A cardiac pacemaker is a small, implanted device that regulates the heart’s rhythm by delivering electrical impulses when the heart’s intrinsic system falters. The electrocardiogram (ECG) is the primary non-invasive tool used to evaluate this device, translating the heart’s electrical activity into a visual tracing. Interpreting an ECG with a pacemaker requires specialized knowledge to assess the complex interaction between the heart and the implanted technology, differentiating between normal function and various forms of malfunction.
Pacemaker Tracing Fundamentals
The visual representation of a paced beat on an ECG relies on three fundamental components. The most recognizable feature is the pacing spike, a sharp, vertical line marking the moment the pacemaker delivers an electrical pulse. This artifact represents the device’s output and precedes the complex it intends to stimulate.
Capture refers to the successful electrical activation of the heart muscle immediately following the pacing spike. If the impulse stimulates the atrium, a P wave follows the spike; if it stimulates the ventricle, a wide QRS complex appears. Failure to see the expected P wave or QRS complex after the spike indicates the heart tissue failed to respond to the electrical stimulus.
Sensing is the pacemaker’s ability to detect the heart’s own intrinsic electrical activity. A functional pacemaker is programmed to deliver a pulse only when the heart rate drops below a pre-set threshold or when a native beat is absent. When the device recognizes a natural P wave or QRS complex, it inhibits its output, allowing the heart to beat naturally. This dynamic interaction allows modern pacemakers to function “on demand.”
Obtaining a Quality ECG
Acquiring a diagnostic-quality ECG tracing requires specific technical adjustments for patients with pacemakers. The standard 12-lead placement is used, but electrodes must not be placed directly over the pacemaker’s pulse generator. The generator, typically located beneath the skin near the collarbone, can create significant electrical interference that obscures the tracing.
To minimize artifact, maintain a distance of three to five inches between the nearest electrode and the device generator. This prevents the large electrical signal from creating a dense noise pattern, sometimes called a “picket fence” artifact, which can hide the pacing spike or distort the cardiac waveform. If the tracing remains noisy, relocating the limb electrodes to the torso can help reduce movement-related artifact, especially if caused by muscle tremor or shivering.
Optimizing the ECG machine’s settings can enhance the visibility of the high-frequency pacing spike. Adjusting the ECG filter settings, specifically increasing the upper cutoff frequency to 150 Hz, helps distinguish the rapid spike from the lower-frequency cardiac waveform. Obtaining a long rhythm strip, particularly in leads II or V1, is necessary to fully assess the pacemaker’s sensing and capture functions over time, as a short strip might miss an intermittent malfunction.
Interpreting Normal Pacing Rhythms
A normally functioning pacemaker displays distinct ECG patterns reflecting the chamber stimulated and the programmed timing intervals.
Single-Chamber Pacing
Single-chamber pacing stimulates either the atrium (A-pacing) or the ventricle (V-pacing). In atrial pacing, a spike precedes a P wave, followed by a naturally conducted, narrow QRS complex, assuming the heart’s atrioventricular (AV) node functions correctly. Ventricular pacing is identified by a pacing spike immediately followed by a wide QRS complex.
This wide appearance occurs because the impulse originates from the lead tip, often in the right ventricle, and spreads slowly across the ventricular muscle outside the native conduction system. This slow, muscle-to-muscle spread results in an electrical pattern that often mimics a left bundle branch block (LBBB) morphology, regardless of the patient’s underlying condition.
Dual-Chamber Pacing
Dual-chamber pacing involves leads in both the atrium and the ventricle, allowing for sequential stimulation. This mode is often referred to by its NBG code, such as DDD, indicating both chambers are paced and sensed. On the ECG, this appears as two distinct pacing spikes: one preceding a P wave (atrial capture), followed by a programmed AV interval delay, and then a second spike preceding a wide QRS complex (ventricular capture).
A key indicator of proper function is 100% capture, meaning every pacing spike is successfully followed by the expected complex. The programmed intervals, such as the minimum heart rate (lower rate limit) and the AV delay, must be consistent with the device’s settings. The absence of pacing when the intrinsic rate is above the lower rate limit demonstrates correct sensing and inhibition.
Recognizing Pacemaker Malfunctions
When a pacemaker malfunctions, the ECG tracing provides visual evidence of problems with impulse generation, delivery, or detection.
Failure to Capture
Failure to Capture is characterized by a pacing spike that is not followed by the expected P wave or QRS complex. This suggests the electrical impulse was too weak to stimulate the heart muscle or that the lead tip has moved.
Failure to Sense (Undersensing)
Failure to Sense, or undersensing, occurs when the pacemaker fails to recognize the heart’s natural electrical activity. Since the device does not detect the intrinsic beat, it delivers an unnecessary pacing spike at an inappropriate time. On the ECG, spikes fall too close to or directly into the patient’s own P waves or QRS complexes. This competition between the device and the heart can potentially trigger a dangerous heart rhythm.
Oversensing
Conversely, Oversensing happens when the pacemaker mistakes non-cardiac electrical signals for a native heartbeat. These signals may include T-waves, skeletal muscle movement, or external electromagnetic interference. The device interprets this noise as intrinsic activity and inappropriately inhibits its output. This leads to long pauses or an unsustainably slow heart rate when pacing is needed. Oversensing is suspected if a rhythm strip shows a prolonged pause without pacing spikes when the intrinsic rhythm is below the programmed rate.
Failure to Pace (Output Failure)
Failure to Pace, or output failure, is the absence of a pacing spike when one is expected. This malfunction results in a pause or a drop in the heart rate to the patient’s underlying, slow rhythm. Causes include battery depletion, a lead fracture, or severe oversensing that suppresses the output. The visual manifestation is a missing spike where the pre-set timing cycle dictates a stimulus should have been delivered.