Review note: AI-assisted draft independently checked against the cited sources by a separate AI reviewer. Not clinician-reviewed. Use with your course materials, supervised clinical training and current local policies.
An ECG records electrical activity detected at the body surface. Its waves mainly reflect atrial and ventricular muscle activity; they are not separate recordings of each node firing. Connecting the tracing to the conduction pathway makes it easier to understand.
Anatomy connection: Watch the impulse travel through the heart with an illustrative ECG
The pathway, step by step
In normal sinus rhythm, the impulse begins in the SA node, spreads through the atria to the AV node, and then travels through the bundle of His, right and left bundle branches, and Purkinje fibers to ventricular muscle.
| Structure or event | Role | Relationship to the surface ECG |
|---|---|---|
| SA node | Normal pacemaker in the upper right atrium, near the superior vena cava | Initiates the impulse; SA-node activity is not a distinct visible wave |
| Atrial muscle | Depolarizes before atrial contraction | P wave |
| AV node | Slows transmission, allowing time for atrial contraction before ventricular activation | Contributes to the delay within the PR interval; the flat PR segment is not a direct recording of the AV node |
| His bundle, bundle branches and Purkinje fibers | Distribute the impulse through the ventricles | Their conduction enables ventricular activation; the QRS represents ventricular muscle depolarization |
| Ventricular recovery | Repolarization | T wave |
Atrial repolarization is usually obscured by the QRS. Electrical activity also does not prove effective pumping: assess the patient and pulse. OpenStax, 19.2
Backup pacemakers
Lower pacemakers can produce escape rhythms when normal impulse formation or conduction fails. OpenStax gives approximate intrinsic rates of 80–100/min for the SA node and 40–60/min for the AV node. Intrinsic firing rates are not the same as the observed resting pulse, which is influenced by autonomic activity. A commonly taught adult resting range is 60–100/min.
A junctional rhythm originates in the AV junction. P waves may be inverted, hidden in the QRS, or follow it. Rate alone cannot identify the rhythm. University of Utah, ECG measurement abnormalities
Timing and measurements
At a recording speed of 25 mm/s, one small box is 0.04 seconds, and one large box is 0.20 seconds. Check the speed before using box counts.
| Measurement | How to measure | Adult teaching point |
|---|---|---|
| PR interval | Beginning of P to beginning of QRS | 0.12–0.20 s, or 3–5 small boxes |
| QRS duration | Beginning to end of QRS | Often 0.06–0.10 s; for basic rhythm classification, narrow is <0.12 s and wide is ≥0.12 s |
| QT interval | Beginning of QRS to end of T | Varies with heart rate; interpret a rate-corrected QTc, not a universal raw-QT cutoff |
The usual QRS range and the wide-complex threshold answer different questions: a QRS under 0.12 seconds is not automatically normal in every respect. University of Utah, normal ECG, AHA 2025 adult advanced life support
QTc interpretation depends on the correction method, age, sex, medications and clinical context. Automated measurements may need confirmation, especially with abnormal T-wave shapes. Avoid memorizing “QT under 0.40–0.44 seconds” as a rule for every heart rate. ESC, measuring the QT interval
A systematic rhythm-strip check
Assess the patient first and throughout interpretation. Check responsiveness, breathing, pulse and signs of poor perfusion. Instability requires immediate help and the appropriate emergency protocol; do not finish a worksheet before responding.
For a stable patient:
- Confirm the lead, recording speed and signal quality.
- Estimate ventricular rate: count QRS complexes in a confirmed 6-second strip and multiply by 10. At 25 mm/s, six seconds equals 30 large boxes.
- Compare R–R intervals for regularity; assess atrial rate and regularity when P waves are identifiable.
- Examine P waves and their relationship to QRS complexes.
- Measure PR and QRS; note whether PR is consistent.
- Inspect ST segments, T waves and QT/QTc; compare with prior tracings when available.
- Relate findings to symptoms and vital signs, and escalate changes appropriately.
This sequence adapts Open RN's systematic ECG approach while placing clinical stability first.
NCLEX-style reasoning
- Natural pacemaker? The SA node.
- A consistently prolonged PR with every P followed by a QRS? First-degree AV conduction delay. The delay is commonly in the AV node, but PR length alone does not localize it. University of Utah
- Seven QRS complexes in six seconds? An estimated ventricular rate of 70/min.
Practice it in 3D
Open Electrical conduction + ECG to relate the illustrated pathway to the tracing. The animation is a teaching model; a clinical surface ECG does not show individual nodes firing. Then try the heart's Practice questions.
Sources
- OpenStax, Anatomy & Physiology 2e, 19.2
- Open RN, Nursing Advanced Skills, 7.3
- Open RN, Nursing Skills 2e, 9.3
- University of Utah, ECG Learning Center, normal ECG
- University of Utah, ECG Learning Center, measurement abnormalities
- AHA, 2025 Adult Advanced Life Support
- ESC, How to Measure the QT Interval
AI-assisted educational content for exam preparation. Clinical assessment and rhythm management follow current training, facility protocols and provider orders.