• Atrial fibrillation (AF) arises from multiple micro-reentrant circuits that create rapid, irregular poorly defined atrial fibrillatory (f) waves going at a rate between 350 and 600 bpm.
• Occasionally an atrial impulse successfully reaches the ventricles via AV-His-Purkinje pathway, resulting in narrow QRS complexes with irregularly irregular rhythm and varying rate:
Slow ventricular response: < 60 bpm, associated with sinus or AV node dysfunction
Rapid ventricular response: > 110 bpm, associated with neurohormonal changes (seen in exercises, stress, infection, low cardiac output, etc) and also with pre-excitation in WPW syndrome (ventricular rate is often > 300 bpm)
• Triggers for atrial fibrillation are often ectopic focal discharges from pulmonary veins (#1), posterior LA, ligament of Marshall, coronary sinus, vena cava, septum, appendages.
• When impulses arising from AF are conducted via an accessory pathway, the resulting rhythm is called "preexcited AF".
• Preexcited AF can appear similar to ventricular tachycardia (VT) or SVT with aberrancy or pre-excitation.
• Comparing to VT, preexcited AF is more irregular and does not degenerate into ventricular fibrillation as quickly.
• When AF rhyhtm is regular, it's called "regularized AF".
• This rhythm arises from an escape junctional or ventricular rhythm in the setting of complete AV block and underlying AF.
• Seen in digitalis toxicity.
• After a long cycle (shown as a long RR interval below), the refractory period of the bundle branch cells is prolonged and since right bundle branch has longer refractory period than left bundle branch, conduction via right bundle branch is slowed down resulting in right bundle branch block (RBBB) morphology on ECG.
• As a result, RBBB follows a short RR interval which is preceded by a long RR interval.
