Middle cerebral artery peak systolic velocity: technique and variability.

OBJECTIVE Assessment of the middle cerebral artery (MCA) peak systolic velocity (PSV) can accurately diagnose fetal anemia and has decreased the number of invasive procedures, such as amniocentesis and cordocentesis. The objective of this investigation was to evaluate the intraobserver and interobserver variability as a measure of reproducibility of MCA PSV. The technique of correctly sampling this vessel is described. METHODS The study population included 30 appropriate-for-gestational-age fetuses. In each fetus, MCA PSV was determined proximal to the transducer at 3 different locations: 2 mm after its origin from the internal carotid artery, at the midlength between its origin and division, and at its division. The peak systolic velocity was also determined at the contralateral MCA 2 mm after its origin. With each measurement (obtained at 2 different institutions), care was taken to ensure that the ultrasound beam was parallel to the artery for its entire length. The reliability of an angle corrector was also assessed. The intraobserver and interobserver reliabilities were determined from the appropriate version of the intraclass correlation. RESULTS Gestational age at study entry ranged from 14 to 37.5 weeks (median, 23.6 weeks). The proximal MCA, 2 mm after its origin from the internal carotid artery, had the best intraobserver and interobserver variability in both institutions. (Intraclass correlation ranged from 0.98 to 0.99.) CONCLUSIONS Our data indicate that fetal MCA PSV is optimally measured soon after the MCA's origin from the internal carotid artery. Given the importance of clinical decision making based on this measurement, sonographers and sonologists interested in measuring MCA PSV should test their variability after a suitable period of training.