What do polarization, depolarization, and repolarization represent on an Electrocardiogram (EKG)?

Understanding Polarization, Depolarization, and Repolarization on the EKG

Depolarization represents the electrical activation of cardiac muscle (seen as the P wave for atria and QRS complex for ventricles), while repolarization represents the recovery phase when cardiac cells return to their resting electrical state (seen as the T wave and ST segment), with polarization referring to the baseline resting state of electrically charged cardiac cells.

Polarization (Resting State)

Polarization describes cardiac muscle cells at rest, maintaining their baseline electrical charge of approximately -85 mV across the cell membrane. 1

• This resting state corresponds to the TP segment on the EKG—the flat baseline between the end of one T wave and the beginning of the next P wave 1
• During this phase, minimal voltage gradients exist between different regions of the heart, which explains why the baseline appears isoelectric (flat) 1
• Cardiac cells remain in this polarized state during electrical diastole, ready for the next activation cycle 1

Depolarization (Electrical Activation)

Depolarization is the rapid electrical activation that triggers cardiac muscle contraction, manifested on the EKG as the QRS complex for the ventricles. 2

Key Features of Ventricular Depolarization:

• The QRS complex represents ventricular depolarization, where the transmembrane potential rapidly shifts from approximately -85 mV toward positive values 1
• Depolarization normally proceeds from endocardium to epicardium (inner to outer heart wall) in the ventricles 1
• This electrical activation is what triggers mechanical contraction of the heart muscle 2
• Abnormalities in depolarization appear as changes in QRS shape, duration, or amplitude and may indicate bundle branch blocks, ventricular hypertrophy, or myocardial scarring 2

Repolarization (Recovery Phase)

Repolarization is the process by which cardiac cells return to their resting electrical state after depolarization, represented on the EKG by the ST segment and T wave. 1

ST Segment (Early Repolarization):

• The ST segment corresponds to the plateau phase (phase 2) of the ventricular action potential, where the transmembrane voltage remains relatively stable at approximately +10 to +10 mV 1
• During this phase, minimal voltage gradients exist between different cardiac regions, which is why the ST segment is normally flat and isoelectric, similar to the TP segment 1
• The ST segment should be at approximately the same level as the baseline (TP segment) under normal conditions 1

T Wave (Rapid Repolarization):

• The T wave represents phase 3 of the action potential—rapid ventricular repolarization—when the transmembrane potential returns from approximately +10 mV back to -85 mV 1
• Repolarization proceeds in the opposite direction from depolarization: from epicardium to endocardium (outer to inner heart wall) 1
• This occurs because epicardial cells have shorter action potential durations than endocardial cells, creating an inverse relationship between activation time and action potential duration 1
• The voltage gradients created as cells undergo sequential repolarization generate the T wave configuration seen on the body surface EKG 1

Clinical Significance: Primary vs. Secondary Repolarization Abnormalities

Primary Repolarization Abnormalities:

Primary repolarization abnormalities result from changes in the shape or duration of the repolarization phases themselves, without changes in depolarization. 1

• These can be caused by:
  • Ischemia (most clinically important) 1
  • Myocarditis 1
  • Electrolyte abnormalities, particularly potassium and calcium 1
  • Drugs and toxins 1
  • Autonomic changes (catecholamines, sympathetic stimulation, hyperventilation, position changes) 1
  • Heart rate changes 1

Secondary Repolarization Abnormalities:

Secondary repolarization abnormalities occur as a direct result of altered depolarization sequence (abnormal QRS), not from intrinsic repolarization changes. 1

• Examples include ST-T wave changes seen with:
  • Bundle branch blocks: ST-T vectors directed opposite to the mean QRS vector 1
  • Ventricular preexcitation: ST-T changes opposite to the delta wave 1
  • Ventricular pacing: Expected repolarization changes from altered activation sequence 1

Common Pitfalls to Avoid

• Do not assume all ST-T wave abnormalities represent ischemia—secondary repolarization changes from bundle branch blocks or pacing are expected and do not indicate primary myocardial pathology 1
• Recognize that repolarization heterogeneity is normal—action potential durations vary significantly across different ventricular regions (epicardium, endocardium, midmyocardium), and this normal variation creates the T wave 1
• When the QRS axis is normal, an abnormal T-wave axis generally indicates primary repolarization abnormalities requiring further investigation 1
• Abnormal voltage gradients during the plateau phase (ST segment) and rapid repolarization phase (T wave) can occur with or without changes in the repolarization sequence itself 1