Cardiac • ECG Fundamentals
ECG Waveform Basics
A quick, factual reference for what each ECG wave/segment represents and how to measure common intervals. Normal ranges vary with age, sex, heart rate, and clinical context.
Quick Jump
Reference pageECG Paper and Calibration
25 mm/s • 10 mm/mV- Standard speed: 25 mm/s (most common).
- Horizontal scale (time): 1 small square = 0.04 s (40 ms); 1 large square = 0.20 s (200 ms).
- Vertical scale (voltage): 10 mm = 1 mV (so 1 mm = 0.1 mV).
- Calibration mark: typically a 1 mV “square” (10 mm tall) printed at the start of the tracing.
- Baseline / isoelectric line: the flat reference level (commonly measured using the PR segment as a reference).
What the ECG Records
Voltage vs time- Depolarization generally produces the main deflections (e.g., P wave, QRS complex).
- Repolarization produces later deflections (e.g., T wave, sometimes U wave).
- The ECG shows voltage differences between electrodes over time, not “mechanical contraction.”
- Wave direction (up/down) depends on the lead axis and the direction of electrical activity relative to that lead.
Waveforms, Segments, and Intervals
Normal features (typical)| Component | What it represents | Typical normal features |
|---|---|---|
| P wave | Atrial depolarization. |
Duration usually < 120 ms (3 small squares). Amplitude in limb leads usually ≤ 2.5 mm (0.25 mV). Often upright in I, II, aVF; typically inverted in aVR. |
| PR interval | From start of P to start of QRS: atrial depolarization + AV node/His-Purkinje conduction time. |
Usually 120–200 ms (3–5 small squares). Measured from the start of P to the start of QRS. |
| PR segment | End of P to start of QRS (AV nodal delay region). Often used as a baseline reference. | Typically near the isoelectric line (flat). |
| QRS complex | Ventricular depolarization (atrial repolarization occurs at the same time but is usually not visible). |
Duration usually < 120 ms (narrow complex). A “Q” wave is the first negative deflection; “R” is first positive; “S” is negative after an R. |
| J point | The junction between the end of the QRS and the start of the ST segment. | Reference point for assessing ST elevation/depression patterns. |
| ST segment | Early ventricular repolarization phase (plateau phase). |
Normally close to isoelectric (flat). Small, benign deviation can occur in some leads/individuals; interpret in clinical context. |
| T wave | Ventricular repolarization. |
Often upright in I, II, V3–V6; typically inverted in aVR and may be variable in V1. T wave direction is often (but not always) concordant with the main QRS direction. |
| QT interval | Start of QRS to end of T: total ventricular depolarization + repolarization time. |
QT varies with heart rate, so clinicians often use QTc (rate-corrected QT). A commonly used reference: QTc roughly < 440 ms (men), < 460 ms (women). Always interpret QT/QTc with clinical context and the method used. |
| U wave | A small deflection after T; mechanism is not always clear (often linked with repolarization phenomena). |
Usually small or absent; can be more prominent at slower heart rates. If present, it follows the T wave and is typically low amplitude. |
How to Measure (Quick Method)
Squares → milliseconds- Pick a clear lead (often II or V5 for intervals).
- Count small squares: 1 small square = 40 ms at 25 mm/s.
- PR: start of P → start of QRS.
- QRS: first deflection of QRS → end of last deflection.
- QT: start of QRS → end of T (use the clearest lead; avoid merging with U wave).
Common Practical Notes
Check multiple leads- Artifact and baseline wander can distort ST/T assessment—check multiple leads.
- Heart rate affects QT; QTc is used to help compare across rates.
- Normal patterns can differ by lead; always interpret waveforms in the context of the 12-lead layout and the patient.