Correlating the Action Potential to the ECG Waveform: P, QRS, and T

Why the ECG Waveform Exists

The ECG (electrocardiogram) does not measure the heart's mechanical pumping directly. It records the electrical activity that triggers each contraction. Every bump and dip you see on an ECG strip is caused by millions of cardiac muscle cells changing their electrical charge in a coordinated sequence. Understanding what is happening at the cellular level makes every waveform meaningful rather than just a shape to memorize.

The Cardiac Action Potential: A Quick Review

A cardiac muscle cell at rest maintains a negative charge inside relative to the outside. This is called the resting membrane potential. When a cell is stimulated, positively charged ions (mainly sodium, then calcium) rush in, making the inside briefly positive — this is depolarization. The cell then works to restore its resting negative charge by moving potassium out and pumping ions back — this is repolarization.

The action potential curve has numbered phases:

• Phase 4 — Resting potential; the cell is negatively charged and ready.
• Phase 0 — Rapid depolarization; sodium rushes in, the cell fires.
• Phase 1 — Brief early repolarization.
• Phase 2 — Plateau; calcium flows in, sustaining contraction.
• Phase 3 — Rapid repolarization; the cell returns to its resting state.

Ventricular muscle cells follow this pattern. Sinoatrial (SA) node cells have a slightly different curve (they depolarize spontaneously), but the big picture is the same.

Mapping the Action Potential to Each ECG Component

P Wave — Atrial Depolarization

The SA node, located in the right atrium, fires first. The electrical impulse spreads across both atria, depolarizing atrial muscle cells (Phase 0 in those cells). This wave of depolarization is seen on the ECG as the P wave — a small, smooth, upright bump. The P wave represents atrial depolarization. Atrial contraction follows a split-second later. Note: atrial repolarization does occur but is usually hidden inside the QRS complex and is not seen as a separate wave.

PR Interval — AV Node Conduction Delay

After the atria depolarize, the impulse reaches the atrioventricular (AV) node, which deliberately slows conduction. This pause allows the atria to finish contracting and push blood into the ventricles before the ventricles fire. On the ECG, the flat line between the end of the P wave and the start of the QRS complex reflects this delay. The entire PR interval — measured from the beginning of the P wave to the beginning of the QRS — captures both atrial depolarization and AV node conduction time. A normal PR interval is 0.12–0.20 seconds.

QRS Complex — Ventricular Depolarization

Once past the AV node, the impulse travels rapidly through the Bundle of His, the left and right bundle branches, and the Purkinje fibers, triggering ventricular muscle depolarization (Phase 0 in ventricular cells). Because the ventricular muscle mass is large, this electrical event produces the tallest, sharpest component of the ECG — the QRS complex. It represents ventricular depolarization. Ventricular contraction (systole) begins almost simultaneously. A normal QRS duration is 0.06–0.10 seconds; a wide QRS suggests delayed conduction.

ST Segment — Early Ventricular Repolarization (Plateau)

Immediately after the QRS, ventricular cells are in the plateau phase (Phase 2) — calcium is flowing in and the cells remain depolarized while actively contracting. The ECG shows a relatively flat line called the ST segment, connecting the end of the QRS to the beginning of the T wave. In a normal ECG, the ST segment sits at the isoelectric (baseline) level. Elevation or depression of the ST segment can be a critical sign of myocardial injury or ischemia — always report abnormal ST changes immediately. This is a key safety point: recognizing ST changes and escalating promptly is part of the EKG technician's responsibility.

T Wave — Ventricular Repolarization

As ventricular cells recover their resting charge (Phase 3), a broader, rounded wave appears — the T wave. It represents ventricular repolarization. The T wave is normally upright in most leads. Inverted or peaked T waves can indicate ischemia, electrolyte imbalances, or other conditions. Because repolarization is slightly slower and less uniform than depolarization, the T wave is wider and more gradual than the QRS.

QT Interval — Total Ventricular Electrical Activity

The QT interval is measured from the beginning of the QRS complex to the end of the T wave. It captures the total time for ventricular depolarization and repolarization — in other words, the entire ventricular action potential. A prolonged QT interval can predispose a patient to dangerous arrhythmias and should always be noted and reported. Normal QT varies with heart rate; the corrected value (QTc) accounts for this.

Connecting the Dots: Electrical Event to Mechanical Action

Remember the sequence: electrical event → mechanical response. Depolarization triggers contraction; repolarization allows relaxation. The P wave triggers atrial contraction; the QRS triggers ventricular contraction. The EKG technician reads the electrical side of this equation. The clinical team uses that information to assess whether the mechanical side is following correctly.

Safety and Scope of Practice

As an EKG technician, your role is to acquire a high-quality tracing and recognize findings that require immediate escalation — such as ST elevation, a very wide QRS, or absent P waves — and report them to the supervising clinician without delay. You do not diagnose, but you must know enough to act quickly when a tracing looks abnormal. Accurate electrode placement and artifact-free recordings are essential: a poor tracing can obscure or mimic true waveform changes.