Pacing Modes and the NBG Code

Each position in the code answers a specific question about the device's behavior.

Position I: Chamber Paced. Where does the device deliver its stimulus? Options: A (atrium), V (ventricle), D (dual, both chambers), O (none).

Position II: Chamber Sensed. Where does the device listen for intrinsic activity? Same options: A, V, D, O.

Position III: Response to Sensing. What does the device do when it detects an intrinsic beat? I (inhibited: it withholds the pacing output), T (triggered: it delivers a stimulus in response), D (dual: both inhibition and triggering, depending on the chamber), O (none: it ignores sensing entirely).

Position IV (optional): Rate Modulation. R indicates rate-responsive pacing, where the device adjusts the pacing rate based on a sensor that detects physical activity. Absent or O means no rate response.

Position V (optional, rarely used): Multisite pacing. This position is seldom referenced in routine clinical practice.

VVI is the simplest and most common backup mode. The first V: paces the ventricle. The second V: senses the ventricle. The I: inhibits pacing when a native ventricular beat is detected.

The device watches the ventricle continuously. If an intrinsic QRS appears within the programmed interval, the timer resets and no stimulus is delivered. If the interval expires without a sensed beat, the device fires a ventricular pacing spike.

VVI is appropriate when atrial pacing is unnecessary or impossible. The most common scenario is chronic atrial fibrillation with a slow ventricular response. The atrial rhythm is chaotic and cannot be tracked, so single-chamber ventricular pacing is sufficient.

The limitation is the absence of AV synchrony. In patients with sinus rhythm, VVI pacing means the atria and ventricles contract independently. The atrial "kick" (the extra filling contributed by atrial contraction just before ventricular systole) is lost. Some patients experience fatigue, dyspnea, and pulsatile neck veins from retrograde VA conduction. This constellation is called pacemaker syndrome.

AAI paces the atrium, senses the atrium, and inhibits on sensing a native P wave. The ventricle is untouched; the device relies entirely on intact AV conduction to activate the ventricles after each atrial event.

The ideal candidate has isolated sinus node dysfunction with completely normal AV conduction. The sinus node is too slow, but once the atrium is paced, the impulse travels through the AV node and His-Purkinje system normally.

AAI is rarely used as a standalone mode in practice. Even patients with apparently normal AV conduction at implant may develop AV block over time. If AV block occurs in a patient with only an atrial lead, the device cannot rescue the ventricle. For this reason, most patients who need atrial pacing receive a dual-chamber system programmed to a mode that can pace the ventricle if needed.

DDD is the workhorse mode for patients with AV block and intact sinus rhythm. It senses and paces both the atrium and the ventricle.

In DDD, the device tracks intrinsic atrial activity. When it senses a P wave, it starts a programmable AV delay (analogous to the PR interval, typically 120 to 200 ms). If the ventricle does not produce a native QRS by the end of this delay, the device paces the ventricle. This preserves AV synchrony: every atrial beat, whether paced or sensed, is followed by a ventricular event at a physiologic interval.

DDD can operate in four combinations depending on what the heart provides. If the sinus node fires on time and AV conduction is intact, the device senses both chambers and does nothing (AS-VS). If the sinus node fires but AV conduction fails, the device senses the atrium and paces the ventricle (AS-VP). If the sinus node is too slow but AV conduction works, the device paces the atrium and the native QRS follows (AP-VS). If both the sinus node and AV conduction fail, the device paces both chambers (AP-VP).

DDD mode has a vulnerability. Because the device tracks every sensed atrial event and paces the ventricle after the AV delay, a rapid atrial rhythm can drive the ventricle fast. If a patient in DDD mode develops atrial fibrillation at 300 bpm, the device would attempt to track every atrial signal up to the programmed upper rate limit, potentially pacing the ventricle at 120 to 130 bpm.

Mode switching solves this problem. When the device detects a sustained rapid atrial rate above a programmed threshold (typically 170 to 200 bpm), it automatically switches from a tracking mode (DDD) to a non-tracking mode such as DDI or VVI. In the non-tracking mode, the device ignores the chaotic atrial signals and paces the ventricle at the lower rate limit.

When the atrial rate drops back to normal (the atrial fibrillation terminates or the atrial flutter breaks), the device recognizes the change and switches back to DDD. This transition is automatic and requires no intervention.

Adding "R" to any mode (DDDR, VVIR, AAIR) enables rate-responsive pacing. The device uses a sensor to detect physical activity and increases the pacing rate accordingly.

The most common sensor is an accelerometer built into the pulse generator. It detects body movement and vibration. When the patient walks, climbs stairs, or exercises, the accelerometer signals the device to shorten the pacing interval and increase heart rate, mimicking the normal chronotropic response.

A second sensor type measures minute ventilation by tracking transthoracic impedance changes with each breath. As the patient breathes faster and deeper during exertion, the device detects the change and raises the rate. Minute ventilation sensing tends to track metabolic demand more closely than accelerometry alone.

Rate response is essential for patients with chronotropic incompetence: the sinus node cannot increase its rate appropriately with exertion. Without rate response, a patient on a treadmill would be stuck at the lower rate limit of 60 bpm despite maximal effort. DDDR or VVIR allows the pacing rate to rise to a programmed upper sensor rate (e.g., 120 to 130 bpm) during exercise.

A pacing spike is a small, sharp vertical artifact on the surface ECG that marks the instant the device delivers its stimulus. An atrial pacing spike appears immediately before the P wave. A ventricular pacing spike appears immediately before the QRS complex.

When the right ventricular apex is paced, the depolarization wavefront begins in the RV and spreads leftward across the septum to the LV. This mimics the activation pattern of a left bundle branch block. The paced QRS is wide (typically >120 ms) and has an LBBB morphology: a dominant S wave in V1 and a broad R wave in the lateral leads.

Fusion beats occur when a native depolarization and a paced stimulus activate the ventricle simultaneously. The resulting QRS is intermediate in morphology between the fully paced and the fully native complex. Fusion confirms that the device is sensing appropriately (it almost inhibited because an intrinsic beat was arriving) and that the pacing stimulus is capturing tissue (it contributed to the QRS). Fusion is a sign of normal device function.