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Patient
65-year-old man, hospitalized for sudden syncope;
Trace
Regular sinus activity; one out of two P waves is blocked: 2:1 second-degree AV block pattern; wide QRS with left axis: right bundle branch block pattern with left anterior fascicular block;
Exergue
In 2:1 block, one P wave is conducted and the following wave is blocked. It is difficult to localize the exact location of the block. The analysis of earlier or later tracings may suggest a supra-Hisian block (the presence of Wenckebach periods) or an intra- or infra-Hisian block (presence of a typical Mobitz 2 tracing), with differentiation between the 2 having significant prognostic importance. Electrophysiological study also allows defining the location of the block (supra-Hisian: blocked P wave not followed by a His potential; intra- or infra-Hisian: blocked P wave followed by a His potential).
2-to-1 AV block deserves to be individualized as it is relatively common and represents a border zone between second-degree block and high-grade block since it comprises the same number of blocked waves as conducted P waves.