ECG criteria
- Premature ventricular contraction (PVC): typically a wide, bizarre QRS (often ≥ 120 ms) unlike a normal sinus complex; the preceding P wave is usually absent or unrelated to the QRS, but this feature is not mandatory — supraventricular extrasystole with aberrancy must be ruled out
- The coupling interval is measured from the onset of the preceding (sinus) QRS to the onset of the ectopic QRS; it is shorter than in a typical late-diastolic PVC. There is no fixed universal threshold in ms for the «R on T» phenomenon — the diagnosis is determined by the actual overlap of the extrasystole's QRS onto the T wave, taking into account heart rate and QT duration
- Visually, the R wave (or another component of the QRS) of the extrasystole is superimposed on the ascending, peak, or descending limb of the T wave
- The vulnerable period corresponds to the relative refractory phase of the ventricles with heterogeneous recovery of excitability — most commonly the peak and descending limb of the T wave
- Evaluate whether the extrasystole is isolated or triggers a run — a burst of ventricular tachycardia, polymorphic VT, or ventricular fibrillation
- Check the background: prolonged QT, ischemia, electrolyte disturbances — factors that increase the likelihood and danger of such timing
What it looks like on ECG
Against the background of sinus rhythm, a normal P-QRS-T complex is registered, and immediately following it, without waiting for the T wave to end, a wide, bizarre QRS of a premature ventricular contraction appears. The R wave of this extrasystole graphically overlaps with the T wave of the preceding contraction — hence the name «R on T».
The key point is a short coupling interval: the distance from the onset of the previous sinus QRS to the onset of the extrasystole is noticeably shorter than in a regular (late-diastolic) premature ventricular contraction, which typically appears after most of the T wave has passed, closer to the next diastole. There is no rigid millisecond threshold for the «R on T» phenomenon — the decisive factor is the actual overlap of the QRS onto the T wave, rather than a specific millisecond value of the coupling interval.
| Sign | Regular VES | PVC of «R on T» type |
|---|---|---|
| Time of occurrence | after most or the end of the T wave | on the T wave of the previous complex |
| Coupling interval | longer | shorter |
| Ventricular cycle phase | closer to the completion of excitability recovery | phase of heterogeneous relative refractoriness (vulnerable period) |
| Risk of VT/VF triggering | usually lower, but not zero | increased |
The phenomenon can be observed in any lead where the T wave of the sinus complex and the morphology of the subsequent extrasystole are simultaneously well visualized. It is useful to compare multiple leads — sometimes the overlap is more noticeable in one of them due to the shape and amplitude of the T wave.
Mechanism
During depolarization (QRS) and early repolarization, the majority of the ventricular myocardium is non-excitable. With further repolarization, predominantly in the middle and late parts of the T wave, a relative refractory period develops with heterogeneous, «patchy» recovery of excitability in different myocardial fibers: some cells are already ready for a new excitation, while others are not yet. This period of electrical heterogeneity is considered the vulnerable period of repolarization; it does not strictly coincide with fixed segments of the QRS or T, but is stretched over time and varies between fibers.
If an ectopic ventricular impulse arrives at this moment, it encounters the myocardium in a state of electrical heterogeneity and can cause conduction block in some areas and wave propagation in others — fractionation of excitation. This creates conditions for re-entry. When the QT interval is prolonged, triggered activity and early afterdepolarizations are added to this mechanism. The result can be polymorphic ventricular tachycardia or ventricular fibrillation. The greatest vulnerability usually corresponds to the peak and descending limb of the T wave; landing in the early ascending part of the T wave more often falls within the still-persisting refractoriness of the majority of the myocardium and may be blocked without causing widespread excitation.
Causes
- Acute ischemia and myocardial infarction (acute phase, reperfusion period)
- Prolonged QT interval — congenital or acquired (drugs, electrolyte disturbances)
- Severe hypokalemia, hypomagnesemia
- Marked left ventricular hypertrophy with electrical instability
- Cardiomyopathies (dilated, hypertrophic, arrhythmogenic right ventricular cardiomyopathy)
- Proarrhythmic effect of certain antiarrhythmics and other QT-prolonging drugs
- Severe heart failure with myocardial electrical instability
Commotio cordis is worth mentioning separately: a blow to the precordial area occurring during the short vulnerable window of repolarization can directly induce ventricular fibrillation. This is neither a premature ventricular contraction nor a classic «R on T» phenomenon, and therefore such a mechanism should not be automatically attributed to this phenomenon.
Differential diagnosis
- Regular (late-diastolic) premature ventricular contraction — occurs after most of the T wave, coupling interval is longer; this does not rule out arrhythmogenic risk in structural heart disease.
- Parasystole — ectopic complexes with a varying coupling interval and signs of entrance block; coincidence with the T wave may be accidental rather than systematic.
- Atrial extrasystole with P′ wave superimposition on the T wave — the P′ wave is usually altered in shape or hidden within the T wave; the QRS is typically narrow and resembles a sinus complex, but with aberrant conduction (especially with a very early P′), it can be wide and bizarre, mimicking a PVC. To differentiate, look for an altered P′ before or within the T wave, compare QRS morphology across multiple leads, and evaluate the activation sequence and the nature of the pause following the complex.
- Artifact or interference distorting the T wave shape — true ectopic QRS is absent, no complex deformation.
Clinical significance and management
The «R on T» phenomenon is traditionally viewed as a marker of increased ventricular electrical instability and a risk factor for triggering ventricular tachycardia, polymorphic VT, and ventricular fibrillation, especially in the setting of acute myocardial ischemia or a prolonged QT interval. The clinical significance of a single episode is determined primarily by the underlying disease rather than the phenomenon as an isolated finding.
Upon detecting such a phenomenon, the physician must:
- evaluate the clinical context: acute coronary syndrome, electrolyte disturbances, use of QT-prolonging drugs;
- measure QTc on the sinus complexes preceding the extrasystole, preferably indicating the correction method (Bazett's or Fridericia's formula), and evaluate the degree of prolongation: QTc ≥500 ms is a particularly alarming value with a high risk of torsades de pointes; QTc >450 ms in men and >460 ms in women is generally considered provisionally prolonged (specific thresholds depend on the source and correction method); an increase in QTc during therapy by approximately ≥60 ms from baseline is also regarded as a significant proarrhythmic signal;
- check potassium, magnesium, calcium;
- in acute conditions, with symptoms, prolonged QT, or repeated/complex ventricular extrasystoles, provide rhythm monitoring and assess whether the extrasystole transitions into runs of VT or polymorphic VT/ventricular fibrillation; the scope and duration of monitoring are determined by the clinical context;
- if acute coronary syndrome is suspected, follow the ACS algorithm immediately; determine reperfusion tactics and timing based on the clinical picture and ECG criteria of occlusion, rather than the mere fact of a premature ventricular contraction with the «R on T» phenomenon;
- correct reversible causes (electrolytes, discontinuation of proarrhythmic drugs) according to the general principles of ventricular arrhythmia management.
Common interpretation errors
- Labeling any premature ventricular contraction as «R on T» without assessing the actual position of the QRS relative to the T wave.
- Failing to measure QTc on sinus complexes, even though a prolonged QT is a key background factor for this phenomenon.
- Confusing it with parasystole without evaluating coupling intervals across a series of complexes.
- Mistaking a wide aberrant QRS of an atrial extrasystole for a ventricular one without searching for an altered P′ wave.
- Drawing prognostic conclusions from a single episode without considering the clinical picture (acute ischemia, electrolytes, medications).
1. Which feature distinguishes the «R on T» phenomenon from a regular late-diastolic premature ventricular contraction?
The key criterion is a short coupling interval, due to which the QRS of the extrasystole superimposes on the T wave of the preceding sinus complex.
2. Why does an extrasystole falling in the area of the peak and descending limb of the T wave increase the risk of ventricular tachycardia?
In this zone, the refractoriness of myocardial fibers recovers extremely unevenly, and an ectopic impulse can trigger fragmentation of the excitation wave and circular movement (re-entry).
3. Which background factor particularly increases the significance of the "R on T" phenomenon?
A prolonged QT and acute ischemia widen the vulnerable period and ventricular electrical heterogeneity, enhancing the proarrhythmic potential of the phenomenon.
Frequently asked questions
How does the «R-on-T» phenomenon differ from a regular ventricular extrasystole?
A typical ventricular extrasystole usually occurs after the greater part of the T wave, when excitability has partially recovered; this does not rule out risk in structural heart disease. With the «R-on-T» phenomenon, the extrasystole lands directly on the T wave of the preceding complex — during the vulnerable period of uneven refractoriness, which increases the risk of re-entry arrhythmias.
Is the «R-on-T» phenomenon dangerous?
It is considered a marker of increased ventricular electrical instability and a risk factor for triggering ventricular tachycardia and ventricular fibrillation, especially in the setting of acute ischemia or prolonged QT. The degree of danger of a specific episode depends on the underlying disease, rather than the phenomenon alone.
Should antiarrhythmic drugs be prescribed immediately for the «R-on-T» phenomenon?
This decision is made by the physician, taking into account the clinical presentation, the cause of the extrasystole, and the dynamics on the monitor. The priority is to identify and correct a reversible cause (ischemia, electrolyte imbalances, proarrhythmic medication) rather than reacting solely to the phenomenon itself.
Can the «R-on-T» phenomenon be seen on a standard resting ECG?
Yes, if the extrasystole happens to occur during the recording, but it is more commonly detected during Holter monitoring or continuous telemetry, as the episode is accidental and short-lived.
How to distinguish the «R-on-T» phenomenon from an atrial extrasystole with aberrant conduction?
In an atrial extrasystole preceding a wide QRS or occurring within the preceding T wave, an altered P′ wave can often be found that deforms the contour of the T wave differently than a simple superimposition of the QRS. Helpful approaches include comparing QRS morphology across multiple leads, assessing the sequence of activation, and evaluating the nature of the post-extrasystolic pause.
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Related terms
More in the «Phenomena and Eponyms» section
Sources
- Goldberger A.L. Clinical Electrocardiography: A Simplified Approach
- Hampton J.R. The ECG in Practice
- 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.