[Influence of intravenous clonidine on normal and pathological sinus and atrioventricular nodes and carotid sinus sensitivity].

A number of clinical observations have suggested that clonidine may be responsible for dizziness and even syncope. The aim of this study was to assess the effects of this drug on normal and pathological sinus and AV nodes and on carotid sinus sensitivity. 19 patients were investigated (average age: 73 years). 14 patients complained of dizziness or syncope, including 3 patients with spontaneous sinus node dysfunction. 5 patients were asymptomatic; 3 were investigated for severe sinus bradycardia (1 on clonidine); 1 patient had sinoatrial block and 1 patient underwent pre-operative assessment for intraventricular block. The sinus node was studied using Mandel's method at 100, 120 and 150/min; the AV node was studied by the extrastimulus method with fixed atrial cycle of 600 ms. The following parameters were measured: Wenckebach point, AH interval in spontaneous and paced cycle length of 600 ms, effective refractory periods. Carotid sinus sensitivity was tested by right and left carotid sinus massage. These parameters were measured under basal conditions and 15 and 30 minutes after IV injection of 0.150 mg of clonidine. Two groups of patients were identified from the results under basal conditions: group 1:11 patients with corrected post-stimulation pauses less than 525 ms, and group 2:8 patients with at least one corrected post-stimulation pause of over 525 ms. Clonidine influenced the post-stimulation pauses significantly in both groups. However, the number of pathological pauses increased much more in group 2 than in group 1.(ABSTRACT TRUNCATED AT 250 WORDS)