Although
atropine is known to increase sinus rate through its vagolytic effect, the effects of
atropine on sinus node sensing are unknown. The purpose of this study was to investigate alterations in sinus node sensing produced by
atropine. Measurement of the zone of sinus node reset and sinoatrial conduction time was performed in 10 patients by programmed premature atrial stimulation. The zone of sinus node reset was determined as the transition point where premature atrial stimuli were followed by a less than compensatory pause. Sinoatrial conduction time was calculated from sinus node return cycles in the area where sinus node reset occurred.
Atropine administration produced a significant increase in the percentage of the sinus cycle length at which
premature atrial contractions penetrated and reset the sinus node. Sinus node reset occurred at a mean percentage of the sinus cycle of 71 +/- 8 per cent before
atropine and 83 +/- 5 per cent after
atropine (P less 0.01). The sinoatrial conduction time was significantly reduced from 109 +/- 29 to 62 +/- 23 msec. (P less than 0.01) from
atropine as sinus cycle length was reduced from 909 +/- 118 to 642 +/- 75 msec. after
atropine. Sinus node echoes were observed in two patients. In one patient
atropine abolished the appearance of sinus node echoes. In the second patient
atropine reduced the coupling interval necessary to produce sinus node echoes but appeared to facilitate sinus node re-entry by the appearance of an additional sinus node echo and a reduction in the echo cycle length. This study demonstrates that
atropine produces significant improvement of sinus node sensing in man.