QT Interval Ventricular Electrical Duration

QT Interval Ventricular Electrical Duration is the measured interval on the surface electrocardiogram extending from the onset of the QRS complex to the termination of the T wave, representing the total time required for the ventricular myocardium to complete both depolarization and repolarization. It encompasses the entire electrical cycle of ventricular activation and recovery, from the earliest activation of the interventricular septum through the final restoration of the resting membrane potential across the ventricular mass.

Physiological Origin

Composite of Depolarization and Repolarization

The QT interval is not a single physiological event but rather the summation of two sequential processes: ventricular depolarization, represented by the QRS complex, and ventricular repolarization, represented by the ST segment and T wave. Its duration therefore reflects the combined time course of ionic currents responsible for both the rapid upstroke and the slower recovery phases of the ventricular action potential.

Cellular Action Potential Duration

At the cellular level, the QT interval corresponds closely to the duration of the ventricular myocyte action potential, from the initial rapid depolarization mediated by fast inward sodium current through the plateau phase sustained by calcium influx, and finally to phase 3 repolarization driven by outward potassium currents. The surface QT interval is, in effect, a body-surface aggregate reflection of these underlying transmembrane events across the ventricular myocardium.

Dependence on Ion Channel Function

Because the interval depends on the coordinated timing of multiple ionic currents, particularly the potassium currents responsible for repolarization, its duration is sensitive to any factor that alters the kinetics of these channels, including changes in autonomic tone, electrolyte concentration, and the intrinsic properties of the channels themselves.

Boundaries of the Interval

Onset

The QT interval begins at the first deflection of the QRS complex, marking the initiation of ventricular depolarization as the electrical impulse first activates working ventricular myocardium.

Termination

The interval ends at the point where the T wave returns to the isoelectric baseline, marking the completion of ventricular repolarization across essentially the entire ventricular mass.

Rate Dependence

Influence of Heart Rate

The duration of ventricular repolarization, and therefore the QT interval, shortens as heart rate increases and lengthens as heart rate decreases, because the duration of the myocyte action potential itself is rate dependent, adapting to the interval available between successive cardiac cycles.

Rate-Corrected QT

To allow comparison across different heart rates, the measured QT interval is mathematically adjusted using the preceding RR interval, yielding a rate-corrected value.

This correction reflects the physiological principle that the electrical duration of the ventricular cycle scales with the cycle length rather than remaining fixed, allowing the underlying repolarization duration to be assessed independent of the immediate heart rate.

Vectorial and Temporal Representation

Composite Interval Diagram

The QT interval spans the entirety of the depolarization vector sequence associated with the QRS complex and the repolarization vector sequence associated with the T wave, making it a temporal rather than a purely vectorial representation, unifying both electrical processes into a single measured duration.

Physiological Determinants

Autonomic Influence

Sympathetic and parasympathetic activity modulate the QT interval indirectly through their effects on heart rate and directly through effects on ion channel conductance, altering the balance of currents that determine both depolarization and repolarization timing.

Electrolyte Sensitivity

The repolarization phase, and thus the QT interval, is particularly sensitive to extracellular concentrations of potassium and calcium, since these ions directly participate in the currents governing phases 2 and 3 of the ventricular action potential.

Functional Significance of the Representation

Marker of Total Ventricular Electrical Activity

The QT interval functions as the electrocardiographic representation of the complete electrical cycle of the ventricles, integrating both the activation sequence and the recovery sequence into a single measurable duration.

Reflection of Refractoriness

Because repolarization determines when ventricular tissue regains excitability, the QT interval indirectly represents the duration of the ventricular refractory period, linking this measured interval to the electrical vulnerability and recovery timing of the ventricular myocardium.