The Vertebral Heart Score (VHS) – also known as Vertebral Heart Size or Vertebral Heart Scale – is a measurement method used in veterinary medicine to assess the heart size of animals based on thoracic radiographs. It allows for the evaluation of heart size independent of the patient's overall body size, similar to the cardio-thoracic ratio in human medicine. The VHS is primarily used in dogs to detect cardiomegaly (heart enlargement), particularly in cases of heart diseases associated with dilation, such as dilated cardiomyopathy. The method involves projecting the long and short axes of the heart onto the thoracic vertebrae starting from the fourth thoracic vertebra and calculating the number of vertebrae these lengths cover. The VHS was established by Buchanan and Bücheler in 1995. A VHS of < 10.5 is generally considered normal for dogs, although higher values are accepted as physiological for certain breeds.
Measurement and normal values
The VHS is typically determined using a thoracic radiograph taken in lateral recumbency. On the radiograph, the distance from the carina (bifurcation of the trachea) to the apex of the heart is measured (long axis), as well as a second distance perpendicular to the first at the widest part of the heart (short axis). For dogs with significant enlargement of the left atrium, Buchanan suggests placing the dorsal end of the long axis at the elevated left bronchus. In older cats, where the long axis of the heart often runs nearly parallel to the sternum, the base of the vein of the cranial lung lobe is recommended as the reference point instead of the tracheal bifurcation. The measurement of the heart axes can also be performed on a radiograph taken in dorsal recumbency (ventrodorsal view); however, in dogs, the left atrium does not contribute to the cardiac silhouette in this projection.
These two lengths are then transferred to the thoracic vertebrae, beginning at the cranial edge of the fourth thoracic vertebra (T4). The number of vertebrae covered by these lengths is determined. If the end of the segment does not align perfectly with the end of a vertebra, the partial vertebra is estimated to the nearest tenth. For example, if the long axis extends from the beginning of T4 to the middle of T9, the value for this axis is 5.5. The length of a vertebral body combined with its associated intervertebral disc serves as a relative unit of measurement reflecting the size of the individual. Since vertebral lengths vary along the spine, it is crucial to always begin measuring from the specified vertebra (T4). The cardio-thoracic ratio, used in human medicine, is not suitable for dogs due to the significant variation in chest shape among different breeds. The Vertebral Heart Score is the sum of the vertebral lengths covered by the long and short axes. A VHS of up to 10.5 (dog) or 8.1 (cat) is considered normal; higher values suggest cardiomegaly. The VHS is not used to diagnose a small heart (microcardia). Instead, microcardia is assessed by the number of intercostal spaces the cardiac silhouette covers. A small heart is typically caused by hypovolemia rather than heart disease. In such cases, the heart occupies fewer than two intercostal spaces.
Breed-specific ranges Certain dog breeds typically present with higher VHS values. For the Boxer, a normal range of 10.8 to 12.4 applies; for French Bulldogs and English Bulldogs, the range is 11 to 14.4; and for the Boston Terrier, it is 10.3 to 13.1. Values up to 11.5 may be considered physiological for other breeds, including the Pomeranian, Cavalier King Charles Spaniel, Pug, Whippet, and Labrador Retriever.
Other species The relationship between heart size and thoracic vertebrae size has also been investigated in other animal species. Onuma et al. determined a mean VHS of 7.55 in domestic rabbits weighing less than 1.6 kg, and 8.0 in heavier animals. The same research group described heart axes in ferrets in relation to the length of the sixth thoracic vertebra (T6). In females, the long axis of the heart was 3.0 to 3.3 times the length of T6, and the short axis was 2.2 to 2.4 times the length. For males, the values were 3.2 to 3.7 times and 2.3 to 2.7 times the length of T6, respectively. A mean VHS of 9.36 was established for Alpaca crias, and 8.9 for the Ring-tailed lemur.
Comparison to human medicine In humans, the heart size is assessed using the cardio-thoracic ratio (CTR). This involves measuring the distance from the midline to the most lateral right and left heart borders on a standing PA (posterior-anterior) radiograph. The sum of these two distances is compared to the transverse diameter of the chest at the level of the right diaphragmatic dome. The ratio should not exceed 1:2 (0.5).
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