An electrocardiogram, or ECG, is a medical test that provides a visual representation of the heart’s electrical activity. For individuals whose hearts do not beat regularly on their own, a small implanted device called a pacemaker can deliver electrical signals to maintain a proper rhythm. One specific function of this device is ventricular pacing, where these signals are sent directly to the heart’s lower chambers. Understanding how to recognize the distinct patterns of ventricular pacing on an ECG is useful for interpreting the heart’s function in a paced individual.
The Basics of Ventricular Pacing
Ventricular pacing is used for individuals who experience conditions that disrupt the heart’s natural electrical conduction system. One common reason is symptomatic bradycardia, a heart rate that is too slow to meet the body’s demands, leading to symptoms like dizziness and fatigue. Another indication is a complete heart block, where the electrical signals generated in the heart’s upper chambers (the atria) are unable to travel to the lower chambers (the ventricles).
The pacing system consists of two main parts: a pulse generator and one or more leads. The pulse generator is a small, battery-powered computer that creates the electrical impulses and is implanted under the skin of the chest. A thin, insulated wire called a lead runs from the generator, through a vein, and into the right ventricle of the heart.
When the pacemaker’s generator detects that the heart has not produced its own beat within a set time, it sends a small electrical impulse through the lead. This impulse directly stimulates the muscle of the right ventricle, causing it to contract and pump blood. This action creates a heartbeat that the body would otherwise have missed.
Key Characteristics on an ECG
The electrical activity from a ventricular pacemaker creates a recognizable pattern on an ECG. The primary characteristics include:
- A pacer spike, which is a sharp, vertical line that appears just before the QRS complex. This spike is the graphical representation of the electrical impulse delivered by the pacemaker.
- A wide QRS complex, which represents the contraction of the ventricles. In a paced rhythm, it is wider than 0.12 seconds because the impulse starts within the ventricular muscle and spreads more slowly than it would through the heart’s specialized conduction pathways.
- A left bundle branch block (LBBB) morphology. Because the pacing lead is most often placed in the right ventricle, this chamber is activated first. The electrical impulse then travels across to the left ventricle, creating this specific pattern.
- ST-T wave discordance. The ST segment and T wave, which show the ventricles resetting electrically after contraction, will point in the opposite direction of the main deflection of the wide QRS complex.
Comparison with Other Pacing Modes
Atrial pacing is used when the heart’s natural pacemaker is not functioning correctly, but the conduction pathway to the ventricles is intact. On an ECG, this appears as a pacer spike immediately followed by a P wave, which represents atrial contraction. The subsequent QRS complex will look narrow and normal, as the signal travels down the heart’s natural conduction system.
Dual-chamber pacing involves placing leads in both the right atrium and the right ventricle. This system is designed to more closely mimic the heart’s natural timing by coordinating the contraction of the atria and ventricles. The ECG can show two pacer spikes: one before the P wave and a second before the QRS complex.
Identifying Pacemaker Malfunctions
While pacemakers are reliable, malfunctions can occur, and the ECG is a primary tool for their detection.
Failure to Capture
This occurs when the pacemaker delivers an electrical impulse, visible as a pacer spike, but the heart muscle does not respond with a contraction. On the tracing, a pacer spike will be seen that is not followed by the expected P wave or QRS complex. This indicates the electrical stimulus was not strong enough to depolarize the heart muscle.
Failure to Sense
Also known as undersensing, this issue happens when the pacemaker fails to detect a native heartbeat and fires unnecessarily. Pacemakers are designed to sense the heart’s own electrical activity and inhibit pacing when a natural beat occurs. The ECG will show pacer spikes occurring at inappropriate times, sometimes appearing shortly after a natural QRS complex.
Oversensing
Oversensing occurs when the pacemaker incorrectly identifies electrical signals that are not heartbeats, such as muscle tremors, as intrinsic cardiac activity. This misinterpretation can cause the pacemaker to withhold pacing when it is needed. On an ECG, this may appear as sudden pauses in the rhythm where pacer spikes that should be present are missing.