Answer

There are clearly visible P-waves in most of the leads (Fig. 1). The P-waves are negative in leads DII, DIII and aVF. Hence, this is not a sinus rhythm. All the P-waves have the same morphology, constant PP interval at a rate of 210 beats per minute (bpm), with an isoelectric baseline between them. Therefore, the underlying rhythm is unifocal atrial tachycardia [1].

Fig. 1
figure 1

The irregular atrial tachycardia

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Lead DII shows 12 P-waves, but some of these waves are barely discernible because they are superimposed on the QRS-T complexes (Fig. 2). In the first part of the strip there are P-waves alternately conducted and non-conducted to the ventricles, characterising a 2:1 AV conduction. In the middle half of the strip there is a longer RR interval where only the ninth P-wave conducts to the ventricles, while the seventh, eighth and tenth P-waves are blocked, realising a 4:1 AV conduction. So, the atrial tachycardia presents an alternating 2:1 and 4:1 AV conduction. The long RR interval is less than twice the short RR intervals and the PR intervals progressively lengthen until a pause with 3 non-conducted P-waves occurs. These elements are consistent with alternating Wenckebach periodicity, a rare phenomenon characterised by a block in two levels of the atrioventricular node; one proximally, giving rise to a 2:1 block and one distally, responsible for the Wenckebach periodicity that explains the progressive PR lengthening until the non-conducted P-wave [2]. Alternating Wenckebach periodicity is encountered primarily in atrial tachyarrhythmias; the most frequent block level is the atrioventricular node, but it has been described in almost every level of the conduction pathways, including accessory ones [3].

Fig. 2
figure 2

The alternating Wenckebach phenomenon visible on the DII lead (see text for discussion) (A atria, AVN atrioventricular node, P proximal block, D distal block, V ventricles, 1–12 P-waves)

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