The pulsatility index (PI) (also known as the Gosling index) is a calculated flow parameter in ultrasound, derived from the maximum, minimum, and mean Doppler frequency shifts during a defined cardiac cycle. Along with the resistive index (RI), it is typically used to assess the resistance in a pulsatile vascular system.

Terminology

Pulsatility is an intrinsic property of the cardiovascular system, governed by the resistance differential across the arteriolar bed, which allows the potential energy stored in the elastic, proximal arteries to propagate throughout the microcirculation at a mean pressure consistent with adequate perfusion. 

When evaluated as a derived flow parameter using pulsed wave Doppler, it is calculated by one of the following equations:

PI = (v_max - v_min) / (v_mean)

PI = (peak systolic velocity - minimal diastolic velocity) / (mean velocity)

The operator typically recognises and demarcates maximum (v_max) and minimum (v_min) velocities, while the mean velocity (v_mean) is calculated by the ultrasound machine. 

Clinical use

Because the calculation of these flow parameters is based on the same Doppler spectrum, possible instrument-dependent errors, or an inappropriate angle of insonation by the user, are mitigated somewhat ¹. 

Clinical scenarios in which a pulsatility index are calculated include:

• malignant ovarian lesions
• transcranial Doppler
• carotid artery evaluation for stenosis
• umbilical vein Doppler
• fetal middle cerebral artery Doppler - fetal middle cerebral artery pulsatility index
• umbilical arterial pulsatility index

History and etymology

The pulsatility index was described in a 1974 paper by Raymond Gosling (1926-2015) ⁴, a British biophysicist. He is more famously remembered as working under the team led by Rosalind Franklin when x-ray crystallography was used to investigate DNA. From this work James Watson and Francis Crick inferred the structure of DNA for which they shared the Nobel prize ^2,3.