Extensive or restricted ultrasound protocols to measure carotid intima-media thickness: analysis of completeness rates and impact on observed rates of change over time

Background: Ultrasound protocols to measure carotid intima-media thickness (CIMT) vary considerably with regard to carotid sites and angles that are assessed. Measurements from the carotid bifurcation and internal carotid artery are thought to be affected by large numbers of missing data. Actual published quantification of completeness rates and the relation with cardiovascular risk factors, however, is scarce. Also, it is currently unknown whether extensive ultrasound protocols including assessment of the carotid bifurcation and internal carotid artery add information in detecting rate of change in CIMT induced by drug therapy. These issues were addressed in this study using data from Measuring Effects on Intima-Media Thickness: An Evaluation of Rosuvastatin (METEOR).

Methods: In METEOR, carotid ultrasound examinations were performed twice before randomization, once each at 6, 12, and 18 months after randomization, and twice after 24 months of study treatment. B-mode ultrasound images were obtained from the near and far walls of the left and right common carotid artery, bifurcation, and internal carotid artery at five predefined angles. Completeness of CIMT data was assessed by carotid site and by angle. A site was considered complete when any of the five angles was measured. The relation between completeness at baseline and cardiovascular risk factors was assessed using logistic regression analyses. Ultrasound protocols with a reduced number of carotid sites and angles were retrospectively constructed, and differences in the rate of change in maximum CIMT between ultrasound protocols were compared.

Results: At each visit, CIMT measurements from all 12 carotid sites were available for >94% of the participants. Incompleteness was the highest for near wall of the internal carotid artery and for the extreme angles (60° and 300°). Of 12 risk factors examined, higher body mass index was related to incompleteness. Ultrasound protocols with a reduced number of angles resulted in similar estimates for the differences in rate of change in maximum CIMT. However, reductions in the number of sites gave results in the same direction but with different magnitudes and larger standard errors.

Conclusions: High levels of complete data can be obtained with extensive ultrasound protocols that include measurement from the carotid bifurcation and internal carotid artery. A high body mass index contributes to incompleteness of CIMT measurements. Extensive ultrasound protocols are required to obtain the highest precision to observe a treatment effect and to fully cover the degree of atherosclerotic burden.