[Studies of nodal conduction].
Nodal conduction has been studied in 44 normal subjects, 21 cases of 1st or 2nd degree AV block, 16 cases of LGL syndrome, and 19 patients with a distal infra-His AV block, by means of stimulation which was increased in regular steps of 5 beats per minute, and of extra-systolic stimulation. These two techniques demonstrate that the nodal response to an extrasystole or a regular tachycardia is not the same, and that different mechanisms are probably at play. With regular stimulation, the normal limits of 1/1 conduction lie between 140 and 200 per minute. A nodal block for a rate less than 130 per minute can be taken as indicative of a nodal defect. Delayed AH conduction is a much less precise criterion. It may reach a high level in normal subjects, and it is not certain that the finding of a PR interval of more than 200 msec. (AH of more than 140 msec.) at a normal rate is neccessarily indicative of nodal pathology. Cases of LGL syndrome are associated with short-circuits which are usually partial. The diverse responses observed in these different groups can be explained by modifications of structure within the A/V node itself.