This ECG is notable for two additional abnormalities. First, each sinus P wave is associated with an atrial depolarisation (P′) wave (arrowheads in figure 2) that is superimposed upon and deforms the ST segment. Each P′ wave shows a very short coupling interval (approximately 240 msec) thereby raising suspicion that it may be a manifestation of atrial reciprocal activation. Yet, this was precluded by the absence of lengthening of the preceding PR interval, which is a prerequisite of its development. Consequently, P′ waves were interpreted as a manifestation of premature atrial depolarisation, and because of a fixed coupling interval they were ascribed to reentry in the atrial myocardium. None of the premature P′ waves is followed by a pause, suggesting, failure to penetrate the sinus node, that is, sinus nodal entrance block. The PR interval is stable at approximately 160 msec and does not prolong after a premature P′ wave, suggesting that the latter fails to penetrate a significant part of the atrioventricular node, that is, atrioventricular nodal entrance block. Premature P′ waves are inverted in leads II, aVF, and III, and upright in leads aVR and aVL, indicating a P′ wave vector directed superiorly in the frontal plane, thus suggesting an ectopic focus in the posteroinferior atrial wall [1, 2]. Furthermore, premature P′ waves are upright in lead V1 and inverted in leads V5 and V6, indicating a P′ wave vector directed anteriorly and rightward in the horizontal plane, thus suggesting a left atrial ectopic rhythm. The second abnormality detected in this ECG is PR segment depression with respect to the TP segment in leads II, aVF, V3 and V4 (arrows in fig. 2), which, together with the non-conducted premature atrial complexes (P′ waves) were ascribed to posteroinferior left atrial infarction [3]. Scrutiny of the presenting ECG (fig. 1) also shows that the ST segment deviation vector is better seen in the horizontal than in the frontal plane and more in leads V3 to V4 than in leads V1 to V2. It is, therefore, inferred that early in an ischaemic process affecting the inferior and inferior-lateral left ventricular walls, the basal (formerly posterior) and mid inferior segments which likely bend upwards, were the segments affected the most [4]. Indeed, this was confirmed in a posterior-lead ECG, which disclosed at least 0.05 mV ST segment elevation in leads V7-V9. The patient underwent emergency coronary angiography, which showed a total occlusion in the proximal segment of a co-dominant left circumflex artery (arrow in fig. 3) tackled successfully with stenting, which achieved a good clinical outcome. Discharge ECG (fig. 4) showed signs of an evolved inferior wall myocardial infarction with extension to the lateral wall.