Which of the following best describes the source of impulses in atrial flutter?

Atrial flutter is characterized by a specific type of abnormal heartbeat where the impulses originate from a reentrant circuit within the atria. This reentrant circuit causes the electrical impulses to move in a circular path, leading to rapid contractions of the atria. The impulse originates from an area known as the cavotricuspid isthmus in the right atrium and continues in a circular fashion, which is a distinct feature of atrial flutter.

This circular propagation results in the characteristic "sawtooth" pattern seen on an electrocardiogram, often referred to as "flutter waves." In contrast, the other options do not accurately describe the mechanism of impulse generation in atrial flutter. The SA node primarily governs normal rhythm; impulses from the ventricles would not contribute to atrial flutter, and multiple ectopic foci would suggest a different arrhythmia such as atrial fibrillation. Therefore, the most accurate description of the impulse generation in atrial flutter is indeed from a source within the atria in a circular pattern.