Atrial Repolarization Wave Mimicking ST Depression

A 25 year old woman presented with a caffeine overdose and chest discomfort.  This is her ECG:

There is sinus tachycardia (rate = 120) with what appears to be diffuse ST depression in leads II, III, aVF and V2-V6.  However, if you look closely, the PR segment is downsloping.  This is due to a pronounced negative atrial repolarization wave (atrial T-wave, or "Ta-Wave").  The wave is still negative at the J-point, and thus depresses the J-point.  

The ST segment is most commonly measured at the J-point and relative to the PR segment, but when there is a Ta-wave, this method is inaccurate.  K. Wang (my mentor) recommends measuring the ST segment relative to the end of the PR segment (this is also called the PQ junction, and the recommended location for measurement according to ACC/AHA)  However, in my experience, when there is a Ta-wave, the PR interval is still downsloping at this point and this method of measurement will underestimate the effect of the Ta-wave.  Also, in my experience, and contrary to research I outline below (and which contradiction I cannot explain), the greatest part of the Ta-wave is back to baseline by 60-80 ms after the J-point. In the case above, if you measure the ST deviation at 60-80 ms after the J-point and relative to the TP segment, you'll see that there is no ST depression.

Many textbooks recommend measuring the ST segment at 60-80 ms after the J-point and relative to the TP segment, presumably because it helps to avoid the issue of the Ta-wave.  However, especially in tachycardia, the TP segment may never come back to baseline after the T-wave; furthermore, the T-wave has often begun by 80 ms after the J-point, as in this case.

Below is a schematization of the Ta-wave:

The atrial  repolarization wave lowers the baseline, but its amplitude, if present at all, is not great (maximum 0.2 mV, or 2 mm at normal recording).  It is not finished until up to 180 ms after the J-point (see references below).   

How do you recognize the Ta-wave?  First, you have to be aware of it and look for it.  Then, you have to imagine a curve, like this drawing:

The Ta-wave inscribes a parabolic curve that can be imagined when viewing the ECG

Learning point: Beware diagnosing ST depression before considering the atrial repolarization wave as the etiology.

More Detail on the Ta-wave:

The Ta-wave is a mean of 320 ms after the end of the p-wave, with a duration of 2-3x that of the p-wave and a polarity always opposite of the p-wave.  The PTa duration (onset of p-wave to end of Ta-wave) is a mean 440 ms, though it varies with heart rate just like the QT interval.  Thus, if the PR interval is 160 ms, the Ta-wave ends about 280 ms later.  If there is normal conduction of 100 ms, the Ta-wave may still be present at 180 ms after the end of the QRS!  Even if the patient has Bundle Branch Block with a duration of 140 ms, then the Ta-wave may still be present 140 ms after the end of the QRS. 

Here is a short explanation of the atrial repolarization wave.

Here are two detailed articles measuring the Ta wave: one by Holmquist et al. and another by Debbas et al.