Sinus node physiology

Sinus rhythm with a rate of less than 60/min is defined as sinus bradycardia. This bradycardia is usually a physiological response. Pathological and/or symptomatic sinus bradycardia may suggest sinus node dysfunction (see sick sinus syndrome). Vagally induced sinus bradycardia may be responsive to atropine, but only needs to be treated if symptomatic. Atropine doses of less than 0.5 mg may cause a paradoxical increase in vagal bradycardia. 

Margolis JR, Strauss HC, Miller HC, Gilbert M, Wallace AG. Digitalis and the sick sinus syndrome. Clinical and electro-physiologic documentation of severe toxic effect on sinus node function. Circulation 1975 52(l) 162-9. 

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