Kidney to aorta ultrasound measurements in Whippets

Kidney size has diagnostic and prognostic value. Since CT and MRI require general anesthesia and are expensive, radiography and ultrasonography (US) represent the preferred techniques in Veterinary practice. Radiographic kidney size is obtained by relating renal length to the L2 body length: a ratio of 2.5 – 3.5 is considered normal. Ultrasonography, other than the profiles, allows to assess internal structure and blood perfusion and can be performed on awake patient, without any risk. The main limit of US is that its accuracy is operator dependent. Therefore, any available quantitative information are useful to reduce subjective assessment. Although several authors tried to relate renal measures to bodyweight or body surface area, to date, the renal US size is still subjectively assessed. Recently, a method in which kidney length (KL) is related to the aortic luminal diameter (AoD) has been proposed (1). The main disadvantage of this method is the wide range of normal values (5.5-9.1) which determines a poor sensitivity and specificity. Therefore, in order to assess a narrower range of KL/AoD normal values and to increase its clinical importance, only one breed - the Whippet - was considered. Differences between right and left kidneys and influence of sex, weight and age were also investigated. Furthermore, longitudinal and transversal scans of AoD were compared. Thirty-six whippets (16 males, 20 females), ranging from 10 months to 14 years old, mean bodyweight 14.12±2.38 kg, clinically normal and without any US renal lesions were included. All US studies were performed on awake dog, in right and left lateral recumbency or in standing position. The US images were obtained using a 3.5-10 MHz microconvex probe and acquired in dorsal and transversal scans. The KL and width (KW) were measured on dorsal scan while depth (KD) on transversal one. The AoD was measured from the left side, both in transversal (AoDT) and longitudinal (AoDL) scans. Measurements were made at the maximal luminal diameter dilatation just caudal to the left renal artery origin. Mean, median, minimum, maximum, SD and 95% C.I. for the KL, KD, KW, AoDT, AoDL, KL/AoDT and KL/AoDL, for each side and for pooled data were calculated. Paired t Student’s test was used to compare right and left side. Mann-Whitney test was used to compare the sexes. Pearson correlation and Spearman's rank correlation were used to correlate bodyweight and age, respectively. BlandAltman plot was used to compare AoDL and AoDT. P was set at <0,05. The 95% C.I. of KL/AoD was narrower than that reported (6.3-7 instead of 5.5-9.1) and showed no differences between sides and sexes. Side did not affect kidney size while males had larger kidneys than females. Kidney size was positively correlated to the bodyweight. In spite of no significant differences and a strong correlation between AoDL and AoDT, the KL/AoDL was significantly higher than KL/AoT and the Bland-Altman plot showed a greater bias for the AoDT. In conclusion, in Whippets, a KL/AoD lower than 6.3 means reduced renal size, while greater than 7, increased renal size. The KL/AoD showed no differences in the two sides and, moreover, although a marked sexual dimorphism, no differences between the two sexes.