QRS complex | Case-based insights: ECG

Bundle branch blocks are discussed in a separate section. The axis of the narrow QRS complex tells a lot about the presence of cardiovascular or pulmonary disease. A horizontal heart may indicate the presence of left-ventricular strain (e.g. hypertension), whereas a vertical heart or right axis deviation may indicate the presence of right-ventricular strain. A SIQIII pattern, like an SI–III axis (sagittal type), is typical of acute or chronic right heart strain, e.g., pulmonary disease or vitiation with left-to-right shunt. Low voltage may indicate pericardial effusion, and high voltage in left and right cardiac leads may indicate asthenic habit and funnel chest. Strange axis orientations, e.g., with QRS axis >120° or <-90°, may be due to situs inversus or an incorrectly connected ECG.

Q waves indicate resolved myocardial infarction. These Pardee Q waves are indicative of myocardial scarring and therefore do not disappear during deep inspiration, in contrast to the Q waves in the SIQIII pattern, in which the Q waves at least become significantly smaller during deep inspiration.

Specific indices of QRS amplitude indicate left- or right-ventricular hypertrophy. They apply only in the absence of bundle branch blocks. The Sokolow index for right-ventricular hypertrophy (R in V₁ + S in V₅ >1.05 mV) is highly specific but not very sensitive. It becomes positive especially in congenital vitiation with right heart strain. The Sokolow index for left-ventricular hypertrophy (S in V₁ or V₂ + R in V₅ oder V₆ >3.5 mV) is sufficiently specific as reported and somewhat more sensitive for left-ventricular hypertrophy than the traditional index (S in V₁ + R in V₅) is. Especially in case of left-anterior hemiblock, the Sokolow index is often false-negative. Therefore, alternative indices are often helpful: the Lewis index from the limb leads: (R – S in I) + (S – R in III), positive from 1.7 mV; also the Sokolow-Lyon II index: R in aVL >1.1 mV. The Cornell index mixes extremity and chest wall leads: R in aVL plus S in V₃ (positive from 2.8 mV for males and from 2.0 mV for females). The precordial index (S in V₂ + V₃, positive from 3.5 mV) may also be helpful.