In the context of vascular ultrasound, how is "pulsatility index" defined?

The pulsatility index is defined as a measure of the variability in blood flow within vessels, particularly in relation to the changes that occur during the cardiac cycle. It is calculated using the formula:

Pulsatility Index (PI) = (Peak Systolic Velocity - End Diastolic Velocity) / Mean Velocity

This value provides insight into the resistance that blood encounters as it flows through a vessel, with higher pulsatility indices often being indicative of increased resistance or variations in blood flow. It is especially useful in assessing conditions such as arterial stenosis or other obstructions, as well as changes in vascular tone.

In contrast, the other options do not accurately represent the concept of pulsatility index. While blood vessel diameter, average blood pressure, and heart rate variability are important clinical parameters, they do not specifically address the concept of flow variability as the pulsatility index does. Therefore, the definition focusing on variability in blood flow within vessels is the most accurate in the context of vascular ultrasound.