The electrocardiogram (ECG) is a diagnostic tool that records the electrical activity of the heart over time, capturing the sequence of muscle contraction and recovery. The QRS complex and the T wave are the most distinct waves, representing electrical events within the heart’s large, muscular ventricles. The QRS complex is a tall, sharp spike, while the T wave is a low, broad hump, despite both representing the same tissue mass. This striking difference results from fundamental differences in the underlying cellular processes of activation and recovery.
Depolarization and Repolarization: The Cellular Events
The electrical rhythm of the heart is driven by the action potential, a rapid sequence of voltage changes across the cell membrane of heart muscle cells. This activity is fundamentally controlled by the movement of charged particles, or ions, across the cell membrane through specialized channels. Depolarization is the process that initiates muscle contraction and is marked by a sudden, massive influx of positive ions, primarily sodium (\(\text{Na}^+\)), into the cell. This rapid flow of positive charge causes the inside of the cell to become electrically positive, triggering the mechanical contraction of the ventricles, which the QRS complex represents on the ECG.
Repolarization is the subsequent recovery phase, preparing the heart muscle for the next beat, and is represented by the T wave. This recovery is achieved by the outflow of positive ions, predominantly potassium (\(\text{K}^+\)), returning the cell’s interior to its negative resting state. The duration of this process is influenced by the opening and closing kinetics of these ion channels. While the QRS complex captures the moment of electrical activation, the T wave captures the electrical recovery that immediately follows the mechanical contraction.
Why the QRS Complex is Tall and Narrow
The QRS complex’s tall amplitude and narrow duration result from the highly efficient and synchronized spread of the electrical impulse throughout the ventricular muscle mass. This rapid electrical transmission is facilitated by a specialized network of conducting tissue known as the Purkinje system. This network ensures that the electrical signal reaches nearly all ventricular muscle cells simultaneously.
When many cells depolarize simultaneously, their individual electrical forces align to form a single, powerful net electrical vector. This synchronized wave produces a large voltage change recorded at the surface, resulting in the tall amplitude of the QRS complex. Ventricular activation is completed extremely quickly, typically lasting only 80 to 100 milliseconds. This speed, facilitated by the Purkinje fibers, makes the QRS complex narrow, representing a brief, intense electrical event.
Delays in this specialized conduction system, such as a bundle branch block, cause the signal to spread slowly through muscle tissue, resulting in a widened QRS complex. The magnitude is also related to the mass of the ventricles, as more tissue generates a larger signal. The QRS complex is a sharp spike due to this rapid, synchronized electrical event.
Why the T Wave is Small and Wide
The T wave’s contrasting appearance—low amplitude and wide duration—is explained by two distinct physiological factors: the slower speed of the repolarization process itself and the asynchronous recovery pattern of the ventricular tissue. At the cellular level, the process of repolarization is inherently slower than depolarization because it relies on the efflux of potassium ions through voltage-gated potassium channels. These potassium channels open and close much more slowly than the sodium channels responsible for the initial rapid depolarization.
Slower Speed and Wide Duration
Since the recovery phase is a drawn-out process, taking significantly longer than the activation phase, the resulting T wave is spread out over a longer duration. Repolarization does not utilize the high-speed Purkinje system, instead relying on slower cell-to-cell conduction, which further contributes to the wave’s prolonged, wide morphology. This slower, less intense movement of ions over a longer period prevents the T wave from being a sharp, narrow spike.
Asynchronous Recovery and Small Amplitude
The T wave also has a significantly smaller amplitude than the QRS complex due to a phenomenon called asynchronous recovery, or dispersion of repolarization. Repolarization does not occur in the same orderly, synchronized manner as depolarization. Instead of repolarizing in the direction of the initial activation (from the inner layer, or endocardium, to the outer layer, or epicardium), the recovery process reverses.
This reversal in the direction of the electrical recovery wave means that the electrical vectors of repolarization are spread out in time and direction across the ventricular wall. Because the recovery is asynchronous, the electrical forces generated by individual cells are not all pointing in the same strong, unified direction. Instead, these multiple, opposing electrical vectors partially cancel each other out. This vector cancellation results in a much smaller net electrical signal reaching the surface electrodes, which is why the T wave is recorded as a low-amplitude, gentle hump.