Peak systolic velocity (Doppler ultrasound)
When traveling with their greatest velocity in a vessel (i.e. during systole), red blood cells exhibit their greatest magnitude of Doppler shift. The spectral Doppler system utilizes Fourier analysis and the Doppler equation to convert this shift into an equivalently large velocity, which appears in the velocity tracing as a “peak” .
By the Doppler equation, it is noted that the magnitude of the Doppler shift is proportional to the cosine of the angle (of insonation) formed between the ultrasound beam and the axis of blood flow . Since the trigonometric ratio that relates these values is the cosine function, it follows that the angle of insonation should be maintained at ≤60 . At angles >60, the cosine function curves much more steeply, leading to a significant reduction in the accuracy of angle correction, and thus the accuracy of blood velocity indices such as PSV and end-diastolic velocity (EDV) .
Factors that influence flow velocity indices
Flow velocity may vary based on vessel properties and pathological changes .
Low resistance vessels (e.g. internal carotid artery, renal artery) supply end organs which require perfusion throughout the entire cardiac cycle. These vessels exhibit high diastolic flow and EDV . In contrast, high resistance vessels (e.g. external carotid artery, limb arteries) are characterized by early reversal of diastolic flow, and low or absent EDV .
In stenosis, a localized reduction in vascular radius increases resistance, causing increased PSV and EDV distal to the stenosed site . High flow velocity causes Reynolds number to increase beyond a critical point, resulting in turbulent flow which manifests as spectral broadening on Doppler ultrasound .
To an extent, an increased degree (%occlusion) of stenosis corresponds to increased PSV and EDV . In near occlusion (>99%), flow velocity indices become unreliable (may be high, low or absent) . In this setting, a significant reduction in post-stenotic flow velocity is termed “trickle flow” . In complete occlusion, PSV and EDV are absent .