Middle cerebral artery (MCA) waveforms are easily obtained transabdominally; a transverse image of the fetal head is obtained at the level of the sphenoid bones. Color Doppler identifies the Circle of Willis and the MCA as the major lateral branch running along the greater wing of the sphenoid bone. Using a small sample gate, velocities are recorded ~ 1 cm distal to the origin from the internal carotid artery (Arduini and Rizzo, 1990) with minimal pressure, as compression of the fetal head alters waveform indices.
Mean MCA PI increases to approximately 28 weeks gestation and then falls to term (Arduini and Rizzo, 1990). Progressive hypoxemia results in a reduction in MCA PI (Figure 32.1) and therefore values <5th centile are regarded as abnormal. The ratio of UA/MCA PI has been widely used as an index of cerebral redistribution. Mean values fall from around 0.7 at 20 weeks to 0.5 at term, while the 95th centile falls from 1.1 to 0.9 (Arduini and Rizzo, 1990).
Was this article helpful?
Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...