The vertebral heart size (VHS) compares the length of the short and long axis of the heart to the length of the thoracic vertebral bodies starting at the fourth thoracic vertebrae.1 The current reported normal reference interval in dogs is 8.7 to 10.7 vertebrae, with an average of 9.7 vertebrae.1 Several studies2–4 have demonstrated that increases in VHS are associated with the progression of underlying myxomatous mitral valve disease. However, the scale does not appear to apply universally to all canine breeds as initially thought as several studies5–11 have reported breed-specific canine VHS reference intervals.
Vertebral left atrial size (VLAS) is a newer diagnostic modality to assess left atrial enlargement on thoracic radiograph and correlates well to left atrial enlargement on an echocardiogram.12,13 Similar to breed-specific VHS reference intervals, breed-specific VLAS reference intervals have been reported dogs as well.5,8,10,11 The authors hypothesize that 3 small-breed canines—Jack Russell Terriers (JRTs), Miniature Pinschers (MPs), and Brussels Griffons (BGs)—have breed-specific VHS and VLAS reference intervals.
Methods
The electronic medical record system of IDEXX was searched for MPs, JRTs, and BGs undergoing thoracic radiography from January 1, 2023, through June 30, 2023. This date range was later expanded for BGs only to increase case numbers to from January 1, 2022, through June 30, 2023. The radiographs had been previously submitted to IDEXX for telemedicine review. Dogs were included if they had 2- or 3-view thoracic radiographs performed (at least a right lateral and 1 ventrodorsal or dorsoventral view) and no evidence of cardiopulmonary or systemic disease detected in the images or history. All dogs with incomplete radiographic studies and/or known extracardiac disease were excluded. Radiographic studies with poor positioning that limited the accuracy of VHS and VLAS measurements were also excluded. All dogs included in the study must have had a normal cardiac auscultation as noted by the original veterinarian performing the physical examination, which was documented in the telemedicine consultation form. All dogs with a reported heart murmur were excluded. If a dog had no reported heart murmur but had cardiomegaly reported by the IDEXX radiologist or cardiologist who initially reviewed the case, was receiving a cardiac medication that could affect cardiac size (ie, pimobendan or diuretics), had a history of receiving a grain-free diet, or had a history of an elevated N-terminal pro–B-type natriuretic peptide measurement, the dog was termed “suspected cardiac disease” and also excluded.
Data collected from the record included age, weight, sex, VHS, and VLAS. The VHS and VLAS measurements were performed by the same board-certified cardiologist in all 3 breeds. As the radiographs were all in a digital format, measurements were performed using a digital caliper, and measurements were performed on the right lateral radiograph. The measurement of VHS was performed using the technique initially described by Buchanan et al9 where the long axis of the heart was measured from the center of the carina to the most distal contour of the ventral apex of the heart. The carina was defined as the radiolucent circular structure within the trachea, which represents the bifurcation of the left and right mainstem bronchi. The short axis of the heart was measured in the central third region of the heart, perpendicular to the long axis. Both axis measurements were then positioned over the thoracic vertebral bodies beginning at the cranial edge of the fourth thoracic vertebra. The sum of both axes was used to determine the number of vertebral units to the nearest 0.1 vertebra.
For the VLAS measurement, a line was drawn from the center of the most ventral aspect of the carina to the most caudal aspect of the left atrium where it intersected with the dorsal border of the caudal vena cava. The measurement was then positioned over the fourth thoracic vertebra as before, and the total vertebral units were used to calculate the VLAS.4
Statistical analysis
Continuous variables were described using median values and IQRs, whereas categorical variables were presented in terms of proportions. Methodologic adherence to the Clinical and Laboratory Standards Institute's EP28-A3 guidelines was maintained by employing a 95% nonparametric bootstrap method with a 90% CI for calculations. All statistical analyses were conducted using R software, version 4.3.4 (R Foundation for Statistical Computing).
Results
During the study period, a total of 991 JRTs, 750 MPs, and 399 BGs were identified. Of these 689 JRTs, 512 MPs, and 267 BGs were excluded. The reasons for exclusion for each breed are shown in Supplementary Table S1. Consequently, there were a total of 302 JRTs, 238 MPs, and 132 BGs available for analysis. For JRTs, there were 26 (8.6%) female intact dogs, 113 (37.4%) female spayed dogs, 35 (11.6%) male intact dogs, 124 (41.1%) male castrated dogs, and 4 dogs (1.3%) where the sex was not reported. For MPs, there were 31 (13.0%) female intact dogs, 87 (36.6%) female spayed dogs, 31 (13.0%) male intact dogs, 78 (32.8%) male castrated dogs, and 11 (4.6%) dogs where the sex was not reported. For BGs, there were 15 (11.4%) female intact dogs, 30 (22.7%) female spayed dogs, 16 (12.1%) male intact dogs, 63 (47.7%) male castrated dogs, and 8 (6.1%) dogs where sex was not reported. Age and weight statistics are reported in Table 1. Median VHS, VLAS, and reference intervals for all 3 breeds are shown in Tables 2 and 3 and Figures 1 and 2.
Age and weight of breeds included in the study.
| Breed/parameter | Median | Q25 | Q75 | Not reported |
|---|---|---|---|---|
| Jack Russell Terrier | ||||
| Age (y) | 11.7 | 7.5 | 13.9 | 3 |
| Weight (kg) | 8.0 | 6.0 | 9.5 | 38 |
| Miniature Pinscher | ||||
| Age (y) | 11.6 | 8.4 | 13.2 | 2 |
| Weight (kg) | 5.9 | 4.5 | 7.7 | 33 |
| Brussels Griffon | ||||
| Age (y) | 10.1 | 5.6 | 12.8 | 2 |
| Weight (kg) | 6.6 | 5.0 | 8.2 | 14 |
Q = Quartile.
Vertebral heart size (VHS) and vertebral left atrial size (VLAS) for Jack Russell Terriers, Miniature Pinschers, and Brussels Griffons.
| Breed/parameter | Median | Q25 | Q75 |
|---|---|---|---|
| Jack Russell Terrier | |||
| VHS (vertebrae) | 10.8v | 10.2v | 11.v |
| VLAS (vertebrae) | 2.1 | 2.0v | 2.2v |
| Miniature Pinscher | |||
| VHS (vertebrae) | 11.0v | 10.3v | 11.2v |
| VLAS (vertebrae) | 2.0v | 1.8v | 2.1v |
| Brussels Griffon | |||
| VHS (vertebrae) | 10.8v | 10.2v | 11.0v |
| VLAS (vertebrae) | 2.0 | 1.9v | 2.1v |
Reference intervals for VHS and VLAS in Jack Russell Terriers, Miniature Pinschers, and Brussels Griffons.
| Lower reference limit | Upper reference limit | |||
|---|---|---|---|---|
| Estimate | 95% CI | Estimate | 95% CI | |
| VLAS (vertebrae) | ||||
| Jack Russell Terrier | 9.7 | 9.5–9.8 | 12.2 | 12.1–12.4 |
| Miniature Pinscher | 9.6 | 9.4–9.7 | 11.9 | 11.6–12.0 |
| Brussels Griffon | 9.3 | 9.3–9.6 | 11.8 | 11.7–11.9 |
| VHS (vertebrae) | ||||
| Jack Russell Terrier | 1.8 | 1.8–1.8 | 2.6 | 2.5–2.7 |
| Miniature Pinscher | 1.7 | 1.6–1.7 | 2.5 | 2.3–2.6 |
| Brussels Griffon | 1.7 | 1.6–1.7 | 2.4 | 2.3–2.5 |
Vertebral heart score (VHS) histogram for all 3 breeds. Solid lines represent the upper and lower reference limits, and the area between these represents the reference interval. The dashed lines represent the confidence interval for those reference limits.
Citation: American Journal of Veterinary Research 85, 12; 10.2460/ajvr.24.07.0209
Vertebral left atrial size (VLAS) histogram for all 3 breeds. Solid lines represent the upper and lower reference limits, and the area between these represents the reference interval. The dashed lines represent the confidence interval for those reference limits.
Citation: American Journal of Veterinary Research 85, 12; 10.2460/ajvr.24.07.0209
Discussion
This study reports breed-specific VHS and VLAS reference intervals for JRTs, MPs, and BGs. This will aid in improving veterinarians’ accuracy at identifying dogs with underlying cardiac disease; otherwise, cardiomegaly may inadvertently be diagnosed in these 3 breeds based on an elevated VHS score, leading to unnecessary testing and pharmaceutical management. The cause of these elevated vertebral scores in these 3 breeds is unclear. One possibility is that it may be related to shorter vertebral bodies compared to body size. One example is that English Bulldogs with fused hemivertebrae could falsely elevate the VHS.7 Another study14 found that lumbar vertebrae in miniature schnauzers may be smaller, which can lead to a misdiagnosis of renomegaly on radiographs. Miniature Schnauzers are another breed with a VHS reference range that exceeds the current reference range as well.8 As such, it is possible that shorter vertebrae in these 3 breeds may have contributed to their elevated VHS and VLAS reference ranges. The relative size of vertebrae in dogs compared to their overall body size may require further investigation. Regardless, clinicians need to be aware of these findings to avoid misdiagnosing a patient with underlying cardiac disease because its VHS exceeds the nonspecific breed scale.
The primary strength of this study is sample size as all 3 breeds have over 130 dogs included to develop a breed-specific reference interval. Current recommendations state that more than 120 animals are ideal to develop any reference interval in veterinary medicine.15 As stated previously, several studies2,3,5–11,16 have published breed-specific VHS reference intervals, but only 1 study8 exceeds the recommended 120-subject threshold. The relatively small sample size of other studies3,16 may be 1 of the reasons that some studies yield conflicting results on breed-specific VHS reference intervals on the same breed. Consequently, larger studies that exceed the 120-subject recommended requirement may be necessary to validate previously published breed-specific VHS reference intervals.
This study has several limitations. The study utilized a large amount of data from telemedicine consultations, and in some cases the demographic data was incomplete (ie, age, weight, and/or sex). The study included cases throughout the country, and therefore radiographic technique and positioning was not standardized, and thus various kilovoltage peak (kVp) and milliampere seconds (mAs) settings were used for radiographic acquisition. Some studies9 have shown that higher body conditions were associated with higher VHS scores, although this analysis was not performed on the data reported here. The dogs included did not have a reported heart murmur or significant systemic disease, but the detection of underlying cardiac disease or systemic disease is dependent on the clinical proficiency of submitting veterinarians. The authors are unable to control for phase of respiration, which may have a mild effect on the accuracy of VHS measurements.17 However, the study population was fairly robust for all 3 breeds, likely minimizing this effect. The heartworm status was not known for all dogs, and heartworm disease causes VHS to increase in affected patients.18 However, to minimize this effect all dogs that were reported to be heartworm positive were excluded from analysis. Other diseases that can affect VHS measurement, such as pericardial effusion,19 cannot be completely ruled out, although it is considered less likely given the absence of clinical signs necessary for inclusion in the study population. Dogs that were reported to receive grain-free diets were excluded given the association with dilated cardiomyopathy.20 However, as diet was not universally reported in all cases, it is possible that a small population of dogs receiving grain-free diets were inadvertently included in the study population. Finally, it also would have been ideal to have 2 different cardiologists measuring VHS and VLAS in the study population and compare the results between cardiologists to look for evidence of interobserver variability, but this was unfortunately not performed here.
In conclusion, the authors report breed-specific VHS and VLAS reference intervals for 3 dog breeds. Clinicians should be aware of these results to avoid erroneously diagnosing cardiomegaly in these breeds. Continued investigation into breed-specific VHS measurements in other canine breeds appears warranted.
Supplementary Materials
Supplementary materials are posted online at the journal website: avmajournals.avma.org.
Acknowledgments
None reported.
Disclosures
The authors have nothing to disclose. No AI-assisted technologies were used in the generation of this manuscript.
Funding
The authors have nothing to disclose.
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