The radiology examination is one of the most effective and widely available modalities for evaluating cardiac changes in small animal patients. The diagnosis of cardiac disease in dogs on radiographs is typically based on the presence of abnormal size and shape of the cardiac silhouette and pulmonary vessels and the presence or absence of pulmonary edema. Initial studies1–3 used a cardiothoracic ratio, but this calculation is limited by variations in the heart axis, chest shape among the different breed conformations, respiration phase, and imprecise measurement points. Vertebral heart score (VHS) was first introduced as a way to accurately identify dogs with cardiomegaly based on radiographs and was found to be a highly reproducible measurement that has been shown to have minimal interobserver variability.4,5
The use of serial measurements of VHS over time has shown that the score often increases in a dog with progressive cardiac disease, and in these dogs, there is a higher chance of developing congestive heart failure (CHF).6 In animals that are not in active CHF, the radiographic examination is focused on the size and shape of the cardiac silhouette to help identify cardiomegaly and if that animal is at risk of developing cardiac disease, CHF, or warrants a larger cardiology workup (eg, echocardiogram). Thus, a canine reference range has been formulated to be able to accurately identify dogs with cardiomegaly.
The initial 1995 study by Buchanan et al4 showed that 98% of the healthy dogs evaluated had less than or equal to a VHS of 10.5, and this value was suggested as the clinically useful upper limit for normal heart size in most breeds. However, exceptions were noted in certain breeds, including dogs with short thoraxes. For example, Dachshunds were found to have a lower VHS, and, most notably for our research, the Miniature Schnauzers in the study by Buchanan et al4 were more likely to have a VHS reference range of 11 to 11.5.4 Several more recent studies7–9 have shown that several breeds without the concurrent cardiac disease have a higher VHS reference range than 10.5.
A newer thoracic radiographic measurement, the vertebral left atrial size (VLAS), has become more prominent in veterinary medicine since its publication in 2018 and has been included in the American College of Veterinary Internal Medicine consensus on the diagnosis of myxomatous mitral valve disease (MMVD).10 The assessment of left atrial (LA) size is considered an important part of the diagnostic evaluation of dogs suspected of having cardiac disease and left-sided CHF. Previously, echocardiography was necessary to measure LA size accurately by indexing it to the aorta. However, echocardiography is not always readily available for all veterinary practitioners. Several studies11–14 have confirmed that VLAS is an accurate method to predict LA echocardiographic enlargement.
Our hypothesis is that Minature Schnauzers have a higher normal VHS than the accepted standard canine reference range of 10.5 and an evaluation of a larger number of Miniature Schnauzers than the original study of Buchanan et al4 would confirm this theory.
Methods
The electronic medical record system of IDEXX was searched for Miniature Schnauzers undergoing radiographs between March 1, 2022, and February 28, 2023. The radiographs had been previously submitted to IDEXX for telemedicine review. Dogs were included if they had 3 view thoracic radiographs performed (left and right lateral, 1 ventrodorsal or dorsoventral view). Radiographs also could not have any evidence of cardiopulmonary disease. This was based on the provided history, physical exam findings (lack of a murmur), and normal radiographic pulmonary parenchyma. All dogs with incomplete radiographic studies or known extracardiac disease were excluded. Radiographic studies with poor positioning, which 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 exam, which was documented in the telemedicine consultation form. All dogs with a murmur were excluded.
The VHS and VLAS measurements were performed by 2 board-certified cardiologists independent of one another. The cardiologists were blinded to the original telemedicine radiology report for each medical record. Since the radiographs were all in a digital format (IDEXX Telemedicine Web PACS Viewer), 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 al4 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 sum of the vertebral units was used to calculate the VLAS.
Statistics
Continuous variables were described using median values and IQRs, while categorical variables were presented in terms of proportions. The agreement in measuring VHS and VLAS among cardiologists was evaluated using a Passing-Bablok regression. The regression’s slope was used to assess proportional bias, aiming for a slope of 1 as the ideal, while the intercept was analyzed to estimate constant bias, with an intercept of zero being the target. Subsequent to the initial evaluations, the arithmetic mean of the measurements taken by the 2 cardiologists was utilized for the establishment of reference intervals. Methodological 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. The life stages for canines were categorized as follows: puppy (≤ 1 year), young adult (>1 to ≤ 4 years), mature adult (> 4 years to ≤ 7 years), senior (> 7 years to ≤ 10 years), and geriatric (> 10 years). All statistical analyses were conducted using R software, version 4.3.4 (R Core Team. R: a Language and Environment for Statistical Computing. 2023; R Foundation for Statistical Computing. https://www.R-project.org/).
Results
A total of 1,000 medical records for Miniature Schnauzers were identified using the IDEXX telemedicine records. Of these 728 were excluded. Reasons for exclusion included the following: wrong breed (n = 1), nonthoracic radiographic study (398), incomplete radiographic study (33), concurrent extracardiac disease (9), presence of a heart murmur (283), and poor positioning (4). This left 272 radiographic studies that met our inclusionary criteria. The median age was 9.2 years (IQR, 6.0 to 12.0 years) with 1.5% (n = 4) not having complete age information. The median weight was 8.0 kg (IQR, 6.0 to 9.5 kg) with 67.2% (183) not having complete weight information. A full list of study demographic information is provided (Table 1). For VLAS, the comparison revealed a slight constant bias, with an intercept of 0.05 vertebrae (95% CI, 0.01 to 0.08) and an estimated slope of 0.92 (95% CI, 0.83 to 1.00). The VHS comparison indicated a constant bias, characterized by an intercept of 0.25 vertebrae (95% CI, 0.45 to 0.95) and an estimated slope of 0.89 (95% CI, 0.82 to 0.96).
Study demographics.
Demographics | % | n |
---|---|---|
Life stage | ||
Puppy | 8.1 | 22 |
Young adult | 12.1 | 33 |
Mature adult | 12.9 | 35 |
Senior | 23.2 | 63 |
Geriatric | 42.3 | 115 |
Unknown | 1.5 | 4 |
Sex | ||
Female | 49.6 | 135 |
Male | 47.8 | 130 |
Unknown | 2.6 | 7 |
Neuter status | ||
Intact | 21.7 | 59 |
Fixed | 75.7 | 206 |
Unknown | 2.6 | 7 |
Canine life stage: puppy (≤ 1 year), young adult (>1 to ≤ 4 years), mature adult (> 4 years ≤ 7 years), senior (> 7 years to ≤ 10 years), and geriatric (> 10 years).
The median and IQR for VHS and VLAS of the demographic subgroups (life stage, sex, neuter status) are provided (Table 2). The median VLAS across all age ranges was 2.0 vertebrae. The lower and upper bound 95% reference limits for VHS are 9.68 vertebrae and 12.07 vertebrae, respectively (Table 3; Figure 1). The lower and upper bound 95% reference limits for VLAS were 1.71 vertebrae and 2.40 vertebrae, respectively. No partitioning of reference intervals by subgroup was done.
Summary statistics of vertebral heart score (VHS) and vertebral left atrial size (VLAS).
VHS | VLAS | |||||
---|---|---|---|---|---|---|
Median | Q1 | Q3 | Median | Q1 | Q3 | |
Life stage | ||||||
Puppy | 10.7 | 10.2 | 10.9 | 2.0 | 2.0 | 2.1 |
Young adult | 10.7 | 10.1 | 11.0 | 2.0 | 1.9 | 2.1 |
Mature adult | 11.1 | 10.5 | 11.3 | 2.0 | 1.9 | 2.1 |
Senior | 11.0 | 10.5 | 11.3 | 2.0 | 2.0 | 2.1 |
Geriatric | 10.9 | 10.5 | 11.3 | 2.0 | 1.9 | 2.1 |
Unknown | 11.0 | 10.5 | 11.3 | 1.9 | 1.8 | 2.0 |
Neuter status | ||||||
Fixed | 10.9 | 10.5 | 11.2 | 2.0 | 1.9 | 2.1 |
Intact | 10.8 | 10.3 | 11.2 | 2.0 | 2 | 2.1 |
Unknown | 11.1 | 11.1 | 11.4 | 2.0 | 1.9 | 2.0 |
Sex | ||||||
Female | 10.8 | 10.4 | 11.2 | 2.0 | 1.9 | 2.1 |
Male | 10.9 | 10.4 | 11.3 | 2.0 | 1.9 | 2.1 |
Unknown | 11.1 | 11.1 | 11.4 | 2.0 | 1.9 | 2.0 |
Canine life stage: puppy (≤ 1 year), young adult (>1 to ≤ 4 years), mature adult (> 4 years ≤ 7 years), senior (> 7 years to ≤ 10 years), and geriatric (> 10 years).
95% Reference interval and 90% CI for vertebral heart score (VHS) and vertebral left atrial size (VLAS).
Lower reference limit | Upper reference limit | |||
---|---|---|---|---|
Estimate | 90% CI | Estimate | 90% CI | |
VLAS (vertebrae) | 1.7 | 1.6–1.8 | 2.4 | 2.3–2.9 |
VHS (vertebrae) | 9.7 | 9.2–9.8 | 12.1 | 12.0–12.5 |
A—Histogram of the vertebral heart score (VHS; mean of 2 cardiologists) and the 95% reference interval (RI). B—Histogram of the vertebral left atrial size (VLAS; mean of 2 cardiologists) and the 95% RI. Blue lines represent the upper and lower 95% RI. Red area and dashed lines represent the 90% confidence bound around the RI.
Citation: American Journal of Veterinary Research 85, 5; 10.2460/ajvr.24.01.0010
Discussion
This study confirmed our hypothesis that Miniature Schnauzers without cardiac disease have a higher VHS reference range and was in agreement with many other studies9,14,15 evaluating individual breed VHS ranges that gender and effect of neuter status did not have significant effects on measured VHS and VLAS. Our findings are particularly relevant to this breed since previous studies16 have shown that these dogs have higher rates of MMVD highlighting the importance of identifying this breed’s normal VHS range. In the last few years, multiple studies7–9 have shown that within individual breeds, there are different VHS ranges with some breeds having smaller or higher values than the accepted normal reference range. This is important research as it will increase a veterinarian’s accuracy in properly identifying dogs at risk for cardiac disease on radiographs, which is one of the most commonly used screening tests for cardiac disease in dogs. The original study of Buchanan et al4 only included up to 4 dogs of each breed evaluated although it postulated that interbreed variations for the VHS likely existed. Current research looking at VHS in multiple individual dog breeds has borne this theory out and Miniature Schnauzers can now be added to that group of breeds where the standard 10.5 VHS range may not be applicable.14,15,17–19
There was a mild positive correlation in this cohort of dogs between VHS and VLAS measurements, although the median VLAS measurement was under 2.6 units, which is considered a useful cut-off for identifying dogs with stage B2 MMVD.12 Based on the normal pulmonary parenchyma and reported normal cardiac and thoracic auscultation, there was no indication that any of the included dogs were suffering cardiac disease or left-sided CHF, which could have increased the measured value of both the VHS and VLAS. The reported correlation in both measurements is likely explained by the fact that both measurements are performed on the same radiographic image and referred to that dog’s vertebral bodies. There is sparse veterinary literature directly assessing the vertebral body length compared to overall body size among different breeds of dogs although it is accepted that chrondrodystrophic breeds, like English bulldogs, with fused hemivertebrae can falsely elevate the VHS.17 Similarly, dogs with severe lordosis or kyphosis could have similar effects on measurements when utilizing the vertebral bodies as a reference.20 One study21 in Miniature Schnauzers specifically showed that at least for the lumbar vertebrae, the breed may have shorter vertebral bodies, which can lead to misdiagnosis of renomegaly on abdominal radiographs. It is unknown if this breed of dog has shorter thoracic vertebral bodies and whether this could have had a similar effect on the measured VHS and VLAS presented here.
This study has some limitations. The study utilized a large amount of data obtained via telemedicine consultations, and in some cases, complete demographic data (exact age and/or weight) were not always provided. Similarly, due to being a study using radiographs obtained via a telemedicine consulting service, the authors could not control the quality of the radiographs although images of poor quality or incomplete sets were excluded. The KvP and Mas used for radiograph acquisition were not standardized amongst the included radiographs. While some studies have shown higher VHS in dogs with higher body condition scores, we did not evaluate any effects of weight on VHS measurements.15 The dogs included reportedly did not have any auscultable murmur or significant extracardiac disease although the authors cannot confirm this fact given the physical exams were performed by the veterinarian submitting the telemedicine consult. Similarly, those veterinarians may have had different experience levels and included specialists and nonspecialists. Given the retrospective nature of this study, we were unable to control for the phase of respiration when the radiographs were performed, and previous studies22 have shown that the respiratory phase may have some mild effects on the accuracy of VHS measurements although any such effects are likely limited by the larger sample size presented here. Radiograph quality was subjective based on the cardiologist’s ability to accurately measure the VHS although poor quality radiographs were specifically not included. To further reduce interobserver variability, all measurements were only performed on right lateral radiographic views although the effect, or lack thereof, of right versus left lateral recumbency on VHS is not consistent across all studies.4,15,23 The study included dogs throughout the continental US and heartworm (HW) status was not known for all dogs. A previous study24 has shown that dogs and cats with HW had higher VHS measurements with a correlation between the mean diameter of the caudal vena cava and VHS. Caudal vena cava measurements were not performed in this study. The lack of known HW status is not believed to have a significant effect given that we purposely excluded dogs with any cardiopulmonary symptoms and radiographs, which showed abnormal pulmonary parenchyma. Similarly, other diseases, like pericardial effusion, which are known to affect VHS measurements, cannot be completely excluded although the fact all dogs were reported asymptomatic likely minimizes any such effect on our results.25
In conclusion, the authors report that the reference interval for VHS in Miniature Schnauzers is greater than the standard canine reference range. Our study is in support of the numerous studies evaluating VHS in individual dog breeds in that the use of one reference range for all dog breeds may lead to an erroneous diagnosis of cardiomegaly. Continued efforts looking at this measurement in more breeds and among different ages within breeds is warranted.
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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