During the cardiac cycle, which wave comes first?

The P Wave is the initial wave in the cardiac cycle, representing the depolarization of the atria, which occurs just before the atria contract. This electrical impulse is triggered by the sinoatrial (SA) node, which is known as the heart's natural pacemaker. The P Wave indicates the beginning of the cycle where blood is pushed into the ventricles from the atria.

Understanding the sequence of the waves in an electrocardiogram (ECG) is crucial, as it reflects the electrical activity of the heart. Following the P Wave, the QRS complex occurs, which corresponds to ventricular depolarization and marks the moment when the ventricles contract. The T Wave follows the QRS complex and represents ventricular repolarization, while the U Wave, often less commonly referenced, may represent repolarization of the Purkinje fibers or other aspects of electrical activity in the heart and is usually seen in specific conditions.

The P Wave's position at the start of this sequence underscores its importance in the overall process of the cardiac cycle.