Physiology of chest leads
Chest leads reflect the electrical activity of different parts of the heart, and their morphology depends on myocardial thickness:
- Right chest leads (V1, V2): record potentials of the right ventricle (RV) and interventricular septum. Since the RV wall is thin (2–3 mm), its excitation occurs rapidly. Normally, an rS complex is formed here with a small r wave (septal and RV excitation) and a deep S wave (left ventricular vector moving away from the electrode).
- Left chest leads (V4–V6): show the activity of the thick left ventricle (LV). Due to significant myocardial mass (especially in the apex region, lead V4), excitation generates high voltage, manifesting as a maximum R wave. Sometimes a small q wave is recorded, reflecting early septal excitation.
Preparation for interpretation
Before starting to evaluate the electrocardiogram itself, you must ensure the recording quality:
- Artifact check: mains interference, patient muscle tremor, or poor electrode contact distort the picture. In case of pronounced artifacts, the tracing must be repeated.
- Calibration: the control millivolt must equal 10 mm.
- Paper speed: it is important to know the paper movement speed (usually 25 or 50 mm/s), as interval duration calculations depend on it.
Furthermore, a clinical conclusion requires comparing the current ECG with old recordings (archive) and mandatory consideration of the patient's symptoms. Isolated evaluation of the tracing is fraught with errors.
General ECG analysis workflow
To make the analysis comprehensive, a five-step plan is followed:
- Rhythm and conduction.
- Electrical axis of the heart (EAH): determined in the frontal plane.
- Atrial complex: detailed analysis of P wave parameters.
- Ventricular complex (QRST): assessment of QRS, ST segment, T wave, and Q-T interval.
- Conclusion: final summary of all identified changes.
Analysis of rhythm, conduction, and P wave
At the first stage, the basic characteristics of cardiac activity are evaluated:
- Regularity: the rhythm is regular if the R-R intervals are equal.
- Heart rate calculation: exact (60 divided by the duration of R-R in seconds), using tables/rulers, or the rapid method (600 divided by the number of large 5-mm squares between R waves).
- Pacemaker: look for the P wave (best seen in standard lead II and V1). In sinus rhythm, it is positive in lead I (the impulse travels from top to bottom). If the impulse is retrograde (from the AV node or ventricles), the P wave becomes negative. Deformations, absence, or irregularity of the P wave indicate a non-sinus rhythm. In some cases, the wave may disappear entirely.
- Conduction: evaluated for each complex, paying attention to the P-Q interval and QRS morphology.
The P wave itself is analyzed by amplitude, duration, polarity, and shape.
Topographic diagnosis of infarction zones
Changes in specific leads can reveal which wall of the left ventricle is damaged:
- Anterior wall and septum: V1–V3.
- Apex: V4 only.
- Lateral wall: I, aVL, V4–V6.
- Extensive anterior (anterior + lateral): I, aVL and all chest leads V1–V6.
- Inferior wall: II, III, aVF.
- Inferior-lateral wall: II, III, aVF along with I, aVL, V4–V6.
Briefly: answers to topical questions
How to determine the position of the electrical axis of the heart (EAH)?
Determining the position of the electrical axis of the heart (EAH) is based on calculating the alpha angle (α) formed between the EAH vector and the horizontal zero line (the axis of standard lead I). Two main methods of determination are used:
- Visual method — the ratio of Q, R, and S wave amplitudes of the QRS complex in standard leads is analyzed.
- Geometric method — graphical construction of the vector in Einthoven's triangle is performed based on QRS wave amplitudes in two leads.
The following rule is used to assess the axis direction:
- Positive complex (R > S) — the axis is directed toward the lead.
- Equiphasic complex (R = S) — the axis is perpendicular to the lead.
- Negative complex (R < S) — the axis is directed away from the lead.
How is cardiac conduction analyzed using an ECG?
Conduction analysis is performed by sequentially reviewing each recorded cardiac complex. The process includes evaluating the following parameters:
- Complex regularity — checking the stability of impulse conduction.
- P-Q interval duration — reflects the state of atrioventricular conduction (its prolongation indicates the presence of AV block).
- QRS complex morphology — analysis of the ventricular complex shape to diagnose intraventricular blocks (e.g., left or right bundle branch block).
In complex arrhythmias that are not diagnosed by a standard ECG, intracardiac ECG and programmed electrophysiological study are indicated to precisely record the spread of excitation.
Class and exam questions
This is due to anatomy. The mass of the right ventricle (reflected in V1) is small, so the r wave is small. The dominant electrical activity of the thick left ventricle is directed away from the V1 electrode, forming a deep negative S wave.
This is due to anatomy. The mass of the right ventricle (reflected in V1) is small, so the r wave is small. The dominant electrical activity of the thick left ventricle is directed away from the V1 electrode, forming a deep negative S wave.
How to quickly calculate heart rate from an ECG strip?
For a rough express assessment, divide 600 by the number of large squares (5 mm in size) fitting between two adjacent R waves.
Where is the P wave best visualized?
For visualization and assessment of the rhythm source, standard lead II and V1 are considered optimal.
What else to study on this topic
- Methods for determining the electrical axis of the heart (EAH)
- Detailed analysis of QRS complex parameters
- Assessment of ST segment shifts
- Characteristics of the T wave and Q-T interval
- Functional ECG tests
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