A heart rhythm is the consistent pattern of electrical activity that triggers the heart muscle to contract and pump blood throughout the body. This electrical sequence normally originates in the heart’s own specialized cells. When this natural system malfunctions, a paced rhythm becomes necessary. A paced rhythm is an artificially generated electrical impulse delivered to the heart to ensure a regular and effective heartbeat.
Defining a Paced Heart Rhythm
The heart’s intrinsic electrical timing is controlled by the sinoatrial (SA) node, often referred to as the natural pacemaker. This node initiates a wave of depolarization that travels through the atria to the atrioventricular (AV) node, ultimately triggering the ventricles to contract. A paced rhythm is required when this sequence is disrupted, resulting in an abnormal heartbeat, known as an arrhythmia.
Common reasons for intervention include bradycardia, where the heart rate slows, preventing sufficient blood and oxygen delivery. Another issue is heart block, where electrical signal transmission between the upper and lower chambers is delayed or completely blocked. In these situations, a specialized device generates an external electrical current to force the heart muscle to depolarize, ensuring a consistent, controlled heart rate.
Components of a Pacing System
The technology responsible for generating a paced rhythm is comprised of two primary components: the pulse generator and the pacing leads. The pulse generator is a small, hermetically sealed device, typically encased in titanium, and implanted beneath the skin, often near the collarbone. It contains the power source, usually a lithium-iodide battery, and the electronic circuitry that functions as the system’s computer.
The leads are thin, insulated wires that connect the pulse generator to the heart muscle. These leads are threaded through a vein and guided into the appropriate chamber or chambers of the heart. At the tip of each lead are electrodes, which serve the dual purpose of sensing the heart’s intrinsic electrical activity and delivering the electrical impulse to the myocardium. This design allows the system to monitor the heart and only intervene when necessary.
Understanding Different Pacing Modes
Pacing modes are specialized settings that determine which heart chambers are stimulated and how the device responds to the heart’s natural activity. These modes are described using a standardized code, where letters indicate the chamber paced, the chamber sensed, and the device’s response to a sensed event. The most common modes fall into either single-chamber or dual-chamber configurations.
Single-chamber pacing involves placing a lead in either the atrium or the ventricle to stimulate that chamber alone. Dual-chamber pacing utilizes leads in both the atrium and the ventricle, allowing the device to coordinate the contractions of the upper and lower chambers. This dual approach is often preferred because it preserves atrioventricular (AV) synchrony, meaning the atria contract just before the ventricles, which optimizes blood flow.
Within these configurations, the device operates in either a “demand” or “asynchronous” response mode. Demand pacing (e.g., VVI or DDD) is the standard for long-term use, as it incorporates a sensing function. The pacemaker monitors the heart’s intrinsic rhythm and only delivers a stimulating impulse if the natural rate falls below a pre-set minimum.
Asynchronous pacing (e.g., VOO or DOO) delivers electrical impulses at a fixed rate regardless of the heart’s own activity. This constant firing prevents the device from sensing the heart’s rhythm. While useful in certain temporary situations, such as during surgery, asynchronous pacing is rarely used for permanent support because the fixed impulses can compete with the heart’s native rhythm, potentially leading to abnormal rhythms.
Identifying Paced Activity on an ECG
A paced rhythm is recognized clinically through an electrocardiogram (ECG) tracing by the presence of a distinct electrical mark. This visual characteristic is a thin, straight, vertical line known as a “pacing spike.” The spike represents the brief electrical impulse generated by the pulse generator as it is delivered to the heart tissue.
The location of the pacing spike immediately indicates which chamber is being stimulated. An atrial-paced rhythm shows the spike occurring right before the P wave, which represents the electrical activity of the atria. Conversely, a ventricular-paced rhythm displays the spike directly preceding the QRS complex, which signifies the depolarization of the ventricles.
In all pacing modes, the spike must be followed by the corresponding electrical wave, a phenomenon called “capture.” If the ventricle is paced, the resulting QRS complex often appears wider and different in shape compared to a natural beat, as the impulse follows an altered path through the heart muscle. Confirming the presence and correct timing of these spikes verifies that the pacing system is functioning as intended.