Pacemaker Recognition — Recognize pacemaker spikes and capture patterns

The heart depends on a precisely coordinated electrical conduction system to generate effective mechanical contractions. When disease, aging, or surgical trauma disrupts this system, patients may experience life-threatening bradycardia, heart block, or asystole. The artificial cardiac pacemaker was developed to deliver precisely timed electrical impulses that restore a viable heart rhythm when the native conduction system fails. For the EKG technician, the ability to recognize pacemaker activity on a tracing is essential: you must distinguish device-generated impulses from intrinsic cardiac activity and determine whether the pacemaker is functioning correctly. Understanding the evolution of pacemaker technology provides critical context for interpreting the variety of pacing patterns seen in modern clinical practice.

With millions of patients worldwide now relying on implanted pacemakers, the EKG technician encounters paced rhythms regularly. The central question this lesson addresses is straightforward yet clinically vital: How do you identify the electrical signature of a pacemaker on the EKG, and how do you determine whether the device is sensing and capturing appropriately?

Before analyzing paced rhythms, you must internalize several foundational concepts. A pacemaker is fundamentally a pulse generator connected by one or more leads to specific cardiac chambers. Its output appears on the EKG as a brief, narrow deflection called a pacemaker spike (also referred to as a pacing artifact). This spike represents the moment the device discharges its electrical pulse into the myocardium. The relationship between the spike and the cardiac waveform that follows it defines whether the pacemaker is successfully depolarizing the target tissue, a phenomenon known as capture. Equally important is the device's ability to detect the patient's own electrical activity, termed sensing, which prevents unnecessary or dangerously timed stimulation.

The following diagram illustrates the three main pacing configurations you will encounter: atrial pacing, ventricular pacing, and dual-chamber (AV sequential) pacing. Each tracing shows the pacemaker spike as a sharp vertical line and the resulting cardiac waveform that indicates capture. Note how the morphology of the paced complex differs from the intrinsic waveform: ventricular-paced beats produce wide, bizarre QRS complexes because the impulse originates from the lead tip in the right ventricle and spreads cell-to-cell rather than through the normal His-Purkinje system.

In the atrial tracing (top row), each narrow spike is immediately followed by a P wave, confirming atrial capture. The QRS complex that follows is narrow because ventricular depolarization proceeds through the intact His-Purkinje system. In the ventricular tracing (middle row), the spike precedes a wide QRS complex (≥ 0.12 seconds) that typically exhibits a left bundle branch block morphology because the pacing lead is positioned at the right ventricular apex. In the dual-chamber tracing (bottom row), two spikes appear for each cardiac cycle — the first producing a P wave (atrial capture) and the second producing a wide QRS (ventricular capture), with a programmed AV interval between them that mimics the native PR interval.

Although pacemaker recognition is primarily a pattern-recognition skill rather than a mathematical exercise, several quantitative relationships are essential for the EKG technician. The most important is the relationship between the pacing interval (the distance between consecutive pacing spikes) and the pacing rate. Understanding this relationship allows you to verify whether the device is pacing at its programmed rate. Additionally, in dual-chamber systems, you must evaluate the AV interval — the time between the atrial spike and the ventricular spike — which is analogous to the PR interval in intrinsic rhythm.

Recognizing normal pacemaker function is only half the equation; the CET must also identify the three cardinal types of pacemaker malfunction. Each malfunction has a distinct EKG signature that can guide the clinical team toward rapid intervention. The three malfunctions are failure to pace (no spike appears when expected), failure to capture (spike appears but no depolarization follows), and failure to sense (the device does not detect intrinsic activity and fires inappropriately).

Consider the following clinical scenario: A 76-year-old patient with a DDD pacemaker presents for a routine EKG. On the tracing, you observe two narrow vertical deflections before each QRS complex, and the QRS complexes are wide. You need to systematically evaluate the tracing for proper pacemaker function.

Different pacing modes produce characteristic patterns on the EKG that the technician must be able to distinguish at a glance. The NBG (NASPE/BPEG Generic) pacemaker code provides a standardized way to describe pacemaker behavior, and understanding this code is essential for interpreting the variety of pacing patterns encountered in clinical practice. The following table compares the most commonly encountered pacing modes, their indications, and the specific EKG features that define each.

As you develop proficiency in basic pacemaker recognition, you will encounter several advanced phenomena that can complicate interpretation. Two of the most important are fusion beats and pseudofusion beats. A fusion beat occurs when the pacemaker spike and an intrinsic impulse simultaneously depolarize the ventricle, producing a QRS complex whose morphology is intermediate between the fully paced and fully native waveform. A pseudofusion beat occurs when the pacemaker spike lands on an intrinsic QRS that has already begun — the spike appears superimposed on the QRS but does not contribute to depolarization. Neither phenomenon represents a malfunction; rather, both indicate that the pacemaker and the native conduction system are active at nearly the same time.

As you advance in your career, you will encounter biventricular pacemakers used in cardiac resynchronization therapy (CRT), which pace both the right and left ventricles to improve synchrony in patients with heart failure. The QRS morphology in biventricular pacing often differs from standard RV pacing — it may be narrower or show a different axis because both ventricles are being simultaneously activated. Additionally, implantable cardioverter-defibrillators (ICDs) contain pacing capability and may produce pacing spikes during bradycardia episodes, but they also have the capacity to deliver high-energy shocks for ventricular tachycardia or fibrillation. These advanced devices build upon the same foundational principles of sensing, pacing, and capture that you are learning now.