Breed‐specific vertebral heart score, vertebral left atrial size, and radiographic left atrial dimension in Cavalier King Charles Spaniels: Reference interval study

Abstract Cavalier King Charles Spaniels (CKCS) are predisposed to developing myxomatous mitral valve disease (MMVD), with radiographs frequently used to screen for evidence of left‐sided cardiomegaly secondary to MMVD. Vertebral heart size (VHS), vertebral left atrial size (VLAS), modified VLAS (M‐VLAS), and radiographic left atrial dimension (RLAD) are reported as objective measurements of global heart size and left atrial size. Normal VHS in CKCS (10.6 ± 0.5) is reportedly higher than the non‐breed‐specific value (9.7±0.5). Breed‐specific VLAS, M‐VLAS, and RLAD cut‐offs have not been reported in CKCS. The aim of this prospective reference interval study was to describe the VHS, VLAS, M‐VLAS, and RLAD values for 30 clinically healthy adult CKCS. Inclusion criteria were unremarkable physical examination, normal echocardiography, and thoracic radiographs without malposition/abnormalities. There were 22 female and eight male dogs. Ages ranged from 1 to 6 years. The VHS mean value in our sample was 10.08 ± 0.56 (95% range, 9.87‐10.29). This was significantly greater than a previously published general canine reference value of 9.7 ± 0.5 and significantly less than a previously published CKCS breed‐specific value of 10.6 ± 0.5 (P < 0.01). Mean VLAS, M‐VLAS, and the RLAD values in our study were 1.79 ± 0.3 (95% range, 1.68‐1.9), 2.23 ± 0.44 (95% range, 2.06‐2.39), and 1.2 ± 0.34 (95% range, 1.07‐1.33), respectively. These were significantly less than previously published reference interval values (P < 0.001). The VHS, M‐VLAS, and the RLAD were not affected by sex, body weight, or BCS; whereas the VLAS was moderately affected by body weight. Findings from this study can be used as background for future thoracic radiographic assessments in CKCS.

or BCS; whereas the VLAS was moderately affected by body weight. Findings from this study can be used as background for future thoracic radiographic assessments in CKCS.

K E Y W O R D S
breed-specific heart scale, Cavalier King Charles Spaniel, radiographic atrial measurements, vertebral heart scale INTRODUCTION Myxomatous mitral valve disease (MMVD) is a common cardiovascular disease affecting dogs, progressing to mitral regurgitation, and eventually heart failure. 1 The incidence is age-related and is particularly high in breeds such as the Cavalier King Charles Spaniels (CKCS). 1,2 Fifty percent of CKCS are affected by the age of 6-7 years, and almost 100% are affected by the age of 11. 1,2 In this breed, thoracic radiographs are frequently used to screen for evidence of left-sided cardiomegaly secondary to MMVD. 3,4 Published radiographic measures of heart size include vertebral heart size (VHS), vertebral left atrial size (VLAS), its modified version (M-VLAS), and radiographic left atrial dimension (RLAD). 3,[5][6][7][8][9] The VHS is influenced by different morphotypes and several studies have described breed-specific reference range. [10][11][12][13][14][15][16][17][18][19][20] Only normal values of VHS in CKCS have been investigated and are reported to be higher than not-breed-specific reference ranges (respectively 10.6 ± 0.5 vs 9.7 ± 0.5). 3,10 The VLAS, M-VLAS, and RLAD have recently been proposed as new radiographic methods for quantifying left atrial size in dogs 7-9 ; however, no published studies have evaluated VLAS, M-VLAS, and RLAD in different breeds. A recent study reported values for VLAS in Chihuahuas. 21 22 The first aim of this study was to describe breed-specific reference interval values for VHS, VLAS, M-VLAS, and RLAD in a sample population of healthy adult CKCS. The second aim was to test the hypothesis that these radiographic measures of cardiac size in CKCS would differ from previously published reference values. The third aim was to test the hypotheses that radiographic measures of cardiac size in CKCS would not be significantly affected by sex, body condition score (BCS), or chest conformation; but would be significantly affected by recumbency.

Ethical approval
This study being a part of a larger project, informed consent was obtained from all the clients before starting any examination procedures for included dogs. The radiographic examinations were carried out as per the hospital's standard procedures.

Case selection
All included CKCS underwent a complete physical examination, thoracic radiography, and echocardiography. Dogs were considered healthy based on the absence of prior clinical conditions/abnormalities documented by the owners and unremarkable physical examination, cardiovascular assessment, and transthoracic echocardiogram. 21 Dogs with cardio-structural heart diseases and cardiac chamber enlargement identified on echocardiogram were excluded from the study, as well as dogs aged <12 months because of the possible influence of young age and skeletal immaturity on the radiographic vertebral-based measurements. Thoracic radiographs that revealed an overt malposition of the patient (as the considerable rotation, the movement due to excessive breathing for tachypnea or the brachial muscles superimposition on the cranial aspect of the thorax) or the presence of thoracic vertebral abnormalities (eg, hemivertebrae) were not included in the study. 23,24 All decisions for dog inclusion or exclusion were made by two veterinarians with more than fifteen years of clinical experience in veterinary radiology (D.Z.) and veterinary cardiology (C.L.), respectively, based on a consensus opinion.

Echocardiographic examination
All echocardiographic examinations were performed in unsedated     Table 1.

Statistical analysis
In our study, the CKCS had a significantly lower VHS (10.08 ± 0.56;

DISCUSSION
The purpose of this study was to describe breed specific reference values for VHS, VLAS, M-VLAS, and RLAD in healthy adult CKCS. [6][7][8][9] Based on the authors' review of the literature, this is the first published study proposing the reference intervals in this breed for VLAS, M-VLAS, and RLAD. In this CKCS sample, the VHS was significantly higher than the not-breed specific reference values initially established by Buchanan and Bücheler in 1995, 6  found a higher VHS in RL recumbency than in LL recumbency. 12,15,17,18 Disagreement between studies may be explained first by differences in thoracic morphotypes among breeds. It was also hypothesized that the larger VHS in RL recumbency may be due to the divergent Xray beam and the larger distance of the heart from the cassette in RL recumbency. 35 In addition, possible variations in radiographic cardiac size during the cardiac cycle (diastolic vs systolic dimensions) need to be considered. In fact, while respiratory cycle can be controlled when radiographs are taken, cardiac cycle cannot. 36,37 The previously reported mean VHS ± SD ranges from 9.9 ± 0.8 to 10.4 ± 0.8 vertebrae between end-diastolic and end-systolic measurements with fluoroscopy at peak inspiration for dogs positioned in right lateral recumbency. 37 On average, mean VHS ± SD is 0.3 ± 0.3 vertebrae greater in diastole than in systole at peak inspiration, with VHS varying up to 0.97 vertebral units over the cardiac cycle in some individuals. 36 Similar influence of cardiac cycle is observed on VLAS but however the same has never been described for RLAD. 37 38 whereas other studies report more variation in this breed. 39 Finally, no correlation was found between the VHS, BCS, and body weight. This result is in accordance with previous studies in Spitzs, mixed breeds, and Labrador Retrievers, 17 but not with the results found in Lhasa Apsos and Norwich Terriers. 16,20 Disagreement between studies may be explained by possible variations in the amount of pericardial fat in different breeds. 18 The VLAS found in our study population is lower than the values pro- The main limitation for the current study was the small sample size.
A larger population could have led to possible differences in the proposed reference range because the CKCS breed is very inhomogeneous in terms of size and morphotype. Body weight in CKCS may vary with different genetic lineages; however, our range of body weight was wide (5-12 kg), and no correlation was found between the VHS and body weight. Thus, it is reasonable to believe that even increasing the sample population, comparable results would be found. In addition, most previous studies reporting breed-specific reference values of the VHS included a maximum of 30 cases per breed, as the present study. 10,11,[15][16][17]19 Also like previous studies on radiographic vertebralbased measurement, 7,16,41,42 our sample population did not include dogs younger than 12 months.
In conclusion, results of this study support previous studies indicating that breed-specific reference values for the VHS are needed.

ACKNOWLEDGMENTS
The authors thank the owners and the breeders for their collaboration and involvement in this study. This study was not supported by a grant.