Interventricular septum
Fascicles, Purkinje, muscle
The first working ventricular muscle to depolarise: septal fibres of the left bundle activate the left side of the septum, so the initial QRS vector points left-to-right and anteriorly. The first 20 ms of every QRS depends on this.
Traced from the start
Detail
- ECG
- The septal vector writes the small r in V1 and the narrow septal q (under 40 ms, under 2 mm) in I, aVL, V5–V6; LBBB abolishes it and RV pacing reverses it
- Arterial supply
- Anterior two-thirds by LAD septal perforators; posterior third by the posterior descending artery (RCA in right-dominant hearts)
- Membranous septum
- Fibrous upper portion beneath the aortic valve; the His bundle runs along its inferior margin
- Conduction
- Activation sweeps left-to-right and apex-to-base at 0.3–1 m/s in working muscle
When it goes wrong
Septal infarction from proximal LAD occlusion
Loss of septal q waves and new Q waves in V1–V2 (V1–V4 in anteroseptal MI); risk of new RBBB, LAFB or complete heart block; days 2–7, ventricular septal rupture with a new harsh pansystolic murmur and an oxygen saturation step-up between right atrium and right ventricle
Asymmetric septal hypertrophy in hypertrophic cardiomyopathy
Deep narrow 'dagger' Q waves in lateral and inferior leads mimicking infarction; substrate for VT and sudden death in the young
Distinguishing pathological from septal Q waves
Pathological: 40 ms or wider, or deeper than 25% of the following R (over 2 mm), in two contiguous leads; septal q waves are narrower and smaller
Practise this structure
1 question in the bank tagged Cardiac Muscle.