AssessmentsPremature ventricular contraction
Premature ventricular contraction
USMLE® Step 1 style questions USMLE
A 55-year-old man comes to his cardiologist for a followup visit. He denies cardiac symptoms, though his EKG shows a ventricular fusion beat. Which of the following is the most likely potential cause of this patient’s fusion beat?
The heart has two lower chambers, called the ventricles, and a premature ventricular contraction is when the ventricles contract earlier than normal in the cardiac cycle. This happens because an abnormal contraction signal, called a depolarization, originates from somewhere in the ventricles rather than coming from the pacemaker cells.
So normally, the sinoatrial, or SA, node sends an electrical signal called a depolarization that propagates out through the walls of the heart and causes both upper chambers to contract. Then, that signal moves to the atrioventricular, or AV, node, where it’s delayed for a split second. Then, the signal travels down into the ventricles, or lower chambers, where it moves down the bundle of His into the left and right bundle branches and into each ventricle’s Purkinje fibers, causing them to contract as well. This trip is called a depolarization wave, and in a healthy heart, it makes sure that the upper chambers contract before the lower chambers contract. On an electrocardiogram, or ECG, which measures the electrical activity of the heart via electrodes that are placed on the skin, the atrial depolarization and its contraction are seen as a P-wave, the ventricular contraction is seen as a QRS complex, and the ventricular repolarization and its relaxation are seen as a T-wave.
This empty space is called the PR segment, and it corresponds to