ECG criteria
- Grade 0 — no VPBs
- Grade I — rare monomorphic VPBs, less than 30 per hour of monitoring (alternatively, less than 1 per minute); the threshold should be specified based on the chosen source
- Grade II — frequent monomorphic VPBs, 30 or more per hour (in some editions, more than 30 per hour)
- Grade III — multiform (polytypic) VPBs, reliably differing in morphology within the same lead, provided artifacts, fusion beats, and conduction variations are excluded
- Grade IVa — paired VPBs (couplets), two consecutive VPBs without an intervening sinus complex
- Grade IVb — runs, three or more consecutive VPBs; at a rate of >100/min (≥100/min in some sources), this constitutes a short run of ventricular tachycardia, while at a slower rate in the appropriate clinical context, an accelerated idioventricular rhythm is possible
- Grade V — early VPBs of the "R-on-T" type, where the ectopic QRS complex begins on or coincides with the T wave of the preceding complex, particularly during its vulnerable period
What it looks like on ECG
The classification relies not on a single ECG strip, but on the analysis of continuous monitoring data — historically, the scale was developed for continuous monitoring of patients with myocardial infarction in intensive care units. 24-hour Holter monitoring is convenient for assessing the frequency and diversity of VPBs, but is not mandatory: morphological grades (multiformity, pairing, R-on-T) can be identified on a standard ECG or shorter monitoring strip if the respective complexes are captured.
| Grade | Description |
|---|---|
| 0 | No VPBs registered |
| I | Rare monomorphic VPBs, <30/h (alternatively, <1/min) |
| II | Frequent monomorphic VPBs, ≥30/h |
| III | Multiform (polytypic) VPBs — confirmed varying QRS morphology of extrasystoles within a single lead |
| IVa | Paired VPBs (couplets) — two consecutive VPBs without an intervening sinus complex |
| IVb | Run of VPBs (3 or more consecutively) — short bursts of ventricular tachycardia at a corresponding rate |
| V | Early "R-on-T" VPBs — the ectopic QRS begins on or coincides with the T wave of the preceding complex, particularly during its vulnerable period |
A VPB on an ECG typically presents as a wide QRS complex (usually ≥120 ms). It is generally unaccompanied by a preceding sinus P wave, although the P wave may be buried in the preceding complex or conducted retrogradely. The ST segment and T wave typically show discordance with the main QRS deflection, though this is not an absolute criterion. A compensatory pause often follows a VPB, but it is not mandatory—interpolated and fusion VPBs can occur, where the pause is absent or non-compensated. Morphology is evaluated in the same lead: if the shape of the VPB reliably changes with the exclusion of artifacts, fusion beats, and intraventricular conduction variations, this indicates multiformity (Grade III) rather than mere recording variability.
Mechanism
Ventricular premature beats arise from an ectopic focus of automaticity, triggered activity (early or delayed afterdepolarizations), or re-entry within the ventricular myocardium or the His-Purkinje system, including focal fascicular and parasylvian sources. The Lown classification does not reflect a specific electrophysiological mechanism, but rather categorizes ECG patterns according to the degree of myocardial electrical instability. Monoform and multiform VPBs are purely descriptive ECG signs of QRS morphology in a single lead: they cannot reliably determine the mechanism of extrasystole or the number of ectopic foci, as identical shapes can occur via different mechanisms, and varied shapes may result from variable intraventricular conduction and other non-mechanistic causes. Paired and consecutive VPBs reflect a tendency toward self-sustaining arrhythmia, while the "R-on-T" phenomenon indicates an extrasystole falling into the vulnerable period of repolarization when the myocardium is partially refractory, facilitating the onset of ventricular tachycardia or ventricular fibrillation.
Causes
- Acute myocardial infarction and post-infarction cardiosclerosis (original scope of application)
- Coronary artery disease
- Cardiomyopathies (dilated, hypertrophic)
- Myocarditis
- Valvular heart disease with ventricular overload
- Electrolyte disturbances (hypokalemia, hypomagnesemia)
- Intoxications (digoxin, sympathomimetics, cocaine)
- Idiopathic extrasystole in individuals without structural heart disease
Differential diagnosis
- Supraventricular extrasystole with aberrant conduction — narrow or moderately widened QRS, often with a deformed P wave preceding the complex; the compensatory pause is usually non-compensated.
- Fascicular or idioventricular tachycardia as standalone rhythms — distinguished by the duration of the run and heart rate, not by the Lown grade.
- Parasystole — characterized by variable coupling intervals and a common multiple rule for inter-ectopic intervals, unlike the relatively fixed coupling interval of typical VPBs.
- Monitor artifacts (motion, electrode) — suspected when findings do not correlate across other channels and leads, with sudden baseline jumps, unrealistic complex morphology, absence of a corresponding pulse or mechanical event (confirmed by pulse oximetry or arterial line), and preserved normal sinus signals in other leads. The presence or absence of a compensatory pause is not a diagnostic criterion, as the pause depends on the underlying rhythm and may be absent even in true VPBs (interpolated or non-fully compensated).
Clinical significance and management
The classification was proposed by Bernard Lown and colleagues in the 1970s as a descriptive system to characterize the complexity of ventricular ectopy in patients post-acute myocardial infarction within intensive care monitoring units. Historically, higher grades (III–V), especially IVb and V, were considered to be associated with an increased risk of ventricular tachycardia, ventricular fibrillation, and sudden cardiac death during the acute phase of infarction.
This is not a linear risk scale: the prognostic significance of individual grades is limited, and an increase in grade should not be interpreted as a sequential escalation of risk. Subsequent studies demonstrated that a high VPB grade alone, without considering left ventricular ejection fraction, the severity of ischemia, and structural heart disease, is a poor predictor of outcomes. Therefore, the classification is primarily used as a descriptive, standardized language when analyzing Holter monitoring. Decisions on further management (echocardiography, stress testing, ischemia evaluation, and if necessary, antiarrhythmic therapy or catheter ablation of symptomatic monomorphic VPBs) are based on the clinical picture, cardiac structure, and overall burden of ectopy rather than the Lown grade alone.
Common interpretation errors
- Assigning frequency-based grades (I, II) based on a single short 10–12-lead ECG recording instead of prolonged monitoring, for which this part of the scale was designed.
- Confusing paired VPBs (IVa, two consecutive complexes) with runs (IVb, three or more) — the distinction lies precisely in the number of consecutive complexes.
- Diagnosing "multiformity" (III) based on varying wave amplitudes due to respiratory baseline wander, artifacts, or fusion complexes rather than true changes in QRS morphology.
- Automatically equating a high grade with high risk without accounting for the ejection fraction and the presence of structural heart disease.
- Confusing the "R-on-T" phenomenon (Grade V) with the normal superimposition of the subsequent sinus P wave onto the T wave.
1. Which Lown grade corresponds to a run of three or more consecutive ventricular premature beats?
IVb — runs of VPBs, three or more consecutive complexes without an intervening sinus beat; at an appropriate rate, this constitutes a short run of ventricular tachycardia.
2. What distinguishes Grade III ventricular extrasystoles from Grades I–II according to Lown?
Grade III is defined by multiformity (polytopy) — reliably different shapes of VPBs within a single lead, in contrast to monomorphic VPBs of Grades I and II.
3. What is the name of the phenomenon corresponding to Lown grade V?
Grade V describes early PVCs where the QRS complex begins on or coincides with the T wave of the preceding beat — the «R on T» phenomenon.
Frequently asked questions
What does Lown grade IVb mean?
Grade IVb represents salvos of ventricular premature beats, i.e., three or more consecutive PVCs without an intervening sinus complex. At a rate exceeding 100 beats per minute, such a salvo constitutes a short run of VT; a slower sequence in the appropriate context may correspond to an accelerated idioventricular rhythm.
How does Lown grade III differ from grade II?
Grade II refers to frequent yet monomorphic (identical in morphology) PVCs, numbering 30 or more per hour. Grade III refers to multiform (polytypic) PVCs: a single lead displays extrasystoles of reliably different morphology, which theoretically may reflect multiple ectopic foci, though this cannot be definitively established from surface QRS morphology alone.
Is Lown grade V extrasystole dangerous?
Grade V (the «R on T» phenomenon) was historically considered the most dangerous, as the extrasystole falls into the vulnerable period of repolarization and can theoretically trigger ventricular tachycardia or ventricular fibrillation. Modern data show that the absolute risk primarily depends on the structural heart disease and ejection fraction, rather than this sign alone.
Is the Lown classification still used in clinical guidelines?
As an independent risk stratification tool, it is practically unused in current ventricular arrhythmia guidelines, but its terminology (paired, salvo, multiform PVCs, «R on T») is still used descriptively in Holter monitoring reports.
Is Holter monitoring necessary to assess Lown grades?
For frequency grades I–II, prolonged monitoring is preferred because PVC frequency is reliably assessed only through extended observation. The remaining features — multiformity, pairing, salvos, R on T — can also be detected on a standard ECG or shorter recording if captured. A 24-hour Holter is not strictly required just to describe a grade.
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Related terms
More in the section "Ventricular Arrhythmias"
Sources
- Goldberger's Clinical Electrocardiography, latest edition
- Surawicz B., Knilans T. Chou's Electrocardiography in Clinical Practice
- 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.