The measurement of blood flow through any vascular structure using ultrasound requires two basic pieces of information: the velocity of blood flow and the cross-sectional area of the vessel. The velocity of blood flow can be measured using spectral Doppler. As pulsed wave (PW) Doppler allows for measurement of velocity at a specific point in the vessel (ie, range resolution), it is the preferred mode of spectral Doppler. When measuring the velocity, the echocardiographer must take care to use an angle of insonation as parallel as possible to the direction of flow. If the ultrasound beam is intercepting the blood flow at an angle, the velocity will be underestimated (see Chapter 6). In clinical practice, an angle of less than 20° is generally considered acceptable, as the effect on velocity is negligible. An angle greater than 20° will require angle correction, which often produces inconsistent results. Another important point in measuring the velocity is that, in the presence of a turbulent flow, the measured velocity by PW Doppler cannot be used to estimate the flow because of aliasing that occurs with higher velocities (see Chapters 1 and 4). Therefore, the operator should assess the flow by color Doppler first to ensure that the flow is laminar before attempting to obtain the velocity by PW Doppler.
In the measurement of the cross section of the vessel, it is assumed that the vessel is cylindrical in shape and therefore the vessel cross section is a circle. By measuring the diameter of the vessel, one can calculate the area using the following formula:
where π is the constant 3.14 and D is the diameter of the vessel.
Blood flow can be calculated using either of these two formulas:
Velocity time integral (VTI) is the area under the curve of the Doppler envelope for each cardiac cycle. Mean velocity is the time-averaged velocity over the entire cardiac cycle (see below).
Important: The assumption that the vessel in cross section is circular is thought to be acceptable for clinical use of flow estimation in the arterial system. However, for the venous system (such as the superior vena cava, due to collapsibility of the veins), this assumption is more problematic. This will be discussed below.
In this chapter, estimation of left and right ventricular output and superior vena cava flow will be discussed. This will be followed by a brief description of mitral and tricuspid flow Doppler. Use of mitral and tricuspid flow Doppler in the assessment of diastolic function will be ...