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Right atrial myocardium

Atrial conduction

Working atrial muscle depolarised first, directly from the SA node, producing the initial half of the P wave. It contains the cavotricuspid isthmus between the inferior vena cava and the tricuspid annulus — the critical isthmus of typical atrial flutter.

Traced from the start

  1. Sinoatrial node
  2. Right atrial myocardium

Detail

Conduction velocity
0.5–1 m/s through working atrial myocardium; right atrial activation is complete within ~50 ms
ECG
P wave under 120 ms and under 2.5 mm, upright in I, II and aVF; atrial repolarisation (Ta wave) is buried in the QRS
Action potential
Fast Na+ upstroke with a shorter plateau and refractory period than ventricular muscle, so the atria can sustain rates of 300–600/min
Arterial supply
Right atrial branches of the RCA (SA nodal and right marginal branches)

When it goes wrong

Macro-re-entry around the tricuspid annulus through the cavotricuspid isthmus (typical anticlockwise flutter)

Atrial rate ~300/min with sawtooth negative flutter waves in II, III, aVF and upright waves in V1; 2:1 AV conduction gives a regular 150/min; adenosine unmasks the flutter waves; cavotricuspid isthmus ablation is curative

Chronic right atrial pressure load — pulmonary hypertension, cor pulmonale, tricuspid or pulmonary stenosis

P pulmonale: peaked P above 2.5 mm in II, III, aVF (above 1.5 mm in V1)

Premature discharge of an atrial ectopic focus

Premature atrial complex: early, abnormally shaped P wave followed by a narrow QRS and a non-compensatory pause; a blocked PAC (P hidden in the preceding T) mimics a sinus pause

Practise this structure

2 questions in the bank tagged Cardiac Conduction.

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