Key Points:
- Every ECG tracing is built from waveforms (deflections), segments (baseline portions between waveforms), and intervals (time that include waveforms plus segments).
- Waveforms describe electrical events (depolarization or repolarization). Intervals tell you how long conduction and recovery take. Segments are where subtle ischemia and injury patterns hide.
- Baseline matters: when judging ST deviation, your default reference is the TP segment (true isoelectric baseline) when it is visible and stable.
- Time-critical bedside interpretation often centers on three things: QRS width and morphology, ST segment and J point, and QT (QTc).
- Small errors in measurement and baseline choice create big clinical errors. Measure deliberately and compare to prior ECGs when possible.
ECGs record cardiac waveforms generated by changes in voltage between different points on the body. These waveforms are displayed on ECG grid paper, which allows for precise measurement of time on the horizontal axis and voltage on the vertical axis.
How to Measure Correctly
Standard calibration
- Paper speed: 25 mm/s
- Voltage: 10 mm/mV
- Rule of thumb at 25 mm/s:
- 1 small box = 0.04 s (40 ms)
- 1 large box = 0.20 s (200 ms)
- 5 large boxes = 1 second
Two practical measurement rules
- Measure intervals in a lead where the onset and offset are clearest, often leads V1/II (P and PR) and V5 or V6 (QRS, QT), but use whichever lead is most clear.
- For QT, measure start of QRS to end of T wave (not the U wave). If there is a prominent U wave, you measure at the end of the T wave, not the end of TU fusion.
Annotated ECG showing labeled intervals and segments: PR interval (P wave + PR segment), QRS duration, ST segment/interval, TP interval (isoelectric baseline), QT interval (QRS onset to T end), and RR interval for rate. Paper speed 25 mm/s; small box ≈ 40 ms, large box ≈ 200 ms. From the LITFL ECG Library: https://litfl.com/r-wave-ecg-library/.
Waveforms: the heart’s electrical “signatures”
A waveform is a deflection away from baseline. Waveforms are described by:
- Amplitude (voltage)
- Duration (time)
- Morphology (shape, direction, symmetry, notching)
Key waveforms & what they represent:
- P wave: atrial depolarization
- QRS complex: ventricular depolarization
- Q wave: first negative deflection of the QRS
- R wave: first positive deflection
- S wave: negative deflection after an R wave
- T wave: ventricular repolarization
Dive deeper into the basics of key waveforms here:
- Delta wave: early ventricular activation via an accessory pathway (pre-excitation)
- J point: where QRS ends and ST begins. This is the hinge point for ST elevation or depression measurement.
- J waves (Osborn waves): prominent terminal QRS notch or slur associated with hypothermia and some other conditions.
- Epsilon wave: small terminal deflection in right precordials, associated with arrhythmogenic cardiomyopathies (ARVC/D spectrum).
- U wave: small deflection after T wave. Often seen in bradycardia or with hypokalemia.
Learn more about these waveforms here:
Segments: areas that connect waveforms
A segment is the flat (or near-flat) portion between waveforms. Typically electrically neutral baselines between depolarization and repolarization. Clinically, segments are where you assess deviation from baseline and detect ischemia or pericarditis patterns.
Key segments:
- PR segment: end of P wave to start of QRS during AV nodal conduction. Atrial repolarization (Ta wave) can influence it, and PR depression can occur in pericarditis.
- ST segment: J point to start of T wave. Represents the early phase of ventricular repolarization. ST deviation is one of the most time-sensitive ischemia and occlusion markers on the ECG.
- TP segment: end of T wave to start of next P wave. This is the true electrical baseline when it is visible and stable, and it is your best reference for ST deviation when available.
Dive deeper into the key segments here:
Intervals: timing across the cardiac cycle
An interval includes one or more waveforms and segments. Intervals are where you diagnose conduction delay and repolarization risk.
Key intervals:
- PR interval: start of P to start of QRS.
- Normal: 120–200 ms
- Prolonged: AV conduction delay (can be benign or pathologic depending on context and QRS width)
- Short PR: consider pre-excitation (if delta wave present) or enhanced AV nodal conduction patterns
- QT interval (and QTc): start of QRS to end of T wave. This is depolarization plus repolarization time for the ventricles.
- Prolonged QTc: risk of torsades, especially with triggers (bradycardia, hypokalemia, hypomagnesemia, QT-prolonging drugs)
- Short QT: consider hypercalcemia or rare congenital short QT syndrome
Dive deeper into the key intervals here:
- QRS duration (technically a duration, often treated as a key interval): narrow vs wide changes your entire differential and treatment pathway.
- RR interval: rhythm regularity and rate calculation.
- JT interval: can help when QRS is wide and QT becomes harder to interpret.
Learn more about these topics here:
Key Clinical Pearls:
- ST segment and J point changes are time-sensitive. Train your eye to see subtle ST elevation less than 1 mm when it is territorial and paired with reciprocal changes.
- PR interval interpretation is incomplete without the QRS. A long PR with a narrow QRS is usually AV nodal delay. A long PR with a wide QRS raises concern for infranodal disease.
- QTc is not optional in syncope, seizure-like presentations, bradycardia, overdose, and electrolyte derangements, or “mysterious polymorphic VT.” It is a risk marker, not a diagnosis.
- Use TP as baseline for ST deviation when possible, especially when PR segment is drifting, depressed, or influenced by atrial repolarization.
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