ECG criteria
- Position I — Chamber Paced: V (ventricle), A (atrium), D (dual), O (none)
- Position II — Chamber Sensed: V, A, D, or O
- Position III — Response to Sensed Event: I (inhibition), T (triggering), D (dual — I and T), O (none)
- Position IV — Programmable Functions: O — none, P — simple programmability, M — multi-programmability, C — communication, R — rate modulation; in clinical notes, usually only R is used or the fourth position is omitted
- Position V — Multisite Pacing (pacing in more than one site of the atrium and/or ventricle): O, A, V, D — rarely used, often omitted in everyday practice
- On the ECG — a pacing artifact (spike) preceding P (atrial pacing) or QRS (ventricular pacing)
- During conventional right ventricular myocardial pacing, especially from the apex, — a wide QRS with LBBB-like morphology, often with leftward/superior axis deviation; with His-bundle pacing or left bundle branch area pacing, the morphology differs and QRS may be relatively narrow
- During biventricular pacing (CRT) — QRS is often narrower than during isolated RV apex pacing, but width and morphology are variable and do not confirm CRT efficacy by themselves
What it looks like on ECG
The NBG code itself is not an ECG sign, but a letter notation of the mode that the physician sees in the discharge summary or programmer, while evaluating its ECG manifestations on the tracing.
| Mode | What is paced | What is seen on the ECG |
|---|---|---|
| AAI | atrium, atrial sensing, inhibition upon native P wave | narrow spike preceding P, native QRS (narrow, barring blocks) |
| VVI | ventricle, ventricular sensing, inhibition upon native QRS | during conventional myocardial pacing (most commonly from the RV apex) — a spike preceding a wide QRS resembling left bundle branch block; during conduction system pacing (His bundle, left bundle branch area), QRS may be narrow or of a different shape. The pacemaker does not track P. In AV block, P and paced QRS are usually dissociated, but with preserved AV conduction, native P waves may be conducted to QRS; fusion and pseudofusion complexes are possible |
| DDD | both chambers are paced and sensed, dual response (I and T) | can be: atrial spike only with native AV conduction; ventricular spike only following native P (atrial-tracking mode); two consecutive spikes with a programmed AV delay — with native AV conduction, the ventricular spike is inhibited |
| VVIR, DDDR | same + rate adaptation via a sensor (accelerometer, minute ventilation) | morphology is the same as in VVI/DDD; R allows the sensor to increase the pacing rate during exertion when needed; in dual-chamber modes, the rate may also be determined by tracking atrial activity |
A pacing spike is a short high-frequency artifact, usually lasting less than a few milliseconds (pulse duration depends on the device and programming, often about 0.2–1.0 ms); spike visibility varies by lead and recording settings. The interval from the spike to the onset of P or QRS reflects myocardial excitation latency following the pulse; its assessment depends on the pacing site, pulse parameters, and recording setup.
Mechanism
The pacemaker delivers an electrical impulse to a heart chamber via a lead, causing depolarization from the lead contact point rather than the physiological pacemaker. During myocardial pacing distant from the conduction system (e.g., right ventricular apex), excitation spreads through the myocardium slower than via the His-Purkinje system, forming a wide deformed QRS; during pacing of the conduction system itself (His bundle, left bundle branch area), excitation follows physiological pathways, and QRS may remain narrow. The sensing function allows the device to 'see' the chamber's intrinsic electrical activity and either inhibit the scheduled pulse (inhibition) or synchronize the next pulse with it (triggering). Rate adaptation (R) adds a sensor that increases the pacing rate during physical exertion, mimicking sinus node function.
Causes
Mode selection is determined by the pacing indication:
- sick sinus syndrome — AAI(R) in patients with reliably preserved AV conduction or DDD(R) with algorithms to minimize unnecessary ventricular pacing; if there is a risk of AV block development, DDD(R) is preferred
- complete AV block in permanent atrial fibrillation — VVI(R)
- complete or high-grade AV block with preserved sinus rhythm usually warrants DDD(R); in bifascicular block, permanent pacing is indicated only in the presence of guideline-recommended symptoms or evidence of significant AV conduction impairment (documented intermittent/high-grade AV block, pathological infra-Hissian conduction on EPS), rather than as an isolated ECG finding
- cardiac resynchronization therapy (CRT) — biventricular pacing, considered based on a combination of guideline criteria: typically in symptomatic heart failure with reduced ejection fraction despite optimal medical therapy, sufficiently wide QRS, especially with typical left bundle branch block morphology; specific thresholds depend on rhythm (sinus/AF) and clinical situation. CRT-P/CRT-D denotes the device type, while the pacing mode is additionally coded via NBG
Differential diagnosis
- Spontaneous wide QRS in left bundle branch block without a spike — differentiate by the absence of a pacing spike preceding the complex
- Ventricular extrasystole — not related to the programmed pacemaker rate and not preceded by a spike
- Pseudofusion and fusion ventricular complexes (fusion beats) — QRS of intermediate morphology between native and paced, not a pacemaker malfunction, but a normal phenomenon during competition between native and paced rhythms
- Failure to capture (absence of myocardial response to a spike) — differentiate by the presence of a spike without a subsequent P or QRS
Clinical significance and management
Understanding the NBG code is necessary for a physician to correctly interpret the ECG in a patient with an implanted device: without knowing the programmed mode, it is easy to confuse normal pacemaker operation (e.g., VVI standby mode with rare spikes) with malfunction. When analyzing an ECG, the physician must: clarify the device type and mode from the discharge summary or interrogate the patient about the model; evaluate the presence and regularity of spikes, their relationship with P and QRS; check for signs of capture failure (spike without response) and sensing failure. Two mechanisms are distinguished here: undersensing — the pacemaker fails to recognize intrinsic electrical activity and delivers a pulse too early or over a native event; oversensing — the pacemaker recognizes noise or an inappropriate signal as cardiac activity and inappropriately inhibits pacing. A single ECG often cannot reliably differentiate these mechanisms — if sensing dysfunction is suspected, urgent device interrogation with a programmer and assessment of the patient's status are required. In pacemaker patients with suspected myocardial infarction, left bundle branch block QRS morphology complicates direct ST-elevation assessment — modified Sgarbossa (Smith) criteria are used for this purpose.
Common interpretation errors
- Confusing a pacing spike with an artifact or a Q wave
- Interpreting normal standby mode operation (rare spikes during native rhythm) as bradycardia or malfunction
- Failing to consider that in DDD, the spike preceding QRS may be absent if only the atrial channel fired and AV conduction is native
- Evaluating ST changes in a paced QRS without applying Sgarbossa/Smith criteria
- Confusing a fusion beat with a capture defect, and oversensing with undersensing without device interrogation
1. What does the letter I mean in the third position of the NBG code?
I (inhibition) means that upon detecting an intrinsic signal in the sensed chamber, the scheduled pacing pulse is inhibited.
2. What QRS morphology is typical for conventional myocardial pacing from the right ventricular apex?
Excitation from the right ventricular apex spreads slowly through the myocardium toward the left ventricle, forming a wide QRS complex resembling left bundle branch block, often with a left/superior axis deviation, although this is not a mandatory sign. With His-Purkinje system pacing, the QRS morphology may differ.
3. Which pacing mode is most likely in a patient with permanent atrial fibrillation and complete AV block?
In chronic atrial fibrillation, atrial pacing and sensing are pointless, so single-chamber ventricular pacing VVI(R) is used.
Frequently asked questions
What do the letters in the VVI code mean?
The first V stands for ventricular pacing, the second V for ventricular sensing, and I for inhibition (the output is inhibited when an intrinsic complex is detected). This is the classic mode for permanent atrial fibrillation with AV block.
How does DDD differ from VVI on an ECG?
In VVI, a spike and a wide QRS appear without tracking the atrial P wave: in AV block, P and QRS are usually dissociated, whereas with intact AV conduction, intrinsic P waves may be conducted to the paced QRS complexes. In DDD, the pacemaker tracks the intrinsic P wave and can synchronously trigger a ventricular output with a programmed AV delay, or operate in both chambers with two consecutive spikes.
Is it possible to determine the full pacemaker code from a single ECG?
No, the active pacing and sensing chambers can be suggested from the ECG by the presence and location of the spikes, but the exact programmed mode and rate response must be confirmed via patient documentation or device interrogation using a programmer.
Why is the letter R needed in the code (VVIR, DDDR)?
The letter R stands for rate response: the device increases the pacing rate during physical exertion based on a signal from a built-in sensor, compensating for the inability of the intrinsic sinus node to accelerate. In dual-chamber modes, the ventricular rate can be determined both by tracking atrial activity and by sensor adaptation.
Is it dangerous if rare pacemaker spikes are visible on the ECG?
Not dangerous on its own: this is normal operation of the demand mode, where the intrinsic rhythm inhibits most of the scheduled impulses. Danger arises when a spike is present but there is no myocardial response (P or QRS) following it—which is a sign of loss of capture—or if the pacemaker inappropriately inhibits impulses due to oversensing.
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Related terms
More in the section "Cardiac Pacing"
Sources
- Goldberger's Clinical Electrocardiography, latest edition
- AHA/ACCF/HRS recommendations for the standardization and interpretation of the electrocardiogram, 2009
- Orlov V.N. Guide to Electrocardiography
Educational material for medical students and physicians. Does not replace clinical guidelines and physical examination; the ECG must always be interpreted in conjunction with the clinical picture.