A pacemaker is a small medical device that regulates heart rhythm. This device delivers controlled electrical impulses to the heart muscle. These electrical signals create distinct patterns on an electrocardiogram (ECG), which professionals use to understand heart activity.
Understanding Paced Rhythms
A paced rhythm occurs when a pacemaker generates electrical impulses to stimulate the heart. The pacemaker delivers a precisely timed electrical discharge, which then spreads through the heart tissue, initiating a contraction. This artificial electrical activity is picked up by an ECG machine, resulting in a unique tracing that differs from a naturally occurring heart rhythm. Pacemakers are often implanted to address conditions such as a very slow heart rate, known as bradycardia, or to manage heart blocks where the heart’s natural electrical signals are disrupted.
Identifying Pacing Spikes and Their Impact
The primary visual characteristic of a paced rhythm on an ECG is the “pacing spike.” This spike appears as a thin, vertical line, typically very narrow and sharp, indicating the moment the pacemaker delivers an electrical impulse. Following this pacing spike, a heartbeat should immediately appear, known as “capture.” This captured beat can be a P wave, a QRS complex, or both, depending on which heart chamber the pacemaker is stimulating.
When the pacemaker successfully captures the heart, the morphology, or shape, of the subsequent P wave or QRS complex may look different compared to natural heartbeats. For instance, if the pacemaker is stimulating the ventricles, the resulting QRS complex will often appear wide and somewhat distorted. This wider appearance is because the electrical impulse originates from a single point within the ventricle rather than following the heart’s natural, more efficient conduction pathways. The consistent presence of these spikes immediately preceding a captured beat indicates pacemaker function.
Different Types of Pacing Patterns
Pacemakers can be programmed to stimulate different chambers of the heart, leading to distinct patterns observed on an ECG.
In atrial pacing, the pacemaker delivers an impulse to the atria, causing a pacing spike to appear directly before each P wave. This indicates that the pacemaker is initiating atrial contraction. The P wave itself might look slightly different from a naturally occurring P wave.
Ventricular pacing involves the pacemaker stimulating the ventricles directly. On the ECG, this is identified by a pacing spike immediately before the QRS complex. The QRS complex that follows a ventricular pacing spike is typically wide and may have an unusual shape, reflecting the abnormal electrical depolarization pathway through the ventricular muscle. This wide QRS morphology is a hallmark of ventricular pacing.
Dual-chamber pacing combines both atrial and ventricular stimulation. Two distinct pacing spikes are visible for each paced heartbeat, one preceding the P wave (atrial capture) and another preceding the QRS complex (ventricular capture). This coordinated pacing ensures that both the atria and ventricles contract in a synchronized manner, mimicking the heart’s natural electrical sequence more closely.
Recognizing Common Pacemaker Issues
A paced rhythm on an ECG may indicate a pacemaker issue. One common problem is “loss of capture,” which occurs when a pacing spike is visible on the ECG, but it is not followed by a P wave or a QRS complex. This means the pacemaker delivered an impulse, but the heart muscle did not respond or contract. The absence of a subsequent heartbeat after a spike suggests a failure in the pacemaker’s ability to stimulate the heart effectively.
Another issue is “undersensing,” where the pacemaker fails to detect the heart’s own natural electrical activity. This can lead to inappropriate firing, delivering impulses even when the heart beats adequately. Conversely, “oversensing” occurs when the pacemaker detects non-cardiac signals (e.g., muscle tremors or electrical noise) as heartbeats. As a result, the pacemaker incorrectly inhibits its pacing output, potentially leaving the heart without necessary support.