The atrial T wave: The elusive electrocardiographic wave exposed by a case of shifting atrial pacemaker
Introduction
It was in the late 19th century that Einthoven designated P, QRS and T as the distinctive features of the electrocardiogram (ECG) [1]. These well-known designations correspond to atrial depolarisation (P wave), ventricular depolarisation (QRS complex) and ventricular repolarisation (T wave). Perhaps its low amplitude and almost complete concealment by the large ventricular deflections ensured that the ECG feature related to atrial repolarisation was neglected. However, a growing research interest in atrial repolarisation necessitated a naming convention and Hering is credited with ascribing the phrase ‘T wave of the auricle’ and its abbreviation ‘Ta wave’ in 1912 [2].
The Ta wave directly follows the P wave but with opposite polarity, and its amplitude is about a third of the P wave [3]. Its duration can extend to more than 400 ms after the end of the P wave, so a substantial part of atrial repolarisation occurs during ventricular depolarisation and repolarisation [4]. As such, the Ta wave can contribute to ST depression or elevation, and concerns about its effect on diagnostic accuracy have been reported [5].
The majority of studies analysing the Ta wave have done so in the context of AV block when the complete atrial depolarisation and repolarisation cycle may occur outside any obscuring ventricular activity [2], [4], [6]. In sinus rhythm without heart block, only a very small segment of the Ta wave is observable from which polarity but not duration can be obtained [3], [7]. Here we present a case of a patient with serial Ta wave (and P wave) polarity changes which clearly illustrates the effects on the ECG of the Ta wave during sinus rhythm without heart block. We also show that these effects are replicated by a simple conceptual model.
Section snippets
Case report and discussion
The case is a 49-year-old male with 12-lead ECG recorded during catheter ablation for atrial fibrillation but in sinus rhythm at the time of the recording. The ECG P wave exhibited distinct polarity changes (Fig. 1), progressing from initially negative (Fig. 1B) to biphasic (Fig. 1C) to positive (Fig. 1D) and resolving to negative polarity (Fig. 1E) after a few seconds. The corresponding PR intervals are 100 ms for the negative P wave and 160 ms for the positive P wave. This is suggestive of a
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