Atrial fibrillation (part IV): ECG

Characteristic findings in the ECG (absence of the P-wave, irregular atrial oscillations replacing the P-waves, and the irregular ventricular response) permit a reliable diagnosis of atrial fibrillation in most cases. The morphology of f-waves (fine or coarse) seems to be of little clinical relevance. The irregular ventricular response is due to varying degrees of penetration of some of the atrial impulses into the AV node resulting in concealed conduction and is modified by the refractoriness of the atrio-ventricular node. Heart rate is further dependent on the quality of AV nodal conduction and is further influenced by the autonomic nervous system and drugs. Regularisation of the ventricular response may occur in depressed AV conduction, and the ventricular response may then be dominated by junctional or ventricular escape rhythms. Wide QRS complexes may be the result of functional ("Ashman-phenomenon") or organic conduction disturbances in the His-Purkinje system or of ventricular ectopy The probability of diagnosis is based on criteria such as the morphology of the QRS complex, the coupling interval and the sequence of RR-intervals. Episodes of atrial fibrillation are triggered in most cases by atrial extra-systoles, sometimes involving vagal mechanisms. Degeneration of other tachycardias into atrial fibrillation may occur (AV reentry, AV node reentry and atrial flutter). Ventricular standstill after the termination of atrial fibrillation is the result of depression of the sinus and subsidiary pacemakers. Atrial fibrillation should be differentiated from atrial flutter and atrial tachycardia with varying A-V response Tachycardia with wide QRS complexes (and an irregular ventricular response) are either due to aberrancy or to anomalous AV-conduction (WPW syndrome). Furthermore an irregular ventricular tachycardia has to be considered.