The mean and range of different ratios like CVC/AO, CVC/VL, CVC/R4, AO/VL, AO/R4 are given below in Table 1. Out of 19 radiographs, the CVC and AO measurements could not be taken in one different case each due to inadequate radiographic sharpness at the particular segment. Therefore, the CVC/AO ratio was eventually calculated in 17 cases and the remaining ratios in 18 cases.
Mean CVC/AO ratio was calculated to be 0.79± 0.22 with a range of 0.42-1.30. Similarly, mean CVC/VL ratio was 0.68 ±0.21 (range: 0.34 -1.11); CVC/R4 was 1.63 ±0.44 (range: 0.65-2.2); AO/VL was 0.84 ±0.12 (range: 0.71-1.18) and AO/R4 was 2.05 ±0.32 (range: 1.53-2.59).
Measurement of caudal vena cava size and its ratio with other surrounding structures can provide information about pathogenesis of right-side heart disease.
Lehmkuhl et al (1997) suggested ratio of CVC/AO > 1.50, CVC/VL > 1.30, or CVC/R4 > 3.50 as a strong indicator of right-sided heart abnormality in dogs whereas, dogs having ratio of CVC/AO < 1.00, CVC/ VL < 0.80, or CVC/R4 < 2.25 could be considered normal definitively. In another study on 200 dogs,
Losonsky et al (1983) reported mean CVC/R4 ratio as 2.07±0.69 in normal sized hearts of dogs. Whereas, it was 2.25± 0.64 in cases of moderately enlarged hearts and 2.60 ±.68 in severely enlarged hearts. Similarly,
Thrall and Calvert (1983) reported CVC/R4 ratio as 2.95 ± 0.76 in 28 dogs with history of heartworm disease and right heart failure. However, their study showed that normal caudal vena cava size was an insensitive indicator of heart worm induced right heart failure in dog.
In the present study, though the higher range of these ratios remained less than those indicative of right-sided heart abnormality as per both of the above-mentioned studies, yet the range of CVC/AO and CVC/VL exceeded the levels suggested to consider the dogs normal in a definitive manner. However, overall, the mean values remained within the normal range. Moreover, in one case, the CVC/R4 ratio was found to be quite close to the mean reported by
Losonsky et al (1983) in cases of moderately enlarged hearts in dogs, whereas, the heart size in that particular dog was considered normal on the basis of VHS and other parameters and it did not show any clinical signs associated with cardiomegaly. This signifies a much greater individual variations of such radiographic biometric parameters in healthy dogs than those reported by
Lehmkuhl et al (1997) or
Losonsky et al (1983). It was also admitted by
Lehmkuhl et al (1997) that such parameters were likely to help only a minority of clinical patients as most patients would fall outside their range.
In a similar yet limited study in the same geographical area,
Sharma (2018) reported range of mean ratio of CVC/VL from 0.66±0.11 to 0.97±0.10 and to that of CVC/R4 from 1.56±0.26 to 2.03±0.16 in different breeds of dog. There were significant differences (P<0.05) in CVC/VL ratios in between different breeds of dogs though no direct correlation was found in between the size of dogs and this ratio. For example, the CVC/VL ratios of Pugs and Rottweilers were found significantly greater than Pomeranians, German Shepherds and mongrels. On the other hand, no significant difference was found in CVC/R4 ratio in different breeds of dogs implying that this ratio might be more useful than CVC/VL for clinical relevance. However, as the mean CVC/VL ratios in that study were found to be higher than collective mean CVC/VL ratio in the present study, it can be concluded that a greater range exists in such measurements in a normal manner.
As regards to AO/R4 ratio is concerned,
Lehmkuhl et al (1997) reported insignificant difference in this ratio in between healthy (2.36 ± 0.52) and right-side heart disease (2.22±0.42) dogs. However, mean AO/VL ratios were found to be lesser in dogs with right side heart disease (0.80 ±0.10) than that seen in healthy ones (0.90±0.15). Similarly,
Vosugh and Nazam (2017) reported low AO/VL ratio in domestic short haired cats with right heart failure (0.68±0.050) as compared to that recorded in healthy subjects (95±0.03). In the present study, the AO/VL ratio of healthy dogs remained in between the mean values for healthy and the ones with right side heart disease as reported by
Lehmkuhl et al (1997).
In the present study, the diameter of CVC showed a wide range of measurements from 6.14 to 19.45 mm in different dogs. The shape and size of CVC differed greatly with in same dog depending upon various factors. Generally, in cases where the heart remained in relatively upright position in thorax, the diameter of CVC remained uniform from heart to diaphragm. Whereas, in cases where it was titled a bit more or there was greater contact of heart with sternum due to normal body conformation, the CVC assumed a somewhat conical shape with lesser diameter towards diaphragm and greater towards heart sometimes with as much as 3 times difference in between them. Similarly, when the lungs were inflated to its fullest extent, the CVC stretched and its diameter reduced greatly (Plate 2). Besides, the margins of CVC weren’t found uniformly clear in all radiographs across its visible segment and hence, the measurements were taken from the clear sections which might not be always truly reflective of their greatest diameters.
Similarly, differences were found in the measurements of R4 width with in the same animal at different segments of R4. Though for maintaining uniformity the measurements of R4 were taken just underneath the vertebrae but in some animals with wide chest where the ribs were too much arched at their origin, the measurements of right and left ribs were not found equal at this level. And, this augment didn’t represent the most proximal aspect of rib as well.
