Traditionally, the systolic blood pressure is taken to be the pressure at which the first Korotkoff sound is first heard and the diastolic blood pressure is the pressure at which the fourth Korotkoff sound is just barely audible. However, there has recently been a move towards the use of the fifth Korotkoff sound.

Reference: https://en.wikipedia.org/wiki/Korotkoff_sounds

Blood Pressure, Heart