Measuring Transmural Dispersion of Repolarization (TDR)
The primary methods to measure TDR include: (1) the T-peak to T-end (TpTe) interval from surface ECG, which provides the most clinically accessible and validated measure; (2) direct measurement from transmembrane action potentials in experimental settings; and (3) analysis of RMS curves from multiple ECG leads, though QT dispersion is NOT a valid measure of TDR.
Clinical ECG-Based Measurement
T-peak to T-end (TpTe) Interval
The TpTe interval is the most practical and validated clinical measure of TDR 1, 2. This measurement works because:
- Full repolarization of epicardium defines the peak of the T wave
- Full repolarization of M cells (mid-myocardial cells) defines the end of the T wave
- The interval between these two points provides a valuable index of transmural dispersion 1
For optimal measurement, use composite leads based on V2, V3, and V4, which show correlation coefficients of 0.99 with simulated global TDR and 0.98 with localized TDR 3. The TpTe measurement demonstrates a correlation coefficient of 0.90 with drug plasma concentration and importantly shows minimal heart rate dependence (correlation 0.02) 3.
TpTe/QT Ratio
The TpTe/QT ratio provides risk stratification, with values >0.28 indicating increased arrhythmia risk 4. In short QT syndrome patients, this ratio averages 0.28 ± 0.03 compared to 0.21 ± 0.02 in controls 4.
Experimental/Research Methods
Direct Transmembrane Action Potential Recording
This gold standard involves simultaneous recording from epicardial, mid-myocardial (M cells), and endocardial layers 5. TDR is calculated as the difference in action potential duration at 90% repolarization (APD90) between these layers. While highly accurate, this method requires:
- Arterially perfused tissue preparations
- Microelectrode recordings from multiple transmural sites
- Controlled experimental conditions 6, 5
RMS Curve Analysis
Root-mean-square (RMS) curves derived from multiple ECG leads provide accurate TDR estimation 7:
- Epicardial lead sets: correlation 0.91 with actual TDR
- Body surface leads: correlation 0.84
- Precordial leads: correlation 0.72
- Optimal 6-lead set: correlation 0.81
The T-wave width from RMS curves significantly outperforms QT dispersion for TDR estimation 7.
What NOT to Use
QT Dispersion - NOT RECOMMENDED
QT dispersion (difference between longest and shortest QT intervals across leads) should NOT be used to measure TDR 8, 9. The AHA/ACCF/HRS guidelines explicitly state:
- Substantial methodological problems exist with QT dispersion measurement
- Correlation between QT dispersion and actual repolarization dispersion is poor (0.11-0.47 depending on lead set) 7
- It is recommended that QT dispersion not be included in routine ECG reports 8, 9
The fundamental issue is that QT dispersion fails to capture the nondipolar signal information necessary to reflect true localized heterogeneity of myocardial repolarization 8.
Additional T-Wave Morphology Parameters
Combined T-wave morphology scoring incorporating:
- T-wave flatness
- T-wave symmetry
- T-wave notching
Shows correlation coefficients of 0.98 with global TDR and 0.92 with localized TDR 3. These parameters are heart rate independent and can augment TpTe interval measurements.
Critical Caveats
When measuring TDR from serial ECGs, standardize:
- Recording equipment (same type if possible)
- Time of day (circadian variation affects measurements)
- Lead placement
- Single reader for consistency 8, 9
The distinction between global versus localized TDR matters—they generate different T-wave morphologies and require different analytical approaches 3.