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 [CVC (max)] – CVC diameter at minimal inspiration [CVC (min)])/CVC (max) × 100%. HYPOTHESIS/OBJECTIVES: To determine whether the CVC could be sonographically measured in healthy foals, determine differences in CVC (max) and CVC (min), 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 CVC (max) and CVC (min) values. ANIMALS: Sixty privately owned foals <1‐month‐old. METHODS: Prospective study. A longitudinal subxiphoid sonographic window in standing foals was used. The CVC (max) and CVC (min) 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 CVC (max) was significantly different from CVC (min) (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 CVC (max) was 0.540 and 0.545, of CVC (min) 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.