Velocity Ratio
Classification of arterial disease requires consideration of other velocity measurements besides maximum velocity.
Velocity ratios compare peak systolic measurements from a stenosis to a normal proximal segment of the vessel. Care must be taken when this measurement is performed that similar angles of insonation are used. If the velocity ratios are calculated using velocity measurements obtained at different angles, significant error is introduced.
Velocity Ratio = Velocity at Stenosis/Velocity Normal Proximal Vessel
Mean velocity
An average of all the velocities recorded in a cardiac cycle can be calculated by the ultrasound system. This value is known as the time average or mean velocity. This measurement is used routinely in Transcranial Doppler examinations.
Resistive Index
Resistive Index is used as an indicator of peripheral resistance. It is calculated by the following formula.
Resistive Index = (Peak Systolic Velocity-End Diastolic Velocity)/Peak Systolic Velocity
Pulsatility Index
Pulsatility Index can also be calculated by the ultrasound system. It quantifies the degree of pulse wave dampening. It is defined by the following equation.
Pulsatility Index = Peak Systolic velocity - End Diastolic Velocity/ Mean Velocity
