Some clinical pictures of the heart can only be diagnosed at a certain heart rate. For an ECG this is stimulated, for example, through an activity which involves a substantial physical effort, like cycling (stress ECG).  

In order to perform a cardiac ultrasound under a simulated stress situation, a drug which briefly increases the heartbeat is administered. This way, symptoms can be diagnosed, especially in relation to the heart valves and the two chambers of the heart, which are not visible in a rest state.