[Partial interatrial block due to a conduction defect of the preferential middle interatrial depolarization pathway]

The authors report the case of partial interatrial block arising in the mid zone of the left atrium as demonstrated by oesophageal recordings. These showed that atrial depolarisation descending normally in the superior part of the left atrium was abnormally retrograde in the inferior part; this suggests a block of the mid-interatrial pathway and conduction via Bachmann's pathway in the upper part of the left atrium and via a retrograde pathway in the inferior region. The surface ECG showed a P wave of normal duration which was diphasic in Leads III and aVF with a normal PR interval. In addition, endocavitary recordings in the inferior part of the right atrium showed the presence of preatrial potentials potentials during three ectopic complexes, the significance of which is uncertain.