Changes in Properties of Heart Muscle Due to Mephentermine

Conduction times were measured from a point of stimulation just below the pulmonary conus to a site 10–12 mm below on the right ventricle and to a second point near the apex on the right ventricle. Atrioventricular conduction time was also observed. Maximal rates of stimuli which the ventricle could follow and ventricular voltage thresholds were measured. Atrial flutter was established by intervenous crush and subsequent atrial electrical stimulation. All these parameters were observed in control periods and after mephentermine, 3 mg/kg by vein. Mephentermine produced the following changes: conduction time via muscle and Purkinje tissue and refractory period of the ventricle were shortened. Atrioventricular conduction time was decreased. The ratio of atrial to ventricular beats in flutter was decreased. The nervous system does not play a role in these changes. It is concluded that the antifibrillatory action of mephentermine depends on decreased ventricular conduction time. This prevents the ventricle from responding to more than one order of excitation. Termination of atrial flutter due to a circus movement also depends on an increased conduction velocity.