ECG Analysis Algorithm
The standard ECG interpretation plan includes several consecutive stages: checking the correctness of recording, analyzing cardiac rhythm and heart rate, determining the pacemaker, assessing the electrical axis of the heart, analyzing waves and intervals (P, PQ, QRS, ST, T), and formulating a conclusion. On modern machines, calibration is performed automatically, so the first point is often omitted. ECG analysis is always carried out from left to right, with a mandatory review of all leads, as pathological changes may appear intermittently.
Rhythm and Heart Rate Analysis
Regular rhythm is characterized by equal R–R intervals with an allowable deviation of up to 10%. Minor fluctuations are associated with respiratory arrhythmia and are considered physiological. To assess regularity, consecutive R–R intervals are measured in millimeters.
Heart rate calculation is performed using the formula:
- HR = 60 / (R–R in mm × T), where T is the duration of one small box;
- at a paper speed of 25 mm/s, one small box equals 0.04 s;
- at a speed of 50 mm/s — 0.08 s.
Normal heart rate values range from 60 to 80 (according to some sources, 60–90) beats per minute.
Determining the Pacemaker
Normally, the pacemaker is the sinuatrial node. Criteria for sinus rhythm:
- presence of a P wave before each QRS complex;
- P wave is positive in lead II;
- P wave is negative in aVR;
- in leads V1–V2, the P wave can be positive or biphasic (notched) due to the sequential activation of the right and then the left atrium.
If all criteria are met, the conclusion states: pacemaker — sinus node (SA).
Electrical axis of the heart
The electrical axis of the heart (EAH) is the sum of myocardial excitation vectors. Excitation spreads from the sinuatrial node through the atria, AV junction, interventricular septum, and ventricles; the resulting vector in a healthy person is directed to the left and downwards.
Shift of the EAH can occur in:
- left ventricular hypertrophy — leftward deviation;
- right ventricular hypertrophy — rightward deviation;
- asthenic body type — vertical axis position (lowering of the apex);
- isolated right ventricular hypertrophy is extremely rare, usually combined.
A simple way to determine the EAH is to compare the amplitude of R waves in standard leads I, II, and III:
- R II > R I > R III (sequence 2–1–3) — normal axis;
- R I > R II > R III (1–2–3, from left to right) — leftward deviation (levogram);
- R III > R II > R I (3–2–1, from right to left) — rightward deviation (dextrogram).
Precise determination of the alpha angle requires summing the waves of the QRS complex in leads I and III using a special table.
Analysis of Waves and Intervals
P wave: duration up to 0.1 s, amplitude up to 2.5 mm (evaluated in lead II). Reflects atrial depolarization.
PQ interval: 0.12–0.2 s (according to Russian sources — up to 0.18 s in English-language sources). Reflects the time of impulse conduction from the atria to the ventricles.
QRS complex: duration up to 0.1 s. Normally, the Q wave is no more than 1/4 of the R wave amplitude and is measured in standard leads I and II (evaluation of the Q wave in chest leads has specific features). The R wave is always positive by definition — a negative ventricular complex is designated as QS. The maximum R amplitude in chest leads is in V4, where the electrode is located over the cardiac apex.
ST segment and T wave
ST segment reflects the period when depolarization forces are balanced with the onset of ventricular repolarization. Normally, the segment lies on the isoelectric line. Permissible deviations:
- in V1–V3 — elevation up to 2 mm;
- in V4–V6 — depression up to 0.5 mm;
- in standard and augmented leads, ST segment changes are less informative; the segment has diagnostic value primarily in chest leads, especially when myocardial infarction is suspected.
It is important to distinguish between the ST segment (the short isoelectric section between the end of QRS and the start of T) and the ST interval.
T wave reflects ventricular repolarization. Normal criteria:
- positive in leads I, II, V2–V6;
- negative in aVR (like the P wave in this lead);
- T in V6 is greater than T in V1;
- T in I is greater than T in III.
Key points
- ECG analysis is carried out according to a strict algorithm from left to right, reviewing all 12 leads.
- Rhythm regularity is determined by the equality of R–R intervals with a ±10% tolerance.
- Sinus rhythm is confirmed by a positive P in lead II and a negative P in aVR.
- A normal EAH corresponds to the ratio R II > R I > R III.
- P wave duration — up to 0.1 s, PQ interval — 0.12–0.2 s, QRS complex — up to 0.1 s.
- Normally, the Q wave does not exceed 1/4 of the R wave height; maximum R is registered in V4.
- In V1–V3, ST elevation up to 2 mm is permissible, in V4–V6 — depression up to 0.5 mm.
- The T wave is negative only in aVR; T V6 > T V1, T I > T III.