Atrial ultrastructure in patients with fixed intra atrial block

This study describes ultrastructural changes seen in the right atria of 10 patients with fixed intraatrial block (FIAB) (P wave duration of 120 msec or more during sinus rhythm). The tissue, which was obtained by biopsy at the time of open heart surgery, is compared to that of 3 patients with normal right atrial morphology. One sample of left atrial tissue from a rheumatic subject is also described. The ultrastructural changes observed in the right atria of patients with P wave durations of 140 msec or more were much more extensive than in the patients with FIAB and P wave durations of 120 to 130 msec. These longer delays in intraatrial conduction may be due to extensive disturbances at the level of the ordinary working atrial myocardium, while the lesser degrees of block may simply be due to sluggish conduction through the anterior internodal tract, as is suggested by the data on intermittent intraatrial block.