Effects of midodrine on experimentally induced postural hypotension and venous pooling in dogs

Effects of (+/-)-2-amino-N-2,5-(dimethoxyphenyl-beta-hydroxyphenyl) acetamide hydrochloride (midodrine) on experimental postural hypotension and venous pooling were examined in the anesthetized dog treated with hexamethonium (20 mg/kg s.c.). A 30 degree-tilt caused a significant decrease in blood pressure. Midodrine, at 0.1 mg/kg, significantly attenuated the decrease in blood pressure induced by the tilt 10, 20 and 30 min after its administration. The heart rate was not affected by the tilt. Cardiac output decreased significantly following the tilt. Midodrine significantly increased the basal rate of cardiac output and attenuated the decrease in cardiac output induced by the tilt. Venous pooling was produced by the occlusion of the circulation of the dog foot, and the increase of limb girth by the cuff inflating was measured as an indicator of venous pooling. Midodrine, at 0.1 mg/kg, significantly attenuated the increase of limb girth, indicating that the drug reduced venous pooling. These results suggest that midodrine (0.1 mg/kg i.v.) constricts the venous bed, resulting in the increase in cardiac output in the anesthetized dog with experimental postural hypotension.