Understanding the T Wave Decline to Baseline After Ventricular Repolarization
The T wave declines to baseline after reaching its peak because ventricular repolarization is completing—as the transmembrane potential returns to the resting state of approximately -85 mV across all ventricular cells, the voltage gradients that generated the T wave disappear, causing the ECG to return to the isoelectric baseline (TP segment) in preparation for the next cardiac cycle. 1
Electrophysiological Basis of the T Wave Decline
The T Wave Represents Voltage Gradients During Repolarization
The T wave corresponds to phase 3 (rapid repolarization) of the ventricular action potential, during which the transmembrane potential repolarizes from its plateau voltage of approximately +10 mV back to the resting level of approximately -85 mV. 1
The T wave is generated by interventricular and intraventricular voltage gradients created as ventricular cells undergo sequential repolarization at different times. 1
Repolarization proceeds from epicardium to endocardium (opposite to depolarization direction), with epicardial cells repolarizing first (at the T wave peak) and M cells repolarizing last (at the T wave end). 1, 2
Why the T Wave Declines to Baseline
As repolarization completes and all ventricular myocardial cells return to their resting transmembrane potential of approximately -85 mV, the voltage gradients that generated the T wave disappear. 1
The absence of significant voltage gradients during electrical diastole (from the end of repolarization to the onset of the next depolarization) explains why the TP segment is normally flat and isoelectric—at approximately the same level as the ST segment. 1
The TP segment represents the period when ventricular myocardial cells are at their resting transmembrane potential of approximately -85 mV, with no voltage gradients present to generate deflections on the surface ECG. 1
Temporal Relationship Between T Wave Components and Cellular Events
Specific Timing of Repolarization Events
The peak of the T wave coincides with repolarization of epicardial action potentials (the earliest cells to repolarize). 2
The end of the T wave coincides with repolarization of M cells (the last ventricular cells to repolarize). 2
The action potential duration of the longest M cells determines the QT interval, and the Tpeak-Tend interval serves as an index of transmural dispersion of repolarization. 2
The Descending Limb of the T Wave
The morphology of the T wave, including its descending limb, results from currents flowing down voltage gradients on either side of the M region during phases 2 and 3 of the ventricular action potential. 2
The interplay between opposing forces from epicardium-to-M-region and endocardium-to-M-region voltage gradients determines the T wave height and the characteristics of its ascending and descending limbs. 2
The Isoelectric Baseline Between T Wave and P Wave
Why the Baseline is Isoelectric
The TP segment is isoelectric because all ventricular cells have completed repolarization and are at their resting potential of -85 mV, creating no voltage gradients. 1
This isoelectric state is similar to the ST segment, which is also normally flat because during the plateau phase (phase 2), transmembrane voltage changes slowly and remains at approximately the same level in all ventricular myocardial cells. 1
Preparation for the Next P Wave
The return to baseline voltage is not an active "preparation" for the P wave but rather represents the natural electrical diastole state of the ventricles. 1
The P wave then appears as atrial depolarization begins the next cardiac cycle, independent of the ventricular resting state. 1
Clinical Relevance and Common Pitfalls
Important Considerations
The ST and TP segments should normally be at approximately the same isoelectric level; deviations suggest pathology. 1
During exercise or tachycardia, the TP segment may become difficult to identify due to close approximation of T and P waves at heart rates >130 beats/min, making the P-Q junction the preferred reference point for ST segment measurement. 1
Abnormalities in T wave morphology or failure to return to baseline may indicate primary repolarization abnormalities (changes in cellular repolarization characteristics) or secondary abnormalities (due to altered depolarization sequence). 1
Distinguishing Normal from Abnormal
The U wave, which may follow the T wave, most likely represents an electromechanical phenomenon occurring after repolarization is completed and should not be confused with the T wave decline. 1, 3
Pathological prolongation of the descending T wave limb overlapping with the next P wave can occur with multiple electrolyte disturbances and represents abnormal repolarization. 4