Читать книгу Protocols for High-Risk Pregnancies - Группа авторов - Страница 72

As mentioned in the overview, pulse‐wave Doppler velocimetry can be used to obtain the following information from a flow velocity waveform.

 Velocity of the blood – requires an angle of insonation of zero degrees between the transducer and the vessel of interest (Figure 7.1). The angle correction function available on most ultrasound machines can be used but the actual angle between the vessel and the ultrasound beam should be less than 30°. For middle cerebral artery peak systolic velocity (MCA PSV), the sample volume should be placed in the proximal third of the MCA as it branches from the circle of Willis.

 Resistance indices (systolic/diastolic or S/D ratio, resistance index, pulsatility index) – these are angle‐independent measurements such that the value obtained for any one of these indices is not dependent upon the angle between the transducer and the vessel being interrogated (Figure 7.2).

 Volume blood flow (milliliters per minute) – this is determined by obtaining the velocity of the blood and multiplying it by the cross‐sectional area of the vessel (obtained by two‐dimensional ultrasound) times 60 seconds:

Volume flow (mL/min) = velocity (cm/sec) × cross‐sectional area (cm2) × 60 seconds

Figure 7.1 Schematic representing zero angle of insonation between the Doppler transducer and the vessel of interest.

Figure 7.2 Flow velocity waveform of the umbilical artery and definitions for the different Doppler indices of resistance.