Premature atrial complex hidden in the T wave

ECG: Atrial activity originated initially from the sinus node with a normal duration of the PR interval and QRS complex; there are two premature ventricular beats corresponding to extrasystoles; the QRS complexes are narrow and identical to the sinus beats, indicating that these are not premature ventricular complexes; a careful analysis (lead III) of the T wave preceding the premature beat shows a slight change in the T wave pattern pointing to the diagnosis of premature atrial complexes;

Comments: A premature atrial complex may occur, as on this tracing, during ventricular repolarization (on the T wave following the preceding QRS complex). The surface electrocardiogram registers on the skin, all of the potentials generated by the heart. Due to a lower mass, the amplitude of the signals generated by the atria is much weaker than that of the signals generated by the ventricles (QRS complex or T wave). In addition, the gentle slope of these two signals (P wave and T wave) is relatively similar and significantly differs from a QRS complex (steep slope). Diagnosing a premature atrial complex may therefore at times be difficult and careful analysis of all leads is necessary.

Focal atrial arrhythmias have been shown to be associated with the initiation of atrial fibrillation. Indeed, the type of premature atrial complexes observed on this tracing, with a characteristic P-on-T pattern and arising mainly from the ostium of the pulmonary veins, represents the initiating trigger necessary for the induction of atrial arrhythmias. The therapeutic implication is straightforward since these premature beats are accessible to radical ablation treatment (disconnection of the pulmonary veins). It is therefore necessary to identify this type of “hidden” premature atrial complexes in the T wave and to differentiate the latter from junctional ectopy which represent the differential diagnosis for a premature beat with narrow QRS.

Take-home message: An early premature atrial complex can be completely or partially hidden in the T wave of the preceding QRS-complex. Careful analysis of all leads is sometimes necessary, as changes in the amplitude of the T wave may be modest. They may be manifested by a simple notching or irregularity of the T wave. When they are not visualized, it can lead to erroneous diagnosis of sinus node dysfunction (when the premature atrial complex is blocked) or premature ventricular contraction (when the atrial extrasystole is followed by aberrant ventricular conduction).