Lead topography and Einthoven's triangle
To interpret any focal or hypertrophic changes, it is essential to clearly understand which lead "looks" at the right heart chambers and which at the left. Based on Einthoven's triangle and the placement of augmented and chest electrodes, the leads are distributed as follows:
- Right heart chambers: III, aVF, V1, V2.
- Left heart chambers: I, aVL, V5, V6.
Chest leads V1–V2 are anatomically projected onto the right hemithorax, and V5–V6 onto the left, which determines their "zone of responsibility". Knowing these two sets allows for the interpretation of virtually any local ECG change—from bundle branch blocks to focal ischemic changes and hypertrophy.
Conduction system blocks
Blocks occur at various levels of the conduction system: SA node, interatrial pathways (Bachmann's bundle), AV node, bundle of His trunk and branches.
- Sinoatrial block. The impulse fails to exit the SA node to the atria. An entire cardiac cycle drops out on the ECG: the P wave and QRS complex are absent.
- Intraatrial block. Delayed conduction through the atria manifests as prolongation (widening) of the P wave.
- First-degree AV block. Delayed conduction through the AV junction — persistent prolongation of the PQ interval with preserved complexes.
- Second-degree AV block, Mobitz I (with Wenckebach periodicity). Progressive prolongation of PQ from cycle to cycle followed by the dropout of a QRS complex. The P wave is preserved since atrial depolarization is not impaired.
- Second-degree AV block, Mobitz II. The PQ interval is fixed and prolonged, with intermittent dropouts of QRS complexes without prior PQ lengthening.
- Third-degree AV block (complete). The impulse is not conducted through the AV junction; the atria and ventricles operate at their own independent rhythms — P waves are registered independently of QRS complexes.
Bundle branch blocks
The key sign of a complete bundle branch block is widening of the QRS complex to more than 0.12 s and a characteristic M-shaped R wave. To determine the side of the lesion, the familiar lead rule is used:
- Right bundle branch block: M-shaped QRS in III, aVF, V1, V2.
- Left bundle branch block: mirror image — changes in I, aVL, V5, V6.
In V1–V2, left bundle branch block often registers a QS-type complex, which must not be confused with an infarction pattern. ST-segment depression in complete blocks reflects secondary repolarization abnormalities and does not indicate ischemia on its own.
Hypertrophy as myocardial adaptation
Hypertrophy is an increase in the myocardial muscle layer in response to chronic overload. Unlike skeletal muscle, the vascular network in the heart grows slower than the muscle mass increases, so pathological hypertrophy leads to the formation of areas of relative ischemia — a mismatch between blood flow and myocardial demand. Ischemia manifests primarily as repolarization abnormalities.
At the ECG level, a hypertrophied area produces two effects: increased electrical activity (increased amplitude of the corresponding wave) and slowed conduction (widening of the wave). At the same time, ECG diagnostics of hypertrophy has limited value: the method of choice is echocardiography, while ECG is used as a screening tool.
Atrial Hypertrophy
The atria have limited capacity for hypertrophy and quickly transition to the dilatation stage, which is why ECG signs of atrial hypertrophy are important as early disease markers.
- Left atrial hypertrophy. The left-atrial (second) phase of the P wave is prolonged and amplified: a notched, widened P wave (P-mitrale) forms in I, II, aVL, V5, V6.
- Right atrial hypertrophy. The right-atrial (first) phase is amplified: the P wave becomes tall and peaked, but not prolonged (P-pulmonale) in II, III, aVF, and sometimes in V1–V2.
Normally, a biphasic P wave can be registered in V1; in left atrial pathology, a notched P wave appears in the left leads, and the combination of signs of both hypertrophies indicates biatrial overload.
Ventricular hypertrophy
The left ventricle has the greatest hypertrophy reserve: its wall can increase to almost double the normal thickness (~2.5 mm). ECG signs of left ventricular hypertrophy:
- Tall R waves in left leads — I, aVL, V5, V6.
- Deep S waves in right leads — V1, V2 (mirror image).
- Increase in the intrinsicoid deflection time in V5, V6 to more than 0.05 s (from the onset of the Q wave to the R peak).
Right ventricular hypertrophy presents a mirror image: tall R in V1, V2 and deep S in V5, V6. Clinically, it is predominantly seen in children with congenital heart defects (specifically tricuspid valve disease), in patients with right-sided infective endocarditis (including intravenous drug use), and in severe cardiac patients with established cardiac remodeling. For the verification of hypertrophy and dilatation, echocardiography is the priority, while ECG plays only a supplementary role.
Key points
- The right heart chambers are reflected in III, aVF, V1, V2; the left in I, aVL, V5, V6.
- SA block manifests as the dropout of an entire cardiac cycle, intraatrial block as P wave widening.
- First-degree AV block is persistent PQ prolongation; second-degree Mobitz I is Wenckebach periodicity; third-degree is complete P and QRST dissociation.
- Complete bundle branch block: QRS > 0.12 s, M-shaped R; the side is determined by "right" or "left" leads.
- Myocardial hypertrophy is accompanied by relative ischemia due to lagging vascular growth.
- P-mitrale (notched, widened) is a sign of left atrial hypertrophy; P-pulmonale (tall, peaked) indicates right atrial hypertrophy.
- Left ventricular hypertrophy: tall R in V5, V6, deep S in V1, V2, intrinsicoid deflection time > 0.05 s.
- Definitive diagnosis of hypertrophy and dilatation is performed using echocardiography rather than ECG.