Sinus node electrograms were obtained in two patients with unexplained
syncope and the cardioinhibitory form of the hypersensitive carotid sinus syndrome. Direct recordings of sinus node potentials were obtained using a transvenous
electrode catheter. Sinus node function was normal in both patients during standard electrophysiologic evaluation. Carotid sinus
massage was performed in both patients and the sinus node electrogram was continuously recorded. After the onset of carotid sinus
massage, prolongation of sinoatrial time, slowing of sinus rate of depolarization,
sinoatrial exit block and finally sinus node arrest were recorded. After termination of carotid sinus
massage, sinus node potentials did not precede the first atrial impulse; subsequent beats showed markedly prolonged sinoatrial times as well as changes in the P wave on the surface electrocardiogram. Sinus rate and sinoatrial time returned to control values gradually, as did the P wave configuration. Intravenous
atropine (1.0 mg) abolished the abnormal response to carotid sinus
massage. It is concluded that the application of carotid sinus
massage in patients with the hypersensitive carotid sinus syndrome produces profound changes in sinoatrial conduction including
sinoatrial exit block, as well as shifts in primary pacemaker site and sinus node arrest. These alterations in conduction and automaticity are reversible with
atropine and may be secondary to
denervation sensitivity to
acetylcholine.