Common Mistakes With External Pacemakers

Transcutaneous pacing is harder than it looks.

ecg rhythm showing pacer capture
Rod Brouhard 

One of the biggest oddities in the emergency treatment of symptomatic bradycardia is the propensity to skip atropine administration and jump straight to external pacing. It's a common choice among paramedics. The thinking is that atropine causes such an increased oxygen demand that it puts undue stress on the heart muscle and could lead to a myocardial infarction. Instead, the thinking goes, use transcutaneous pacing to increase the heart rate without the ill effects of atropine.

Without getting into the debate about whether that is the appropriate choice or not, it's important to recognize the pitfalls of using external transcutaneous pacing. It's far from a panacea. It's a high-acuity, low-frequency procedure that brings more than its share of confusion to the emergency. To properly pace a patient in symptomatic bradycardia, one must make sure he fully understands the mechanics and the clinical use of an external transcutaneous pacemaker.

History of Pacing

First of all, pacemakers have been around as long as the human heart as been around. It comes with its own natural pacemakers—indeed, every cardiac muscle cell can fulfill this role if necessary—but the use of electricity to trigger a cardiac contraction has been around since the late 1700s, albeit on frogs.

Therapeutic pacemakers hit the clinical scene in the mid-1900s and have been getting smaller and smarter ever since. There are implantable pacemakers that are used for patients with chronic cardiac arrhythmias. The use of transcutaneous external pacemakers that use electrodes embedded in adhesive patches have been used in and out of the hospital since 1985.

The Machine

There are several brands and models of transcutaneous external pacemakers, but they all follow the same basic design. A cardiac monitor capable of at least a basic, continuous, single-view electrocardiogram (ECG) is paired with a pacemaker that comes with two electrodes. The electrodes are usually embedded into single-use, pre-gelled adhesive pads. In most modern models, the pacemaker portion and pads double as a defibrillator.

Most of these also come with a printer to record the patient's ECG rhythm and any attempts to pace or defibrillate it. Many devices are capable of other vital sign monitoring, such as non-invasive blood pressure (NIBP), pulse oximetry, end-tidal capnography, etc. There are some tricks we can do using these other vital signs to help identify proper pacing. More on that later.

Transcutaneous pacemakers have two variables that the caregiver has to control: the strength of the electrical impulse and the rate of impulses per minute. Rate is pretty self-explanatory. This is a treatment for symptomatic bradycardia, so the rate setting should be faster than the patient's arrhythmia. Usually, we shoot for a number around 80 per minute. This varies by locale, so be sure to check with your medical director for guidance on proper pacing rate.

Electrical impulse strength is measured in milliamperes (milliamps for those in the know). It takes a minimum amount of energy to break through a patient's threshold to trigger a contraction. That threshold is different for every patient and the most common mistake in using a transcutaneous pacemaker is failing to crank up the energy high enough. To make things even more complicated, there are different thresholds for the conduction pathways of the heart and the actual heart muscle, which means it's possible for the ECG to look like the pacemaker is working, but the heart muscle isn't actually responding.

Attaching the Device

Each model is different and it's really important that each caregiver spends time to familiarize herself with the device she'll be using in the field. That being said, the procedures are very similar across multiple brands.

The pacer pads must be attached along with the monitoring electrodes. When transcutaneous pacemakers and defibrillators were separate devices, the pacer pads had to be placed out of the way of the defibrillator paddles in case of cardiac arrest, a legitimate concern when playing around with a patient's cardiac conduction system. Now that most transcutaneous pacemakers double as defibrillators, the patches are often placed the same for both uses. Again, follow the manufacturer's directions.

The patient must be connected to the cardiac monitor. This is important. For those who are familiar with the way most manual cardiac defibrillators work, it's a common mistake to assume that the pacemaker electrodes (pacer pads) will be able to also monitor the patient's heart rhythm. That's how defibrillators work, but defibrillators deliver a single shock, and then go back to monitoring the rhythm. A transcutaneous pacemaker is continuously delivering impulses and doesn't really have a chance to monitor anything through the pacer pads.

Make sure the ECG monitor is set to read a lead through the monitoring electrodes and not through the pacer pads. Because a combination defibrillator/pacemaker uses the same patches for both electrical therapies, it's very easy to set this incorrectly. If it is set to read through the pads, many devices simply won't work when pacing is tried.

Pacing a Patient

Once the device is properly applied and activated, look for pacer spikes in the ECG tracing. Once we have that, it's time to pace the patient:

  1. Set the rate to the desired beats per minute. Most devices default to a rate between 70-80, but the rate is adjustable by the caregiver.
  2. Increase the energy level until the impulses trigger a QRS complex, which is known as capture. The ECG monitor will show a solid spike for each impulse and when each spike is followed immediately by a QRS complex, capture is achieved (see the image above).
  3. Feel for a radial pulse. There must be a radial pulse for each QRS complex, or this thing isn't helping. If the patient is not perfusing the radial pulse, the blood pressure is still too low to be sustainable.
  4. Bump up the energy 10 milliamps past the point of capture. This reduces the likelihood of losing capture in the future.

Once the pacemaker is working and the patient's condition is improving, consider sedation. This thing hurts like crazy. There will be a lot of skeletal muscle contraction of the chest wall with each impulse. The patient can tolerate it for a few minutes, but not for too long. If this is applied in the field, the patient still has to be transported to the hospital before something more invasive (and less painful) can replace the transcutaneous pacemaker.

Pitfalls of Transcutaneous Pacing

Three words: Capture! Capture! Capture! The most common error that I've witnessed in out-of-hospital transcutaneous pacemaker application is failure to capture. The biggest reason is misreading the ECG and believing that capture has occurred.

When the pacer spikes seem to be hitting right before the QRS complexes, it can appear that the device is helping (see the image above). There are a few indicators to help avoid this mistake:

  • Compare the pre-paced rhythm with what the caregiver believes is the "paced" rhythm. True capture will show a different formation of the QRS complex because the focal point of the impulse is coming from a different place (a giant patch on the chest that's as big as the heart instead of some pinpoint location along the cardiac conduction pathway). If the formation of the QRS hasn't changed, capture is very unlikely.
  • If the pacer spikes outnumber the QRS complexes, we haven't achieved capture yet. In the image above, there are three spikes, but only two QRS complexes in the part of the strip without capture.
  • If the pacer spikes are at variable distances from the QRS complexes, no capture.
  • If the energy is below 40 milliamps for an adult patient, it's very unlikely that capture can happen. Most patients have a threshold above this level. Turn it up a notch. Most devices increase the energy in five- or ten-milliamp increments.

A QRS for each spike; eureka! We have capture!

Not so fast...do we have a pulse with that? Electrical capture is identified on the ECG, but physical capture is assessed through vital signs. The second most common mistake I see is failure to confirm physical capture. Look for these signs:

  • A radial pulse for each QRS is the best indicator. This tells the caregiver that each cardiac contraction is achieving a systolic blood pressure of at least 80-90 mmHg.
  • A hack for difficult patients is to watch the pulse oximetry waveform. If the waveform matches the QRS rate—which must be the rate set on the device, or we don't really have capture—then we know the heart is contracting with each QRS. Take a blood pressure to see if the pressure is sustainable. If it is low, a fluid bolus might help correct the problem. Be sure to consult medical direction.

Avoid using the carotid pulse as an indicator of physical capture. The skeletal muscle contractions that come with transcutaneous pacing make it really hard to identify carotid pulses. They're there, but maybe not as fast as the pacer, which is the whole reason to check the pulse in the first place.

Lastly, treat the pain. There is at least one example of a patient sustaining burns from pacer pads and patients almost universally complain of pain from skeletal muscle stimulation with transcutaneous pacing.

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