What is the significance of a high R on T (R wave on the T wave) in an electrocardiogram (ECG)?

The Significance of R on T Phenomenon in ECG

The R on T phenomenon represents a potentially dangerous arrhythmogenic pattern where a premature ventricular complex (PVC) occurs during the vulnerable period of ventricular repolarization (T wave), which can trigger life-threatening ventricular arrhythmias, particularly in patients with underlying cardiac disease.

Electrophysiological Basis

The R on T phenomenon involves two critical components:

1. Timing: A premature ventricular complex (PVC) occurs during the vulnerable repolarization period of the preceding beat (T wave)

2. Mechanisms: Two distinct processes can occur 1:

   • R-to-T mechanism: A PVC arising focally from one region propagates into another region with delayed recovery
   • R-from-T mechanism: The PVC is generated directly from the repolarization gradient manifesting as the T wave

Clinical Significance

High-Risk Scenarios

• Acute Myocardial Infarction: R on T phenomenon can trigger polymorphic ventricular tachycardia (PVT) during the hyperacute phase of myocardial infarction 2

  • These "ischemic PVTs" are typically triggered by PVCs with very short coupling intervals
  • They are not pause-dependent and occur with normal QT intervals

• Structural Heart Disease: Particularly dangerous in patients with:

  • Ventricular scarring
  • Cardiomyopathies
  • Severe left ventricular hypertrophy

• Repolarization Abnormalities: Especially concerning in:

  • Long QT syndromes (congenital or acquired)
  • Electrolyte disturbances (particularly hypokalemia)
  • Brugada syndrome
  • Catecholaminergic polymorphic ventricular tachycardia

Risk Stratification

While early observations suggested R on T was highly likely to initiate sustained ventricular tachyarrhythmias, more recent evidence provides a more nuanced view 3:

• R on T represents only a small proportion of total PVCs in post-MI patients (approximately 1.8% of total PVCs over 24 hours) 4

• R on T-initiated ventricular tachycardias tend to be faster than non-R on T-initiated VTs (374 ± 56 ms vs 411 ± 69 ms) 4

• The phenomenon appears more frequently during thrombolysis than after completion in acute MI patients 4

Diagnostic Considerations

When evaluating an ECG with R on T phenomenon:

1. Assess QT interval:

   • Normal QT: Consider ischemia, electrolyte abnormalities, Brugada syndrome
   • Prolonged QT: Consider torsades de pointes, congenital/acquired LQTS

2. Evaluate for T-wave abnormalities:

   • T-wave inversions in lateral leads (V5-V6) are particularly concerning 5
   • T-wave alternans indicates latent instability of repolarization predictive of malignant arrhythmias 5

3. Look for other high-risk markers:

   • Pathological Q waves (Q/R ratio ≥0.25 or ≥40 ms in two or more contiguous leads) 5
   • ST-segment abnormalities
   • QRS widening or bundle branch blocks

Management Implications

1. Acute Setting:

   • Immediate correction of electrolyte abnormalities (especially potassium)
   • Discontinuation of QT-prolonging medications if applicable
   • Antiarrhythmic therapy as appropriate (beta-blockers, amiodarone)
   • Treatment of underlying ischemia if present

2. Long-term Management:

   • Risk stratification for sudden cardiac death
   • Consider implantable cardioverter-defibrillator in high-risk patients
   • Address modifiable risk factors

Clinical Pitfalls to Avoid

1. Not all R on T PVCs lead to sustained arrhythmias: The phenomenon itself is not always dangerous, particularly in the absence of structural heart disease

2. Context matters: R on T is more concerning in acute ischemia, long QT syndromes, and structural heart disease than in otherwise healthy individuals

3. Differentiate types of polymorphic VT: "Ischemic PVT" differs from "infarct-related torsades de pointes" in terms of pathophysiology and ECG manifestations 2

4. Look beyond simple interval measurements: Modern risk assessment includes evaluation of interlead heterogeneity of R- and T-wave morphology 6

The R on T phenomenon remains an important ECG finding that warrants careful clinical evaluation, particularly in patients with known or suspected cardiac disease, as it may represent an early warning sign of electrical instability and risk for sudden cardiac death.