Left ventricular myocardium
Fascicles, Purkinje, muscle
The dominant electrical mass: after the septum, the LV free wall depolarises from endocardium to epicardium and from apex to base, generating the main QRS vector (normal axis −30° to +90°). Repolarisation runs the opposite way, epicardium to endocardium, so the normal T wave points the same way as the QRS.
Traced from the start
Detail
- Conduction
- Working myocardium 0.3–1 m/s through connexin-43 gap junctions in the intercalated discs; faster along the fibre axis than across it, an anisotropy that promotes re-entry in scar
- ECG timing
- QRS under 120 ms; ventricular activation is complete about 230 ms after sinus discharge; the posterobasal LV is last
- Action potential
- Fast Na+ upstroke (phase 0), plateau via L-type Ca2+ entry (phase 2, coupling to contraction), K+ repolarisation (phase 3); an effective refractory period of ~250 ms prevents tetany
- Arterial supply
- LAD (anterior wall, apex), LCx (lateral and posterior walls), RCA/PDA (inferior wall)
When it goes wrong
Re-entry around surviving muscle strands in an old infarct scar
Sustained monomorphic VT: regular broad-complex tachycardia with AV dissociation, capture and fusion beats, extreme axis and precordial concordance; DC cardioversion if unstable, amiodarone if stable, ICD for secondary prevention
Acute ischaemia, electrolyte derangement or degeneration of VT
Ventricular fibrillation: chaotic baseline with no output — immediate defibrillation
Hyperkalaemia raising the resting potential and inactivating Na+ channels
Peaked T waves, then flattened P and prolonged PR, then widened QRS, then a sine wave and asystole or VF; IV calcium to stabilise the membrane first, then insulin–dextrose and salbutamol
Class Ic sodium-channel blockers (flecainide) or tricyclic overdose
Use-dependent QRS widening (over 100 ms predicts seizures in TCA overdose), right axis and a terminal R in aVR; sodium bicarbonate
Pressure-load hypertrophy from hypertension or aortic stenosis
Voltage criteria for LVH (S in V1 plus R in V5/V6 of 35 mm or more; R in aVL of 11 mm or more) with a strain pattern of ST depression and asymmetric T inversion in lateral leads
Practise this structure
1 question in the bank tagged Cardiac Muscle.