Guide — Cardiac
ECG Interpretation Fundamentals
This guide builds on basic rhythm recognition and goes deeper into ECG analysis — precise interval measurement, conduction abnormalities including bundle branch blocks and AV blocks, ST segment and T wave interpretation, and a nine-step systematic method. If you are new to EKG reading, start with the EKG Basics Guide first.
14 min read · Advanced Cardiac Interpretation
Educational use only. ECG interpretation requires clinical context, licensed supervision, and institutional protocols. Always correlate findings with patient assessment and full clinical picture. This material supports nursing education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional policy, or medical direction. Always follow facility protocols and current provider orders.
Normal ECG Intervals at a Glance
| Parameter | Normal Range | What It Measures |
|---|---|---|
| Heart Rate | 60–100 bpm | Ventricular depolarization frequency |
| PR Interval | 0.12–0.20 sec (3–5 small boxes) | SA node to ventricular activation — AV conduction time |
| QRS Duration | < 0.12 sec (< 3 small boxes) | Ventricular depolarization — His-Purkinje conduction time |
| QT Interval | ≤ 0.44 sec in men; ≤ 0.46 sec in women (rate-corrected) | Ventricular depolarization + repolarization time |
| ST Segment | Isoelectric (flat at baseline) | Early ventricular repolarization phase |
Step 1 — Rate Calculation
Heart rate can be calculated several ways depending on whether the rhythm is regular or irregular.
Regular Rhythm — 300 Method:
Count the large boxes between two consecutive R waves. Divide 300 by that count.
Large box count → Approximate rate:
1 box = 300 bpm | 2 = 150 | 3 = 100 | 4 = 75 | 5 = 60 | 6 = 50
Irregular Rhythm — 6-Second Method:
Count the R waves in a 6-second strip (30 large boxes) and multiply by 10. Use this for atrial fibrillation and other irregular rhythms.
Step 2 — Rhythm Assessment
Determine whether the rhythm is regular or irregular by comparing R-R intervals across the strip.
- Regular: R-R intervals vary by < 2 small boxes. Seen in NSR, SVT, and ventricular tachycardia.
- Regularly irregular: A consistent pattern, such as grouped beating in Wenckebach (Mobitz I).
- Irregularly irregular: No discernible pattern — classic for atrial fibrillation.
Step 3 — P Wave Assessment
Ask four questions to characterize the P waves completely:
- Are P waves present? Absent in VF, asystole, and junctional rhythms.
- Are they regular? In atrial fibrillation, there are no true P waves — only a fibrillatory baseline.
- Is there one P wave per QRS? More P waves than QRS = AV block.
- Are they upright in lead II? Confirms SA node origin. Inverted P waves in lead II = junctional or ectopic atrial source.
Step 4 — PR Interval & AV Block Patterns
The PR interval measures AV conduction time — from the start of the P wave to the start of the QRS complex. Abnormalities indicate AV node dysfunction.
| Finding | Interpretation |
|---|---|
| PR 0.12–0.20 s | Normal AV conduction |
| PR > 0.20 s (constant) | First-degree AV block — delayed conduction, no dropped beats |
| PR progressively lengthens, then QRS drops | Second-degree Mobitz I (Wenckebach) — usually benign, often inferior MI or high vagal tone |
| PR constant but QRS randomly drops | Second-degree Mobitz II — more serious; risk of progression to complete block |
| P and QRS independent of each other | Third-degree (complete) AV block — ventricular escape rhythm; urgent intervention required |
| PR < 0.12 s | Pre-excitation (WPW syndrome) or junctional rhythm with retrograde P waves |
Step 5 — QRS Duration
The QRS complex represents ventricular depolarization. Measure from the beginning of the Q wave to the end of the S wave.
- Normal: < 0.12 sec — conduction through the normal His-Purkinje system
- Wide (≥ 0.12 sec): Bundle branch block, hyperkalemia, ventricular rhythm, or aberrant conduction
- Wide + fast: Treat as ventricular tachycardia until proven otherwise
Step 6 — QT Interval
The QT interval measures ventricular repolarization. It is rate-dependent; the QTc corrects for heart rate.
- Normal QTc: ≤ 0.44 sec in men, ≤ 0.46 sec in women
- QTc > 0.50 sec: High risk for Torsades de Pointes — potentially fatal polymorphic VT
- Common causes of prolongation: Hypokalemia, hypomagnesemia, antiarrhythmic drugs (amiodarone, sotalol), antipsychotics, some antibiotics (azithromycin, fluoroquinolones)
- Nursing priority: Check QTc before and after QT-prolonging medications. Report QTc > 500 ms to the provider immediately.
Bundle Branch Block Identification
A bundle branch block occurs when conduction through the right or left bundle is delayed or blocked, causing the ventricles to depolarize asynchronously. The hallmark is a wide QRS (≥ 0.12 s).
Right Bundle Branch Block (RBBB)
- QRS ≥ 0.12 sec
- rsR' pattern (“rabbit ears”) in V1–V2
- Wide, slurred S wave in leads I, V5–V6
- May be normal variant or associated with right heart strain
Left Bundle Branch Block (LBBB)
- QRS ≥ 0.12 sec
- Broad, notched R wave in leads I, V5–V6
- Absent normal septal Q waves in lateral leads
- New LBBB with chest pain = treat as STEMI equivalent until proven otherwise
LBBB can mask ischemic changes on ECG — always interpret in the clinical context of the patient's symptoms.
ST Segment & T Wave Analysis
The ST segment and T wave reflect ventricular repolarization. Deviations from normal are important markers of ischemia, injury, and electrolyte abnormalities.
| Finding | Clinical Significance |
|---|---|
| ST elevation (≥ 1 mm in 2+ leads) | Myocardial injury — triggers STEMI protocol; activate cath lab |
| ST depression | Subendocardial ischemia or reciprocal changes from elevation elsewhere; NSTEMI/unstable angina |
| T wave inversion | Ischemia, CNS events, bundle branch block, or electrolyte abnormalities |
| Peaked, tall T waves | Hyperkalemia (early sign); also seen in hyperacute MI |
| Flattened T waves | Hypokalemia, digoxin effect, or non-specific finding |
ST changes must always be interpreted in combination with patient symptoms, prior ECGs, and troponin trends — no ECG finding is interpreted in isolation.
Systematic ECG Analysis — Nine-Step Method
- Assess the patient first. A rate of 180 means something different in a well-appearing athlete versus a diaphoretic patient in distress. ECG findings are always interpreted in clinical context.
- Calculate the rate. Use the 300 method for regular rhythms; 6-second count for irregular rhythms.
- Assess rhythm regularity. Regular, regularly irregular, or irregularly irregular?
- Evaluate P waves. Present, regular, one per QRS, upright in lead II?
- Measure the PR interval. Within 0.12–0.20 sec? Constant or variable?
- Measure the QRS duration. Narrow (< 0.12 sec) or wide (≥ 0.12 sec)? If wide, is it RBBB, LBBB, or ventricular origin?
- Evaluate the ST segment. Elevated, depressed, or isoelectric? In which leads?
- Assess the T wave. Upright and symmetric? Peaked, inverted, or flattened?
- Measure the QTc. Within normal limits? Any QT-prolonging medications on the MAR?
Related Resources
Standards & sources
Fact-checked Jun 20, 2026This page is written to align with American Heart Association (AHA) · American College of Cardiology (ACC) · AHA ACLS Guidelines. It is an educational summary, not a citation of any single document — always verify specific doses, values, and protocols against current guidelines and your facility policy. How we source content →