The ultrasonographic assessment of the caudal vena cava-to-aorta ratio (CVC:Ao) appears to be a promising method for early recognition of alterations of intravascular volume status in veterinary medicine. The primary objective of this study was to establish the reference intervals of the CVC:Ao ratio with ultrasound in nonsedated healthy dogs. Secondary objectives were to determine the influence of the respiratory cycle and to evaluate correlations between ultrasonographic measurements, signalment and physical exam findings. Ultrasonographic measurements of Ao and CVC diameters were successfully obtained for all sixty dogs included. No evidence of a difference was observed between the measurements of Ao and CVC diameters, and CVC:Ao ratio between inspiration and expiration (P = .373, P = .318, and P = .537, respectively). The reference interval for CVC:Ao ratio (95% CI), generated from US measurements performed at any moment of the respiratory cycle was defined as 0.93 (0.91-0.95) −1.32 (1.30-1.34). The CVC:Ao ratio was significantly negatively correlated with age (r = −0.341, P = .008) and positively correlated with respiratory rate (r = 0.423, P < .001), but not with heart rate (P = .573) or arterial systolic blood pressure (P = .166). A low inter- and intraoperator variability in repeated measurements was observed for each operator and between operators. The ultrasonographic measurement of the CVC:Ao ratio appears as a simple method with low inter- and intraoperator variability using the ultrasonographic protocol described in the current study. With the reference interval established in the present study in healthy nonsedated dogs, further studies should evaluate the utility of this simple method in assessing and monitoring volume status in hypo- and hypervolemic dogs.

尾腔静脉与主动脉比率 (CVC:Ao) 的超声评估似乎是兽医学中早期识别血管内容量状态变化的一种有前景的方法。本研究的主要目的是确定未使用镇静剂的健康狗的超声 CVC:Ao 比率的参考区间。次要目标是确定呼吸周期的影响并评估超声检查测量、信号和体检结果之间的相关性。成功获得了所有 60 只狗的 Ao 和 CVC 直径的超声测量结果。没有观察到 Ao 和 CVC 直径测量值以及吸气和呼气之间 CVC:Ao 比率之间存在差异的证据（分别为P  = .373、P  = .318 和P  = .537）。根据呼吸周期任何时刻进行的 US 测量生成的 CVC:Ao 比值 (95% CI) 的参考区间定义为 0.93 (0.91-0.95) -1.32 (1.30-1.34)。CVC:Ao 比率与年龄显着负相关 (r = -0.341，P  = .008)，与呼吸频率正相关 (r = 0.423，P < .001)，但与心率无关 ( P  = .573)或动脉收缩压（P  = .166）。对于每个操作员以及操作员之间的重复测量，观察到操作员之间和操作员内部的变异性较低。使用当前研究中描述的超声检查方案，CVC:Ao 比率的超声测量似乎是一种简单的方法，操作者之间和内部的变异性较低。根据本研究在健康未镇静狗中建立的参考区间，进一步的研究应该评估这种简单方法在评估和监测低血容量和高血容量狗的容量状态方面的效用。