Haemodynamic Changes Following Separate and Combined Administration of Beta‐Blocking Drugs and Quinidine*

Haemodynamic parameters were measured in dogs after the administration of quinidine, propranolol and practolol, alone or in combination. Following the single administration of these drugs, left ventricular dp/dt decreased by 38%, 48%, and 39%, respectively. Following the simultaneous administration of the same amount of quinidine and propranolol, the decrease was only 15%; following the combined administration of quinidine and practolol, the decrease was 16%. The administration of quinidine 20 minutes after pretreatment with propranolol, resulted in an 8% decrease in left ventricular dp/dt. Similarly, the negative effect of these drugs on right ventricular force became significantly reduced when the drugs were administered jointly. The significance and possible mechanism leading to the interaction between beta‐blocking agents and quinidine, are discussed.