What is this ST Depression? (Hint: there are 2 etiologies)

Discussion from above article by Manne JRR.Atrial repolarization wave (Ta wave) is usually not perceptible on the ECG as it has low magnitude of 100 –200 microvolts and is usually concealed by the ensuing QRS complex [1]. Occasionally, they are seen as shallow negative deflections right after the P wave in conditions with prolonged PR interval, but they are best seen in patients with complete heart block, when the Ta waves and QRS complexes are uncoupled [2]. In contrast to the QRS complex and T wave which under normal conditions have the same polarity, the polarity of the P wave is always opposite to that of the Ta wave in all leads [2]. The duration of Ta wave (average duration of 323  ± 56 ms) is generally 2-3 times longer than the P wave (average duration of 124 ± 16 ms) [2].Identifying the discernible Ta wave and its location is relevant as it has some important clinical and diagnostic implications. In conditions with short PR interval like sinus tachycardia, the Ta wave can blend into the ST segment and cause ST segment depression mimicking myocardial ischemia. The Ta wave voltage of an inverted or retrograde P wave is always larger than that of a sinus P wave [3]. In low atrial rhythm, the atrial activation initiates from an ectopic focus rather than the sinoatrial node, and it spreads from below to upwards in the atria. The retrograde conduction to the sinoatrial node causes the inverted P waves in inferior leads whil...
Source: Dr. Smith's ECG Blog - Category: Cardiology Authors: Source Type: blogs