Influence of the Respiratory Cycle on Caudal Vena Cava Diameter Measured by Sonography in Healthy Foals: A Pilot Study

Background: Intravascular volume assessment in foals is challenging. In humans, intravascular volume status is estimated by the caudal vena cava (CVC) collapsibility index (CVC-CI) defined as (CVC diameter at maximum expiration [CVCmax] CVC diameter at minimal inspiration [CVCmin])/CVCma x 9 100%. Hypothesis/Objectives: To determine whether the CVC could be sonographically measured in healthy foals, determine differences in CVCmax and CVCmin, and calculate inter-and intrarater variability between 2 examiners. We hypothesized that the CVC could be measured sonographically at the subxiphoid view and that there would be a difference between CVCmax and CVCmin values. Animals: Sixty privately owned foals <1-month-old. Methods: Prospective study. A longitudinal subxiphoid sonographic window in standing foals was used. The CVCmax and CVCmin were analyzed by a linear mixed effect model. Inter-rater agreement and intrarater variability were expressed by Bland-Altman and intraclass correlation coefficients, respectively. Results: Measurements were attained from 58 of 60 foals with mean age of 15 +/- 7.9 days and mean weight of 75.7 +/- 17.7 kg. The CVCmax was significantly different from CVCmin (D = 0.515, SE = 0.031, P < 0.001). Inter-rater agreement of the CVC-CI differed by an average of -0.9% (95% limits of agreement, -12.5 to +10.7%). Intrarater variability of CVCmax was 0.540 and 0.545, of CVCmin was 0.550 and 0.594, and of CVC-CI was 0.894 and 0.853 for observers 1 and 2, respectively. Conclusions and Clinical Importance: These results indicate it is possible to reliably measure the CVC sonographically in healthy foals, and the CVC-CI may prove useful in assessing the intravascular volume status in hypovolemic foals.