Differential response to carotid sinus pressure during sinus rhythm and atrial fibrillation.

In a patient who presented with syncope and palpitation left carotid sinus massage resulted in prolonged ventricular asystole during atrial fibrillation. In the same patient left carotid sinus stimulation during sinus rhythm produced no change in sinus rate and only slight PR interval prolongation. This very different response may be the result of factors which are relevant only during atrial fibrillation such as an increase in the atrial depolarisation frequency and a prolongation of the duration of the atrioventricular nodal concealment zone secondary to increased vagal activity. An increase in atrioventricular nodal refractoriness could not explain this differential response.