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Vertebral scale system to measure heart size in growing puppies Margaret M. Sleeper, VMD, DACVIM, and James W. Buchanan, DVM, MMedSci, DACVIM

Objective—To determine relative heart size in clinically normal puppies and assess whether relative heart size changes with growth. Design—Prospective radiographic study. Animals—11 puppies without evidence of disease. Procedure—Standardized measurements of the long and short axes of the heart, midthoracic vertebrae, and other structures were made at 3, 6, 12, and 36 months of age. Measurements were recorded in millimeters and number of thoracic vertebral lengths spanned by each dimension, measured caudally from T4 on lateral radiographic views. The long and short axis measurements of the heart, expressed in vertebral lengths, were added to yield vertebral heart size. Results—Mean ± SD vertebral heart sizes on lateral radiographic views at 3, 6, 12, and 36 months of age were 10.0 ± 0.5, 9.8 ± 0.4, 9.9 ± 0.6, and 10.3 ± 0.6 vertebrae, respectively. Significant differences were not detected. Conclusions and Clinical Relevance—Vertebral heart size measurements in puppies are within the reference range for adult dogs (9.7 ± 0.5 vertebrae) and do not change significantly with growth to 3 years of age. Standards for determining cardiac enlargement are similar in puppies and adult dogs. (J Am Vet Med Assoc 2001;219:57–59)

A

vertebral scale system for measuring heart size on radiographic views has been described in adult dogs and cats.1,2 The method is based on the fairly constant relationship between vertebral length and heart size in clinically normal dogs weighing 6.6 to 143 lb (3 to 65 kg). A midthoracic 5-vertebrae segment of vertebral length is used as an index of body size, and heart size is represented by the sum of the long and short axes measurements of the heart scaled against vertebrae (v) on lateral radiographic views. In 100 clinically normal adult dogs, mean (± SD) vertebral heart size (VHS) was 9.7 ± 0.5 v. In 100 clinically normal adult cats, mean (± SD) VHS was 7.5 ± 0.3 v.2 In newborn human infants, the heart weight/body weight ratio (6.3 g/kg) is greater than that of adults and decreases to adult proportions (4.6 g/kg) by 3 years of age.3 Radiographic cardiothoracic ratios in infants also suggest there is a relative reduction in heart size with growth; however, some of the reduction in cardiothoracic ratio may be attributable to lowering of the diaphragm and consequent change in the cardiac axis.3 This is similar to what is seen in cats; mean heart weight/body weight ratio in neonatal kittens was 9.3 From the Department of Clinical Studies, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, PA 19104. Address correspondence to Dr. Buchanan. JAVMA, Vol 219, No. 1, July 1, 2001

g/kg, whereas the mean for adults was 4.0 g/kg.4 In contrast, the heart weight/body weight ratio in neonatal puppies is 4.7 g/kg and increases to 7 to 8 g/kg in adult dogs.5 To our knowledge, the age at which this change in ratio is radiographically evident has not been determined. One source states that young animals up to approximately 3 months of age have relatively rounder hearts than adults.6 The objectives of the study reported here were to determine relative heart size radiographically in clinically normal puppies and to determine the influence of age on relative heart size. Materials and Methods Dogs—Eleven 3-month-old puppies (6 mixed-breeds, 2 Golden Retrievers, 1 Labrador Retriever, 1 Beagle, and 1 German Shepherd Dog) were selected for the study. Seven were females, and 4 were males. Examination by a cardiologist revealed no evidence of cardiac or pulmonary disease. Radiographic heart size was considered subjectively normal in all animals. The puppies were raised in household environments and returned to the hospital for thoracic radiographic assessment at 6 and 12 months of age. Radiographs also were obtained in 6 of the puppies at 3 years of age. Clinical abnormalities were not detected in any of the dogs, and heart sizes remained subjectively normal throughout the study. Radiographic measurements—Radiography was performed without sedation, using standard exposure techniques; left lateral and dorsoventral (DV) radiographic views were obtained. In left lateral radiographic views, the long and short axes of the heart were measured, using the vertebral scale system.1 The long axis was measured from the ventral border of the left main stem bronchus to the most distant ventral contour of the cardiac apex, using an adjustable caliper. The caliper was then repositioned over the vertebral column beginning with the cranial edge of T4. The distance spanned by the caliper was estimated to the nearest 0.1 v. The caliper was then placed on a metric ruler and the interval recorded to the nearest millimeter to obtain more precise measurements for statistical analysis. The maximal short axis of the heart in the central third region, perpendicular to the long axis, was measured in the same manner, beginning at the cranial edge of T4. The long and short axes dimensions were then added to obtain a vertebrae/heart sum that indicated heart size relative to body length. Thus, overall VHS was expressed as total units of vertebral length to the nearest 0.1 v. Heart size and vertebral length also were determined in millimeters. On DV radiographic views, the long and short axes of the heart were determined with calipers and were recorded as the number of vertebrae spanned on the lateral view beginning at T4. This measurement also was determined in millimeters. Depth and width ratios of the thorax were determined to assess thoracic conformational change with growth. The depth of the thorax was measured on lateral radiographic views from the cranial edge of the xiphoid process to the ventral border of the vertebral column along a line perpendicuScientific Reports: Original Study

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lar to the vertebral column. Thoracic width was measured on DV radiographic views as the distance between the medial borders of the eighth ribs at their most lateral curvatures. Statistical analyses—Linear random effects were fitted for the time periods for each of the dogs. Student t-tests were performed to compare the means against zero for the slope and intercept of these linear functions. Significance was set at P < 0.05. All analyses were performed using SAS statistical software.a

Results Differences among vertebral scale measurements at each time point were not significant. Mean VHS on lateral radiographic views were 10.0, 9.8, 9.9, and 10.3 v at 3, 6, 12, and 36 months of age, respectively. In DV radiographic views, mean VHS were 10.3, 9.7, 9.8, and 10 v at 3, 6, 12, and 36 months of age, respectively (Table 1). All absolute measurements increased with age as expected (Table 2). Mean weight of 3-month-old puppies increased from 11.7 to 22.2 lb (5.3 kg to 10.1 kg) at 6 months of age (91% increase) and 26.4 lb (12.0 kg) at 12 months of age (18% increase). Increases of 21 to 30% were observed in all measurements at 6 months of age, and an additional increase of 4 to 14% was observed at 12 months of age. Additional increases of 1 to 6% were observed in measurements of VHS in 6 of the dogs at 3 years of age. Discussion Heart/skeletal ratios to quantify heart size have been used in recent years to overcome the limitations caused by variable chest conformation and animal size.1,2,7 The vertebral scale system has gained popularity, because it is simple to explain and use, and there is little interobserver variation.1,2,8 In addition to providing a reference for normal heart size, it can be used to

document sequential changes in heart size in response to treatment or with progression of heart disease.8-10,b In adult dogs, variables such as a wide or deep thoracic conformation, sex, respiratory phase, and positioning in right or left lateral recumbency for radiography were found to be insignificant.1 Thus, many factors that have limited other systems for measuring heart size are no longer troublesome with this technique. Results of our study indicated that heart size in proportion to vertebral length does not change in dogs older than 3 months of age, and the vertebral scale system can be applied in puppies as well as adult dogs. This is in contrast to results of a study performed in kittens, in which heart size also was measured, using the vertebral scale system described in this report.11 Clinically normal kittens had relatively larger hearts (or smaller vertebrae) at 3 (VHS > 8.5 v) and 6 (VHS > 8.0 v) months of age than at 9 or 12 months of age, at which time their VHS values decreased to reference ranges in adult cats (VHS, 7.5 ± 0.3 v).2,11 It has been suggested that 3-month-old puppies have more rounded hearts than adult dogs6; this was not supported by the results of our study. Long and short axes measurements of the heart, using both the vertebral scale method and measurement of heart size and vertebral length in millimeters, had essentially the same proportions at 3, 6, 12, and 36 months of age. This indicates that diameters of the heart were relatively unchanged, and hearts were not more rounded at 3 months than at 6, 12, or 36 months of age. The vertebral scale system is helpful for inexperienced observers to ascertain cardiac enlargement and quantify it radiographically. However, it is not possible to evaluate subtle changes, and therefore one must keep in mind that considerable changes in contour of the heart can exist without cardiac enlargement.1,10,12 For example, cardiac concentric hypertrophy, which

Table 1—Measurement of relative heart size in growing puppies, using the vertebral scale system (data are reported as mean
SD No. of vertebrae) Age Variable Lateral long axis of heart Lateral short axis of heart Lateral axis sum Dorsoventral long axis Dorsoventral short axis Dorsoventral axis sum

3 mo (n = 11)

6 mo (n = 11)

12 mo (n = 11)

36 mo (n = 6)

5.4
0.32 4.6
0.26 10.0
0.52 5.8
0.33 4.5
0.35 10.3
0.64

5.2
0.28 4.6
0.02 9.8
0.42 5.4
0.32 4.3
0.28 9.7
0.54

5.3
0.31 4.6
0.32 9.9
0.60 5.5
0.35 4.3
0.28 9.8
0.66

5.5
0.34 4.8
0.36 10.3
0.58 5.6
0.43 4.4
0.34 10.0
0.75

Table 2—Thoracic and cardiovascular measurements (mm) for determination of relative heart size (mean
SD) Age

58

Variable

3 mo (n = 11)

6 mo (n = 11)

12 mo (n = 11)

36 mo (n = 6)

Lateral long axis of heart Lateral short axis of heart Lateral axis sum Length of 5 vertebrae

74.0
12.9 63.0
10.1 137.0
22.8 67.0
8.6

91.0
17.7 81.0
12.5 172.0
30.6 87.0
13.2

99.0
18.2 86.0
15.7 185.0
34.2 92.0
13.2

108.0
19.7 95.0
17.4 203.0
36.1 97.0
13.1

Thoracic depth Thoracic width Depth/width ratio Dorsoventral long axis Dorsoventral short axis Dorsoventral axis sum

105.0
21.5 120.0
15.8 0.86
0.07 78.0
13.9 61.0
11.4 139.0
25.1

130.0
29.7 145.0
28.3 0.90
0.08 97.0
17.7 76.0
14.2 173.0
32.5

146.0
33.7 159.0
29.4 0.92
0.07 104.0
21.2 79.0
14.9 183.0
35.5

165.0
39.7 168.0
24.1 0.97
0.11 109.0
19.2 86.0
14.8 195.0
33.7

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a

SAS, version 6.12, SAS Institute, Cary, NC. Nayakama H, Nakayama T, Nishijima Y, et al. Correlation of cardiac enlargement between the vertebral scale system (Buchanan index) and routine radiographic, echocardiographic, and electrocardiographic findings in dogs with evolving cardiomegaly (abstr). J Vet Intern Med 2000;14:375.

b

References 1. Buchanan JW, Bucheler J. Vertebral scale system to measure canine heart size in radiographs. J Am Vet Med Assoc 1995;206: 194–199. 2. Litster AL, Buchanan JW. Vertebral scale system to measure heart size in radiographs of cats. J Am Vet Med Assoc 2000;216: 210–214. 3. Holzmann M. Diseases of the heart and blood vessels. In: Schinz HR, Baensch WB, Friedl E, et al, eds. Roentgen diagnostics vol 3. New York: Grune & Stratton, 1953;2819. 4. Harpster NK, Zook BC. The cardiovascular system. In: Holzworth J, ed. Diseases of the cat: medicine and surgery. Philadelphia: WB Saunders Co, 1987;820.

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5. Lee JC, Taylor FN, Downing SE. A comparison of ventricular weights and geometry in newborn, young and adult mammals. J Appl Physiol 1975;38:147. 6. Ettinger SJ, Suter PF. Radiographic examination. In: Ettinger SJ, Suter PF, eds. Canine cardiology. Philadelphia: WB Saunders Co, 1970;48–49. 7. Van Den Broek AHM, Darke PGG. Cardiac measurements on thoracic radiographs of cats. J Small Anim Pract 1987;28:125–135. 8. Beardow AW, Buchanan JW. Chronic mitral valve disease in Cavalier King Charles Spaniels: 95 cases (1987–1991). J Am Vet Med Assoc 1993;203:1023–1029. 9. Buchanan JW, Sammarco CD. Circumferential suture of the mitral annulus for correction of mitral regurgitation in dogs. Vet Surg 1998;27:182–193. 10. Buchanan JW. Vertebral scale system to measure heart size in radiographs. Vet Clin North Am Small Anim Pract 2000;30: 379–393. 11. Gaschen L, Lang J, Lin S, et al Cardiomyopathy in dystrophin-deficient hypertrophic feline muscular dystrophy. J Vet Intern Med 1999;13:346–356. 12. Lamb CR, Tyler M, Boswood A, et al. Assesment of the value of the vertebral heart scale in the radiographic diagnosis of cardiac disease in dogs. Vet Rec 2000;146:687–690. 13. Buchanan JW, Patterson DF. Selective angiography and angiocardiography in dogs with congenital cardiovascular disease. J Am Vet Radiol Soc 1965;6:21–39.

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typically develops secondary to pressure overload, may develop at the expense of cardiac chamber volume, and the size of the heart may appear normal radiographically.13