Discussion:
This was a prospective interventional study exploring the idea that
administration of intravenous adenosine bolus at 10 minutes post
radiofrequency ablation of an AP might identify dormant pathway
conduction which will otherwise require a waiting period of 30 minutes.
It was hypothesized that absence of conduction with adenosine at 10
minutes would predict the same at 30 minutes. We found that adenosine
testing at 10 minutes had a low sensitivity of 37.5% and a negative
predictive value of 94.9%. Hence, it does not appear to be reliable
enough to allow an abbreviation of the waiting period.
Alvarez et al utilized adenosine triphosphate [ATP] at the end of 5
minutes after ablation to unmask dormant accessory pathway
conduction.5 The present study utilized intravenous
adenosine as it is widely available while Alvarez et al utilized ATP
which is currently not widely available. We postulated that
administration of adenosine at 10 minutes rather than at 5 minutes would
increase the predictive ability of AP recurrence at 30 minutes. In that
study 108 ATP tests were performed in 100 patients and had 9 positive
tests, 82 negative tests and 17 tests were indeterminate. They reported
a sensitivity of 69%, which is higher than in our study, but still far
from ideal. There were 4 early recurrences among the 82 patients with
negative tests (4.9%) in their study while ours had 5 early recurrences
among 99 patients with negative tests (5.1%).
In the present study, there were 8 indeterminate tests out of 110 tests
(7.2%) while Alvarez et al had 17 (15.7%).5 The
smaller number of indeterminate results in the present study could have
been due to administration of a higher dose of adenosine 18mg through a
large bore peripheral intravenous line which was immediately followed by
a bolus of 20 ml normal saline.
Adenosine prolongs conduction in the AV node and causes AV block
transiently. Adenosine reduces the refractoriness of AP
conduction.15 Spotniz et al proposed three models for
unmasking dormant accessory pathway conduction: slow conduction,
linking, and excitation recovery model.10 Of these,
90% of the unmasking of AP conduction was due to excitation recovery
model wherein the administration of adenosine caused membrane
hyperpolarization and thereby reduced pathway refractoriness. The early
recurrence of pathway conduction is a marker of late recurrence, and it
can represent about 50% of total recurrences.4 Hence,
it is of paramount importance to detect early recurrence by monitoring
the patient for 24 hours post-procedure.
When there is a reversible injury to the accessory pathway as in the
perilesional zone of radiofrequency ablation, the pathway is partially
depolarized, making it unexcitable, and refractory to impulse
conduction. Adenosine by causing membrane hyperpolarization makes the
pathway re-excitable and thereby unmasks dormant accessory pathway
conduction. After any reversible injury, the pathway regains its
electrophysiological properties after a variable period, which are
hastened by adenosine. However, this recovery time is ill-defined, and
10 minutes is probably insufficient when compared to 30 minutes.