Vertebral heart scale (VHS) is an objective radiographic measurement of the cardiac silhouette that can be useful when cardiac disease is suspected and other imaging modalities (eg, echocardiography, thoracic-focused point-of-care ultrasonography) cannot be performed.
Appropriate radiographic technique and patient positioning should be used to prevent measurement errors due to malpositioning. Positioning should be consistent if serial radiographs are taken to allow accurate comparison.
VERTEBRAL HEART SCALE IN A CAT WITH A GRADE I-II/VI MURMUR
A 12-year-old neutered male domestic shorthair cat was presented for evaluation of a heart murmur diagnosed 3 months prior. On physical examination, a grade I-II/VI right parasternal systolic murmur was auscultated. All other physical examination findings were within normal limits. There is history of intermittent vomiting.
Thoracic radiographs revealed a normal-sized cardiac silhouette with VHS 7.7 and no signs of CHF (Figure 1), as well as a redundant aorta caused by exaggerated horizontal positioning of the cardiac silhouette that increased the prominence of the aortic arch, a real and common age-related change considered normal in older cats.25 Remaining thoracic structures were unremarkable. Although heart disease could not be ruled out without echocardiography, there was no evidence of structural changes causing cardiac enlargement.
VHS calculation in a clinically normal cat using a left lateral radiograph (A). L (ie, long axis; 4.4) is drawn from the carina to the most ventral aspect of the apex. S (ie, short axis; 3.3) is drawn perpendicular to L at the widest aspect of the heart, extending to the cranial and caudal borders. S and L are transposed along the spine from the cranial aspect of T4 using calipers. The number of vertebrae traversed (rounded to the nearest tenth) are summed to calculate VHS (7.7). Orthogonal radiograph of the patient is also shown for more complete evaluation of cardiac silhouette (B).
Normal Vertebral Heart Scale Values
The published range for normal VHS in cats is 6.7 to 8.1 on lateral radiographs (average, 7.5).1 For simplicity, VHS <8 is generally considered normal (see Vertebral Heart Scale in a Cat with a Grade I-II/VI Murmur and Vertebral Heart Scale in a Cat with a Grade III-IV/VI Murmur).
In general, there is little breed variation of VHS in cats2; however, Persian cats have an average VHS of 8.16, which may be due to abnormalities associated with brachycephalic conformation. Although studies in brachycephalic cats are limited, studies in dogs have shown brachycephalic breeds have a larger than normal VHS that may be attributed to congenital vertebral malformations, narrowed intervertebral disk space secondary to intervertebral disk disease, and right-sided cardiac remodeling secondary to respiratory difficulty and resultant pulmonary hypertension.3-5 A study of VHS in healthy stray cats found slightly smaller VHS values (7.3 in right and left lateral views), but this was attributed to a less variable sample population.6
VERTEBRAL HEART SCALE IN A CAT WITH A GRADE III-IV/VI MURMUR
A 12-year-old neutered male domestic shorthair cat was presented for evaluation of a previously diagnosed heart murmur. On physical examination, a grade III-IV/VI left parasternal systolic murmur was auscultated. All other physical examination findings were within normal limits. There is history of right thoracic limb lameness and mild lytic lesions on the right humerus. Thoracic radiographs revealed mild to moderate cardiac silhouette enlargement with VHS 8.5, suggestive of cardiac disease (Figure 2). Additional cardiac diagnostic investigation (eg, total thyroxine, echocardiography) is recommended, and medical treatment should be considered.
VHS calculation in a cat diagnosed with HCM using a right lateral radiograph (A). L (ie, long axis; 5.0) is drawn from the carina to the most ventral aspect of the apex. S (ie, short axis; 3.5) is drawn perpendicular to L at the widest aspect of the heart, extending to the cranial and caudal borders. S and L are transposed along the spine from the cranial aspect of T4 using calipers. The number of vertebrae traversed (rounded to the nearest tenth) are summed to calculate VHS (8.5). Pulmonary vasculature is prominent but within normal limits, and there is a ballistic metallic foreign body (likely a bullet) in the dorsal subcutaneous tissue of the caudal thorax. Moderate spondylosis deformans exists at T13-L1 and L1-L2, and there is bridging spondylosis deformans at T10-T11. Intervertebral disk space at T10-T11 is collapsed, and there is fusion at the 2 vertebral bodies. Orthogonal radiograph of the patient is also shown for more complete evaluation of cardiac silhouette (B).
Uses of Vertebral Heart Scale
Although echocardiography remains the gold standard for diagnosing heart disease in cats, radiography is the most common diagnostic method used to evaluate cardiac size and shape.7 Radiography can be important for diagnosing heart disease because many cats are subclinical until they show clinical signs consistent with congestive heart failure (CHF), femoral arterial thromboembolism, or fatal arrhythmias.<sup8,9 sup>
Cardiomegaly is a consistent finding in cats with left-sided CHF, and VHS can be used to differentiate cardiac and noncardiac causes of dyspnea.10,11 Lack of cardiomegaly or left atrial enlargement on thoracic radiographs does not rule out heart disease or failure.11,12 Thoracic radiography is specific but has low sensitivity for identifying cardiomegaly in cats with mild structural cardiac disease13; therefore, cardiomegaly may not be detected radiographically during early stages of hypertrophic cardiomyopathy (HCM) before significant left atrial enlargement and left ventricular hypertrophy are present.14 In addition, cardiomegaly may not be detected in disease states (eg, endocarditis, myocarditis, arrhythmias) that do not produce exaggerated structural changes to the heart.15 VHS is significantly higher in cats with heart disease that causes moderate to severe left atrial enlargement compared with cats with heart disease that causes mild left atrial enlargement or no enlargement, emphasizing the importance of echocardiogram in patients without obvious radiographic evidence of left atrial enlargement.13
Although a valentine-shape heart on thoracic radiographs is highly suggestive of cardiomyopathy in cats, it is not a specific indicator of biatrial enlargement or HCM, and cardiomyopathy is not guaranteed, which highlights limitations of subjective radiographic evaluation in diagnosing heart disease.16 Determination of cardiac size via VHS is a more objective means of identifying cardiomegaly; however, up to 33% of cats with normal VHS have been diagnosed with heart disease.8
Cats with VHS >9.3 have a high risk for heart disease and likely do not require further cardiac diagnostics to determine the cause of respiratory distress, whereas cats with VHS 8 to 9.3 likely require further investigation (eg, echocardiography) to diagnose heart disease and differentiate cardiac and noncardiac causes of dyspnea.7,10 Cats with VHS >8 have a 2.2 times greater risk for HCM.8
In a study of 59 apparently healthy cats, 41% had heart disease, with HCM being the most common cause8; only 52% of apparently healthy cats with VHS >8 were diagnosed with HCM. This finding supports use of echocardiography for definitive diagnosis of heart disease in cats with VHS 8 to 9.3.7,10
Measurement of plasma N-terminal pro-brain natriuretic peptide (NT-proBNP), a peptide secreted in response to cardiac stretching, can be used to distinguish cardiac and noncardiac causes of dyspnea; cats with cardiac causes have higher levels of NT-proBNP.17,18 An available point of care test for cats measures circulating NT-proBNP concentration using serum or EDTA plasma; values >150 pmol/L are abnormal and suggest CHF in cats with respiratory signs.19 This test can be used in conjunction with VHS results to further confirm cause of dyspnea.
Noncardiac Factors That Can Affect Vertebral Heart Scale
Interobserver Variability
Although there are few studies comparing interobserver variability of VHS measurement in cats, studies in dogs have mixed results. One study found a high level of reproducibility among expertise levels, but another study found a mean difference of ≈1 vertebral unit among observers and recommended serial VHS measurements be completed by the same clinician.20,21
Obesity
Obese cats tend to have slightly larger cardiac silhouette measurements, but a study found these differences were not significant once pericardial fat was differentiated and excluded.22,23 Obese cats usually have excessive pericardial fat, especially if they also have increased falciform fat. Care should be taken when measuring VHS in obese cats because difficulty differentiating the true cardiac silhouette from surrounding pericardial fat can lead to overestimation of cardiac size.22
Right Versus Left Lateral Recumbency
Current studies do not indicate significant differences in VHS measurement based on recumbent patient position.6 Consistent patient positioning is recommended in serial VHS measurements but may not be clinically significant.
Sedatives
Dexmedetomidine, an alpha-2 agonist, is commonly used for chemical restraint and sedation during radiography; however, use of this drug has been associated with small but significant VHS increase in cats.24 Based on this finding and undesirable cardiovascular effects, dexmedetomidine is not recommended as a sedative in patients with known or suspected cardiac disease.
STEP-BY-STEP
MEASURING VERTEBRAL HEART SCALE
WHAT YOU WILL NEED
Right or left lateral thoracic radiograph
Calipers (electronic calipers can be used with digital films)
STEP 1
Use calipers to measure the long axis of the heart from the carina to the most ventral point of the apex on the right or left lateral thoracic radiograph.
STEP 2
Transfer the long axis measurement to the cranial aspect of the fourth thoracic vertebrae (T4), extending caudally along the vertebrae at midbody position. Count the vertebrae transversed, and round to the nearest tenth (ie, L).
STEP 3
Use calipers to measure the short axis of the heart at the widest point perpendicular to the long axis.
STEP 4
Transfer the short axis measurement to the same cranial aspect of T4 as in Step 2, count the vertebrae transversed, and round to the nearest tenth (ie, S).
STEP 5
Sum L and S to calculate VHS.
Author Insight
Normal VHS does not rule out cardiac disease or dysfunction. VHS, in conjunction with other diagnostics (eg, physical examination, patient history, ECG, echocardiogram), is important to assess cardiac status and determine whether treatment for cardiac disease or further testing is needed based on clinical suspicion of cardiac involvement in disease.