Happy New Year from everyone at SCST! Let’s kick off 2022 with Dr Dave Richley’s ECG of the Month

This ECG was recorded from a 38-yearold female who was complaining of a fast heart rate. There was no chest pain or breathlessness and no history of heart disease. There is ST elevation in many leads. Is this evidence of a silent developing myocardial infarction, or pericarditis, or is there some other, more likely, explanation?

The Answer

It’s time to discuss the atrial repolarisation (Ta) wave. The main ECG deflections, obviously, are the P wave, QRS complex and T wave, corresponding to atrial depolarisation, ventricular depolarisation and ventricular repolarisation. The Ta wave is generally ignored and this is because it tends to be very small and of no importance.

The Ta wave is usually opposite in polarity to the P wave and of low amplitude such that in most leads, when it can be seen, it is a shallow, negative deflection, broader than the P wave and immediately after it. Effectively it is buried in the PR segment and QRS complex but can be seen when a P wave is not followed by a QRS complex, as in second-degree or complete AV block.

Figure 1. Isolated P wave with visible Ta wave (ringed in red).

Figure 1 is a short extract from lead II in an ECG that showed second-degree AV block. An atrial repolarisation wave, which drops to 1 mm below the TP baseline, is clearly visible immediately after the blocked P wave. The depth of a Ta wave is proportional to the height of the P wave that precedes it, so when there is a tall P wave, as in the ECG in figure 2, from a patient with right atrial abnormality, the Ta wave is more deeply inverted than normal and therefore more clearly visible.