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Patient
63-year-old man with no particular prior history;
Trace
Atrial activity originates from the sinus node with a normal duration of the PR interval and QRS complex; on this tracing there are 2 premature atrial contractions (PACs) arising from the lower aspect of the atrium (probably in proximity to the coronary sinus) given the negative atrial activity in the inferior leads; the QRS complexes following these PACs are narrow and identical to the sinus complexes; the P'R is relatively short, the ectopic focus being located near the atrioventricular junction; partially compensated pause during the cycle following the PAC;
Exergue
A premature atrial complex corresponds to an ectopic and premature atrial electrical activity. Usually, the following QRS is narrow but the QRS may be broad in the case of a pre-existing bundle branch block or due to aberrant conduction (functional block).
A PAC (P' wave) corresponds to ectopic atrial activity, meaning that it occurs early relative to the sinus activity (P) and originates from outside the sinus node.