The Internal Carotid Artery (ICA) ascends through the neck to supply the majority of blood flow to the brain’s cerebral hemispheres. Assessing the speed of blood moving through this artery is a standard medical procedure to evaluate its health. This measurement is performed using Carotid Duplex Ultrasound, a non-invasive technology combining conventional ultrasound imaging with Doppler technology. The resulting velocity provides dynamic information about the vessel’s internal condition.
The Role of Carotid Velocity Measurement
Measuring the speed of blood flow in the ICA is a fundamental step in screening for Carotid Artery Disease, which is typically caused by the buildup of plaque in a process known as atherosclerosis. While ultrasound imaging visualizes the vessel wall and any plaque, the velocity measurement offers a dynamic assessment of how that plaque is affecting flow efficiency.
When plaque narrows an artery, the heart must pump blood faster to push the same volume through the smaller opening, which increases the blood’s velocity. Two primary metrics are recorded: the Peak Systolic Velocity (PSV) and the End Diastolic Velocity (EDV). The PSV captures the maximum speed during the heart’s contraction phase, while the EDV measures flow speed during the heart’s relaxation phase. Both metrics determine the severity of any blockage within the ICA.
Defining Normal Velocity Parameters
A normal ICA velocity is a range of values that indicates the absence of significant arterial narrowing or disease. Most vascular laboratories adhere to criteria established by consensus guidelines, such as those published by the Society of Radiologists in Ultrasound (SRU). According to these standards, a normal ICA Peak Systolic Velocity (PSV) is less than 125 centimeters per second (cm/s).
The End Diastolic Velocity (EDV) is also expected to be low, typically less than 40 cm/s, in a healthy artery. These numerical thresholds apply to an ICA that shows no visible plaque or only minimal plaque buildup on the corresponding ultrasound image.
Another metric for defining normal flow is the ICA/CCA PSV ratio, which compares the PSV in the ICA to the PSV in the Common Carotid Artery (CCA). The CCA is the vessel that feeds into the ICA and serves as an internal control for the patient’s overall cardiac output and blood pressure. A normal ICA/CCA ratio is less than 2.0, providing a relative measure of flow acceleration compared to the main artery.
Interpreting Elevated Velocity Readings
An elevated velocity reading is the primary sign used by clinicians to diagnose and grade the severity of arterial narrowing, or stenosis. As the artery’s diameter decreases due to plaque, the blood must accelerate to pass through the constricted area, causing a measurable increase in both the PSV and EDV. Velocity thresholds are utilized to categorize the degree of stenosis.
A PSV of 125 cm/s or greater, coupled with an EDV of 40 cm/s or greater, generally marks the transition to a hemodynamically significant stenosis, typically corresponding to a 50% or greater reduction in the vessel’s diameter. As the stenosis becomes more severe, velocities continue to rise; for example, a PSV exceeding 230 cm/s and an EDV over 100 cm/s often indicates a severe stenosis (70% or greater). The ICA/CCA ratio also increases, often exceeding 4.0 in severe cases.
Velocity readings are subject to various influences beyond physical narrowing. High cardiac output, such as that seen in patients with severe anemia or heart conditions, can cause a generalized elevation in blood flow velocity, potentially leading to a falsely elevated reading without significant stenosis. Conversely, in cases of near-total occlusion, the velocity can paradoxically drop (“pseudo-normalization”) because the flow volume is restricted. Therefore, interpretation requires correlation with ultrasound images and clinical context.