Final answer:
The dicrotic notch is formed during the isovolumic ventricular relaxation phase of the cardiac cycle when the semilunar valves close to prevent backflow of blood from the aorta and pulmonary trunk into the ventricles.
Step-by-step explanation:
The dicrotic notch occurs as a result of the isovolumic ventricular relaxation phase, the initial phase of ventricular diastole, when pressure within the ventricles falls below the pressure in the aorta and pulmonary trunk. As the ventricles relax, the pressure decreases, and the semilunar valves close to prevent blood from flowing back into the heart, creating the notch on blood pressure tracings.
The cardiac event responsible for the dicrotic notch is the isovolumic ventricular relaxation phase of the cardiac cycle. During this phase, the ventricular muscle relaxes, causing the pressure within the ventricles to drop below the pressure in the pulmonary trunk and aorta. As a result, blood flows back toward the heart, producing the dicrotic notch in blood pressure tracings.