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Left atrial myocardium

Atrial conduction

Activated 30–40 ms after the right atrium, with the impulse arriving from the SA node via Bachmann's bundle, and generating the terminal half of the P wave. The myocardial sleeves extending into the pulmonary veins are the dominant source of the ectopic triggers that start atrial fibrillation.

Traced from the start

  1. Sinoatrial node
  2. Left atrial myocardium

Detail

ECG
Writes the second half of the P wave; its late activation projects as the terminal negative deflection of the P in V1
Arterial supply
Left atrial branches of the LCx (and the SA nodal artery when it arises from the LCx)
Pulmonary vein sleeves
Atrial myocardium extending 1–3 cm into the pulmonary veins with automaticity and micro-re-entry — the target of pulmonary vein isolation
Haemodynamics
Atrial contraction ('atrial kick') supplies 20–30% of ventricular filling, more in a stiff ventricle

When it goes wrong

Rapid pulmonary-vein ectopic firing on a dilated, fibrotic left atrial substrate

Atrial fibrillation: irregularly irregular narrow-complex rhythm with no P waves and a fibrillatory baseline; stasis in the left atrial appendage causes cardioembolic stroke — anticoagulate by CHA2DS2-VA (Australian guideline, no sex point): 0 none, 2 or more anticoagulate, NOAC preferred to warfarin

Left atrial pressure load — mitral stenosis or regurgitation, hypertensive or aortic-stenosis LVH, hypertrophic cardiomyopathy

P mitrale: P above 110–120 ms, bifid in II with peaks more than 40 ms apart, terminal negative portion in V1 deeper than 1 mm and wider than 40 ms

Loss of coordinated atrial contraction at the onset of AF

Loss of the atrial kick: acute decompensation in aortic stenosis, hypertrophic cardiomyopathy or diastolic heart failure; loss of the 'a' wave in the JVP

Practise this structure

2 questions in the bank tagged Cardiac Conduction.

A 24-year-old man presents with two hours of palpitations. He is alert, has no chest pain, and his blood pressure is 124/76 mmHg. The radial pulse is regular at…

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A 46-year-old labourer is freed after six hours trapped beneath a collapsed brick wall. Both thighs are tense, swollen and tender, and he cannot move his toes.…

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