Sinus bradycardia during injections directly into the sinus node artery

In over 90% of 800 experiments in 75 dogs injection directly into the sinus node artery produced sinus bradycardia. Studies to explain this phenomenon included control of temperature, pH, osmolarity, oxygen, and ionic content of injecting solutions. Although unphysiologic variations of any of these are known to produce marked alterations in the sinus mechanism, sinus bradycardia from injection still occurred when they were controlled. Neurogenic mechanisms were excluded by bilateral cervical vagotomy, and by direct perfusion of the sinus node with atropine, hexamethonium, and trimethaphan. Because of the unique anatomic structure of the sinus node, which completely surrounds its nutrient artery, it is suggested that the sinus bradycardia may simply be the consequence of distending the sinus node artery. Implications concerning autoregulation of the normal heart rate are discussed.