Why is a flat line registered on the ECG during the plateau phase?

During the plateau phase, the membrane potential of cardiomyocytes remains stable, and the heart's structure is fully depolarized. As a result, there is no potential difference between myocardial regions, the net electrical field around the heart disappears, and an isoelectric line is recorded on the ECG.

Where does the calcium for myocardial contraction come from during this phase?

Unlike skeletal muscle, where calcium is released primarily from the sarcoplasmic reticulum, the myocardium experiences a significant transmembrane calcium influx. During the plateau, $Ca^{2+}$ ions enter cardiomyocytes from the extracellular fluid via the T-tubule system.

Which ECG segments correspond to the plateau phase?

In the atria, the plateau phase (corresponding to slow repolarization) is represented by the PQ segment with a duration of 60 ms. In the ventricles, the plateau corresponds to the ST segment with a duration of 120 ms.

Action Potential Plateau: Cardiac Phase, ECG & Calcium

Action Potential Plateau

plateau phase

For medical students2 min readUpdated 2026-10-10

The action potential plateau is the period during which the membrane potential of cardiomyocytes remains relatively constant. During this phase, calcium ions enter the cardiomyocytes from the extracellular space, which determines the specific contractile features of the myocardium. On the electrocardiogram (ECG), this process is registered as an isoelectric line because there is no potential difference between different regions of the myocardium.

ECG RepresentationIsoelectric line (ST and PQ segments)

Primary IonCalcium ($Ca^{2+}$), partially entering from the extracellular space

Duration in Ventricles120 ms (corresponds to the ST segment)

Contraction LinkActin-myosin interaction regulated by $Ca^{2+}$

What Happens During the Plateau Phase?

During the action potential plateau, cardiomyocyte membranes maintain a sustained depolarization state. At this moment, the entire heart muscle is fully depolarized.

Because excitation simultaneously encompasses a significant portion of the muscle fibers, the net electrical field around the heart is zero. There is no potential difference between different regions of the myocardium. Unlike the rapid depolarization and repolarization phases that form the ECG waves, the plateau phase forms the isoelectric line—the flat baseline between the waves.

Contraction Mechanism and the "Calcium Influx"

The general mechanism of myocardial contraction, similar to skeletal muscle, is based on the interaction of actin and myosin proteins triggered by $Ca^{2+}$ ions. However, the sources of this calcium differ:

In skeletal muscle, $Ca^{2+}$ ions are delivered to the contractile proteins predominantly from the sarcoplasmic reticulum.
In the myocardium, calcium comes partially from the sarcoplasmic reticulum and partially from the extracellular environment through the T-tubule system.

The influx of $Ca^{2+}$ ions into cardiomyocytes from the extracellular space is known as the calcium influx (or calcium trigger). It occurs during the plateau phase of the action potential and accounts for the unique electromechanical coupling features of the myocardium.

ECG Representation: Atria and Ventricles

On the electrocardiogram, the plateau phase is recorded as segments—portions of the isoelectric line.

PQ segment (in the atria): Reflects slow repolarization (the plateau phase) in the atria. The duration of the segment is 60 ms. During this period, the atria are depolarized, and the atrioventricular delay occurs—slowing the conduction of excitation from the atria to the ventricles. Together with the P wave, this segment forms the PQ interval (100 ms), corresponding to atrial systole.
ST segment (in the ventricles): Reflects the action potential plateau phase of ventricular cardiomyocytes. The duration of the segment is 120 ms. The entire ventricular myocardium is depolarized, and the potential difference is zero. The segment lies between the QRS complex and the T wave.

Clinical Significance

The ST segment, formed during the ventricular plateau, is a critical diagnostic criterion for myocardial ischemia.

Selective potassium channel blockers, such as ibutilide and nibentan, selectively prolong the effective refractory period of the bundle of His fibers. This blocks premature impulses from propagating from the atria to the ventricles in supraventricular tachyarrhythmias. Normally, phase 3 of the action potential in these fibers is short, ensuring a brief refractory period and high conduction velocity.

Frequently asked questions

Why is a flat line registered on the ECG during the plateau phase?: During the plateau phase, the membrane potential of cardiomyocytes remains stable, and the heart's structure is fully depolarized. As a result, there is no potential difference between myocardial regions, the net electrical field around the heart disappears, and an isoelectric line is recorded on the ECG.
Where does the calcium for myocardial contraction come from during this phase?: Unlike skeletal muscle, where calcium is released primarily from the sarcoplasmic reticulum, the myocardium experiences a significant transmembrane calcium influx. During the plateau, $Ca^{2+}$ ions enter cardiomyocytes from the extracellular fluid via the T-tubule system.
Which ECG segments correspond to the plateau phase?: In the atria, the plateau phase (corresponding to slow repolarization) is represented by the PQ segment with a duration of 60 ms. In the ventricles, the plateau corresponds to the ST segment with a duration of 120 ms.

Go deeper

Cardiomyocyte Action Potential
Slow Diastolic Depolarization (SDD)
Electrocardiography (ECG): Waves, Segments, and Intervals
Mechanism of Muscle Contraction