The use of Doppler ultrasound has become a standard, non-invasive method for assessing blood flow within the body’s arteries. This technology allows healthcare providers to measure the speed of blood movement, providing valuable insights into cardiovascular health. Two measurements obtained from this process, the Peak Systolic Velocity (PSV) and the End Diastolic Velocity (EDV), are the primary metrics used to characterize arterial flow. These values help determine if blood is moving normally or if there is evidence of blockage or disease.
Defining Peak Systolic Velocity and End Diastolic Velocity
The heart’s continuous pumping action creates a pulsatile flow in the arteries, which has distinct phases corresponding to the cardiac cycle. Peak Systolic Velocity (PSV) is the maximum speed that blood reaches in an artery during the heart’s contraction phase, known as systole. This measurement captures the highest point of the blood flow wave generated when the left ventricle forcefully ejects blood into the aorta.
End Diastolic Velocity (EDV) measures the speed of blood flow at the very end of the heart’s relaxation and filling phase, known as diastole. This is the lowest velocity recorded just before the next heart contraction begins. The PSV represents the maximum driving force, while the EDV reflects the continuous flow required by the downstream tissues.
The Role of PSV and EDV in Assessing Arterial Health
The relationship between PSV and EDV offers a window into the resistance of the vascular bed being supplied by the artery. The difference between the two velocities helps determine whether the downstream tissues represent a high-resistance or low-resistance flow environment. Arteries supplying organs that require continuous perfusion, such as the brain and kidneys, typically exhibit a high EDV, indicating low resistance.
In contrast, arteries supplying muscles and peripheral limbs that do not require constant flow often have a low or even absent EDV, signifying a high-resistance bed. This comparison is often quantified using derived indices, such as the Resistivity Index (RI) or the Pulsatility Index (PI). The Resistivity Index is calculated by dividing the difference between PSV and EDV by the PSV, providing a single number that reflects the downstream impedance.
Normal Velocity Ranges Based on Artery Location
The concept of a “normal” PSV and EDV is entirely dependent on the artery’s location and the tissue it feeds. The internal carotid artery (ICA), which supplies the brain, is a classic example of a low-resistance vessel. In a healthy ICA, the PSV is less than 125 cm/s, and the EDV is less than 40 cm/s, with continuous forward flow.
The external carotid artery (ECA), which supplies the face and scalp, is a high-resistance vessel, exhibiting a much lower EDV, sometimes showing a brief flow reversal. The renal arteries, which supply the kidneys, are another low-resistance system, characterized by a rapid systolic upstroke and continuous forward flow during diastole. A normal PSV in the main renal artery is often less than 150 cm/s, with a continuous EDV of 20 to 50 cm/s.
Conversely, in peripheral arteries of the limbs, a normal waveform is triphasic, featuring a sharp systolic peak, a brief reversal of flow in early diastole, and often a final small forward flow component. This high-resistance pattern means the EDV is often very low or near zero in healthy peripheral arteries.
When PSV and EDV Indicate a Problem
Deviations from these expected normal ranges are often the first clue that a vascular problem exists. An abnormally high PSV at a specific point in an artery is a primary indicator of stenosis, or vessel narrowing. This occurs because blood accelerates as it is forced through a smaller opening, much like water speeding up when a hose nozzle is constricted. For instance, in the internal carotid artery, a PSV exceeding 230 cm/s suggests a severe stenosis of 70% or more.
A low EDV in an artery that should normally have high diastolic flow, such as the renal or internal carotid artery, suggests increased resistance downstream. This could be due to disease affecting the smaller vessels within the organ or a blockage located further away. Conversely, an abnormally high EDV in a normally high-resistance artery, like a peripheral artery, can suggest vasodilation or the presence of an abnormal connection between an artery and a vein. Interpretation requires careful consideration of the specific artery being measured and the overall clinical context of the patient.