Autonomic reflexes in patients with cardioinhibitory carotid sinus syncope.

Autonomic reflexes were investigated in patients with the cardioinhibitory carotid sinus syndrome. Heart rate, blood pressure and forearm blood flow responses were recorded during prolonged head-up tilt, the Valsalva manoeuvre, lower body negative pressure and sequential respiratory cycles in ten patients and nine age-matched controls. The mean maximum R-R interval prolongation during carotid sinus massage was 4.4 s. Three patients had syncope during prolonged head-up tilt. Heart rate and blood pressure responses were similar for patients and controls who completed tilt. Responses to lower body negative pressure and Valsalva manoeuvre were similar for both groups. Respiratory sinus arrhythmia was significantly less marked in patients, 7 bpm (0 to 20 bpm) versus 15 bpm (6 to 35 bpm; p = 0.05). Therefore, neck massage and deep breathing produce abnormal cardiac vagal responses, whereas other reflexes, including response to lower body negative pressure and the Valsalva manoeuvre are within the normal range in cardioinhibitory carotid sinus syndrome.