Rhythm regularity assessment
ECG interpretation begins with checking recording correctness and assessing rhythm regularity. For this purpose, consecutive R–R intervals are measured and compared with each other. The rhythm is considered regular if the difference between the intervals does not exceed 10% upward or downward.
For example, if two adjacent R–R intervals are 24 mm, there is no deviation — the rhythm is regular. Only after confirming regularity does it make sense to proceed with heart rate calculation.
Heart rate calculation and rhythm driver determination
Heart rate is calculated using the formula: HR = 60 / (R–R in seconds). At a recording speed of 25 mm/s, one small grid square corresponds to 0.04 s, and a large one to 0.2 s. With an R–R interval equal to 24 mm (approximately 0.96 s), HR ≈ 62 bpm.
Next, the rhythm driver is determined. A sinus rhythm (source — sinoatrial node) is indicated by the following signs:
- positive P wave in I, II, aVF, V2–V6;
- negative P wave in aVR;
- P wave in V1 is positive or biphasic (notched);
- each P wave corresponds to a QRS complex, and the shape of the P waves is constant.
P wave analysis
After identifying the rhythm driver, P wave morphology and parameters are evaluated. Normal values:
- duration — no more than 0.1 s;
- height (amplitude) — no more than 2.5 mm;
- polarity — positive in all leads except aVR (where it is negative); biphasic is acceptable in V1.
Calculation example: P height — 2 mm (normal), width — 3 grid squares. Duration = 3 × 0.04 = 0.12 s, which exceeds normal and must be reflected in the conclusion. P wave prolongation is characteristic of left atrial overload.
PQ interval and QRS complex
The PQ interval reflects atrioventricular conduction. Normally, its duration is 0.12–0.20 s. Prolongation indicates AV block, while shortening indicates ventricular pre-excitation syndromes.
The QRS complex normally does not exceed 0.1 s. Widening to 0.11 s or more indicates intraventricular conduction disturbance (bundle branch blocks). During analysis, the following are evaluated:
- maximum amplitude of the R wave — normally registered in V4; shifting of the maximum to V5–V6 may indicate left ventricular hypertrophy;
- Q wave — considered pathological if Q depth is greater than 1/4 of the R amplitude in the same lead and/or width is greater than 0.03 s;
- R wave progression in chest leads from V1 to V4–V5.
ST segment and T wave
The ST segment is normally located on the isoelectric line. The following are permissible:
- elevation up to 2 mm in right chest leads (V1–V3);
- depression up to 0.5 mm in left chest leads (V4–V6).
ST elevation greater than 2 mm in V2–V3, as well as any elevation in V4–V6, is a pathological sign requiring the exclusion of acute myocardial infarction. The localization of changes across leads indicates the injury zone: V1–V4 — anterior wall, II, III, aVF — inferior, I, aVL, V5–V6 — lateral.
When analyzing the T wave, polarity is taken into account: negative in aVR, positive in most other leads; in V1 and III, a negative or flattened T is permissible. T inversion in other leads is considered a sign of ischemia or repolarization disturbances.
QT interval and conclusion formulation
The QT interval reflects ventricular electrical systole and is evaluated with a correction for heart rate (corrected QTc). QT prolongation increases the risk of life-threatening ventricular arrhythmias.
The conclusion consistently reflects: rhythm regularity, rhythm source, heart rate, deviations of wave and interval parameters from normal. Only pathological findings are indicated; normal parameters are not specifically described. Formulation example: “Regular sinus rhythm, heart rate 62 bpm. P wave widening up to 0.12 s. QRS widening up to 0.11 s. R maximum in V5. ST elevation in V2–V3 up to 3 mm, elevation in V4–V6”.
Key points
- ECG analysis begins with assessing rhythm regularity using R–R intervals (deviation up to 10%).
- HR is calculated using the formula 60 / (R–R in seconds); at 25 mm/s, a grid square = 0.04 s.
- Sinus rhythm: P is positive in I, II, aVF, V2–V6 and negative in aVR.
- P wave norm: ≤ 0.1 s and ≤ 2.5 mm; PQ interval — 0.12–0.20 s.
- QRS normally ≤ 0.1 s; pathological Q — greater than 1/4 of the R height.
- The R wave maximum is normally registered in lead V4.
- ST elevation > 2 mm in V2–V3 or any elevation in V4–V6 is a sign of myocardial injury.
- The conclusion records only deviations from the norm with specific values indicated.