. If you have mechanical capture, the pulse ox waveform should show definite pulses and the patient's ETCO2 should increase because of increased perfusion.

What should I do if I see artifact instead of electrical capture?

If the limb electrodes are located near the pacing electrodes, move them as far away as possible on the limbs. Increase the current until a QRS and T wave are seen and peripheral pulses match the TCP rate.

Remember to warn the conscious patient as you increase current. When a QRS complex with T wave are seen, evaluate the patient’s extremity pulses manually to determine that they match the pacemaker rate.

When pacing with a TCP, do not rely on electronic vital-signs measurements and heart-rate monitoring to determine the patient’s condition. Artifact can trigger the pulse and rate indicators on the monitor, showing a rate faster than the actual heart rate.

Evaluate the patient's blood pressure after two minutes of a normal pulse rate before treating hypotension with fluids, as correcting the rate may be all you need.

If the patient is unresponsive, slow the pacemaker to look for the presence of ventricular fibrillation, which can be masked by TCP artifact.

Don’t be fooled

Remember to evaluate the complex to determine if it is a QRS-T or just a wide artifact. Check the patient’s peripheral pulse manually and observe the patient’s condition.  When the patient's heart rate is improved with pacing, give the patient a few minutes to improve hemodynamically before deciding to give large amounts of fluid.

Additional resources

Learn more about transcutaneous packing from these resources.

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