Cardiovascular – Heart Rotation & Electrical Axis

Heart Rotation & Precordial QRS Morphology

• Progressive increase in R-wave height from $V1 \rightarrow V6$ reflects greater left-ventricular myocardial thickness and endocardium-to-epicardium depolarisation.

• S-wave depth decreases from $V1 \rightarrow V6$.

• Initial QRS deflection:
– Positive (upward R) in $V1–V3$.
– Becomes negative (Q) by $V6$.
– Caused by physiological rotation of the heart around a near-vertical axis (left hip → right shoulder); not itself pathological.

• Electrode placement matters: If $V4–V6$ electrodes sit further from myocardium, their R waves may appear smaller.

• Hypertrophy effect: Larger R and S deflections when ventricular wall thickens (eg, LVH from hypertension or aortic stenosis). Thin patients may show seemingly ‘high’ R waves over $V4–V6$ without disease.

Electrical Axis of the Heart

• Depolarisation produces vector loops; movement toward an electrode ⇒ positive deflection, away ⇒ negative.

• Degree of deflection depends on the angle between the depolarisation wave and the electrode’s viewpoint.

• Hexaxial reference system assigns each frontal-plane lead a fixed angle; example angles:
$+90^\circ\,(II),\ +60^\circ\,(aVF),\ 0^\circ\,(I),\ -30^\circ\,(aVL)$.

• Axis estimation: Compare R-wave height vs. S-wave depth across leads; the lead with the largest positive QRS indicates the general axis direction (larger deflection ⇒ axis points toward that lead).

• Example (text): Larger deflections in leads II & aVF suggest downward/rightward axis; larger in I & aVL suggest leftward axis.

• Conceptual summary: Electrical axis represents the summed ventricular depolarisation direction; deviations from normal may signal chamber enlargement or conduction defects.