Step 1. Determining heart rate
ECG analysis begins with calculating the HR. There are two calculation methods.
- R-wave counting method: on the long bottom strip (rhythm strip), count the number of QRS complexes and multiply by 6. This method is applicable to a standard 10-second recording.
- Division method: at a paper speed of 25 mm/s, divide 300 by the number of large squares between two adjacent R waves; at 50 mm/s, divide by 600.
The main objective of this step is to categorize the rhythm as tachycardia (over 100 bpm) or bradycardia (under 60 bpm). The entire subsequent algorithm is based on this distinction.
Step 2. Tachycardia: QRS width and rhythm regularity
In tachycardia, assess the width of the QRS complex: narrow is less than 120 ms (less than 3 small squares), wide is 120 ms or more. Then determine the regularity of the R–R intervals.
Narrow QRS, regular rhythm — three variants:
- Sinus tachycardia: positive P in lead II, negative in aVR, each P is followed by a QRS.
- Atrial flutter 2:1: characteristic HR of about 150 bpm, absence of normal P waves in II, III, aVF, sawtooth F waves; conduction ratio is assessed in V1.
- Paroxysmal supraventricular tachycardia (AVNRT): no visible P waves.
Narrow QRS, irregular rhythm:
- Atrial fibrillation — chaotic f waves, absence of P waves.
- Atrial flutter with variable block — sawtooth waves, changing conduction ratio.
- Multifocal atrial tachycardia — P waves of varying morphology, often associated with COPD.
Wide QRS, regular rhythm — monomorphic ventricular tachycardia. Wide QRS, irregular rhythm — polymorphic ventricular tachycardia (torsades de pointes, associated with QT prolongation) or ventricular fibrillation with a completely chaotic curve.
Step 3. Bradycardia and PR interval assessment
In bradycardia, proceed directly to the PR interval. If the PR is within normal limits, it is sinus bradycardia. If the PR is pathological, atrioventricular blocks are diagnosed.
- First-degree AV block: PR greater than 200 ms (more than 5 small squares), all P waves are conducted, no dropped QRS complexes.
- Second-degree AV block, Mobitz type I (with Wenckebach periodicity): progressive PR prolongation followed by a dropped QRS complex.
- Second-degree AV block, Mobitz type II: constant PR interval with intermittently dropped QRS complexes.
- Third-degree AV block (complete): atria and ventricles beat independently, P and QRS are unrelated.
A shortened PR interval (less than 120 ms) with a delta wave and QRS widening is characteristic of WPW syndrome (an AVRT variant in tachycardia).
Step 4. Electrical axis of the heart
The axis is determined using leads I and aVF. The direction of the main QRS deflection in these leads dictates the quadrant:
- QRS upward in I and in aVF — normal axis.
- QRS upward in I, downward in aVF — check lead II: if QRS is positive, the axis is still normal; if negative, left axis deviation.
- QRS downward in I, upward in aVF — right axis deviation.
- QRS downward in I and in aVF — extreme axis deviation.
Step 5. QT interval and P waves
The QT interval is compared with the R–R interval: if QT occupies half of R–R or more, it is prolonged. QT prolongation increases the risk of torsades de pointes; typical causes include hypokalemia, hypomagnesemia, hypothermia, and drug effects. QT shortening occurs in hypercalcemia.
Analysis of the P wave is performed in lead II and V1.
- P mitrale (left atrial hypertrophy): notched P in II, biphasic P in V1 with a predominant negative phase.
- P pulmonale (right atrial hypertrophy): tall peaked P in lead II, associated with pulmonary hypertension.
Step 6. QRS complex: bundle branch blocks and hypertrophy
Key leads are V1, V2, V5, V6.
- Left bundle branch block: deep S in V1–V2, notched or M-shaped R in V5–V6.
- Right bundle branch block: rSR' type complex in V1, wide S in V5–V6.
- Complete block — QRS greater than 120 ms; incomplete — QRS of normal width with a characteristic morphology.
Left ventricular hypertrophy is determined by the Sokolow–Lyon index: the sum of S in V1 and R in V5 (or V6) greater than 35 mm. Right ventricular hypertrophy: tall R in V1–V2 and deep S in V5–V6.
Step 7. ST Segment and T Wave
Evaluation of the ST segment is performed from the J point (the transition of QRS into ST).
ST depression can be horizontal, downsloping, and upsloping; the first two are more specific for ischemia. Depression indicates ischemia, unstable angina, or NSTEMI (differentiated by troponin). A scooping, "mustache-like" depression is characteristic of digitalis toxicity (digoxin); depression with T wave inversion occurs in hypokalemia.
ST elevation:
- Concave ("smiling") — pericarditis, early repolarization; in pericarditis, elevation is diffuse, often accompanied by PR depression.
- Convex ("frowning"), localized — acute STEMI.
Localization of myocardial infarction by ST elevation:
- V1–V4 — anterior myocardial infarction;
- II, III, aVF — inferior myocardial infarction;
- I, aVL, V5, V6 — lateral infarction.
The T wave may be inverted (reperfusion, ischemia) or hyperacute — tall and peaked (an early sign of ischemia, precursor to ST elevation). Tall peaked T waves are also characteristic of hyperkalemia.
Key points
- The ECG interpretation algorithm is based on 7 consecutive steps: heart rate, rhythm/QRS, axis, PR and QT intervals, P wave, QRS complex, ST segment.
- Heart rate calculation determines the further branch of the algorithm: tachycardia requires analysis of QRS width, bradycardia — evaluation of PR.
- The combination of QRS width and rhythm regularity in tachycardia covers the main emergency arrhythmias.
- AV blocks are classified according to PR behavior and the presence of dropped QRS complexes into first-degree, second-degree (Mobitz I and II), and third-degree.
- The cardiac axis is quickly determined by the direction of QRS in I and aVF without constructing triangles.
- P mitrale and P pulmonale in lead II and V1 indicate left and right atrial hypertrophy, respectively.
- A Sokolow–Lyon index greater than 35 mm confirms left ventricular hypertrophy.
- Convex localized ST elevation is a sign of STEMI; the localization of infarction is determined by the group of leads with elevation.