Abstract
Objective
To determine normal reference ranges for end-inspiratory and end-expiratory planimetric cardiothoracic ratios in apparently healthy domestic shorthair cats using plain digital thoracic radiographs.
Methods
The planimetric cardiothoracic ratio, calculated by comparing the areas of the cardiac and thoracic cavity silhouettes, was used to assess the cardiac size.
Results
Planimetric cardiothoracic ratios varied significantly across radiographic views and respiratory phases. In the right lateral view, the mean end-inspiratory ratio was 22.17% (range, 17.42% to 27.02%), increasing to 25.51% (range, 20.47% to 32.6%) at end expiration. The left lateral view showed a similar pattern, with mean values of 21.15% (range, 18.07% to 25.5%) at end inspiration and 24.28% (range, 20.34% to 29.19%) at end expiration. Dorsoventral and ventrodorsal views exhibited higher ratios, with mean end-inspiratory values of 28.31% (range, 24.43% to 38.85%) and 27.96% (range, 22.96% to 33.57%), respectively, increasing to 32.70% (range, 27.91% to 42.92%) and 31.56% (range, 24.71% to 45.24%) at end expiration.
Clinical Relevance
This study provides reference values for cardiac size based on the planimetric cardiothoracic ratio. Given the distinct contrast between the cardiac and thoracic silhouettes and the ease of calculation, this ratio may serve as a useful tool for assessing cardiac size in cats.
Conclusions
Potential influence of general anesthesia and the specific phase of the cardiac cycle on the cardiac silhouette measurements, as well as the possibility of subtle misalignments or rotational errors during image acquisition, could compromise the accuracy of cardiothoracic measurements. The reliability of the planimetric cardiothoracic ratio in reflecting cardiac size changes in feline heart disease necessitates additional study.
Plain radiography serves as an essential, noninvasive imaging modality for global assessment of the cardiac size and shape in the diagnosis of feline cardiac disease.1 The normal feline cardiac silhouette resembles an ovoid on the lateral and dorsoventral views.2 It occupies two-thirds of the height of the thorax and is typically less than or up to 2 and a half intercostal spaces wide on the lateral view and is relatively wider, occupying up to two-thirds the width of the thorax, with the apex close to the midline, on the ventrodorsal and dorsoventral views.2
Several radiographic studies3–5 have resorted to various objective methods to evaluate the cardiac size in cats. Absolute cardiac and thoracic measurements and their correlations are considered reliable indicators of cardiac size in normal cats and those with cardiac disease.3 The vertebral heart size method allows objective assessment of relative heart size, meeting the desired criteria of consistency, precision, and simplicity.4 In a similar vein, measuring the size of the left atrium using a modified vertebral heart size on the right lateral thoracic radiograph is regarded as a valid, albeit less sensitive, indicator of left atrial enlargement in cats.5
Traditional cardiac radiographic parameters, such as vertebral heart size and the unidimensional cardiothoracic ratio, are constrained by their linear nature. These measurements do not fully account for the entire cardiac silhouette and may fail to detect changes in heart size that occur outside the long and/or short axes. Therefore, a method for evaluating cardiac size that considers the entire cardiac circumference may provide significant clinical advantages, notably in terms of objectivity. The aim of this study was to establish and compare the normal reference ranges for the end-inspiratory and end-expiratory planimetric cardiothoracic ratios on 4 standard thoracic radiographic projections (right lateral, left lateral, ventrodorsal, and dorsoventral) in a cohort of clinically healthy domestic shorthair cats. To the best of the author’s knowledge, no such data has been previously reported in the literature.
Methods
Animal recruitment
This prospective, observational, descriptive study was approved by the Animal Welfare and Ethical Committee of the Department of Animal Husbandry, Government of Jammu and Kashmir, Srinagar, India.
The study initially enrolled 64 apparently healthy client-owned domestic shorthair cats presenting for elective sterilization over a 9-month period. The following inclusion criteria were applied to the initial 64 cats, resulting in the enrollment of 50 cats: (1) the cats were assigned a score ranging from 4 to 6 based on the evaluation of a 9-point body condition scoring system (https://wsava.org/wp-content/uploads/2020/08/Body-Condition-Score-cat-updated-August-2020.pdf), (2) the cats exhibited no prior history or clinical signs of cardiopulmonary disorders as thoracic auscultation did not reveal any abnormal sounds associated with the heart or lungs, (3) the subjective assessment of the cardiac size as well as the linear cardiothoracic ratios and vertebral heart sizes measured on lateral, ventrodorsal, and dorsoventral radiographs were within normal ranges,3,4 and (4) the 2-D grayscale echocardiography did not show any indications of subclinical cardiomyopathy.6
Animal restraint
To ensure optimal safety and minimize radiation exposure, feline subjects were chemically restrained using a combination of xylazine and ketamine. The xylazine (Xylaxin; Indian Immunologicals; 20 mg/mL) and ketamine (Aneket; Neon Laboratories Ltd; 50 mg/mL) combination was mixed and administered IM at 0.8 and 15 mg/kg to achieve approximately 45 to 60 minutes of general anesthesia.7 Overnight fasting was implemented to mitigate the risk of pulmonary aspiration and maintain a consistent diaphragmatic silhouette.
Radiographic imaging procedure
All feline subjects underwent tabletop radiography with the aid of a computed x-ray device manufactured by Toshiba Electron Tubes & Devices Co Ltd. Image acquisition was conducted at 60 to 65 kVp/2 to 4 mAs without a grid. All radiographs were acquired at a fixed focus-to-film distance of 80 cm. The object-to-film distance was kept constant for each cat. A foam positioning sponge was used to gently raise the sternum and realign it with the spine to rectify any inadvertent misalignments or rotations and mitigate the distortion artifacts caused by the sagging of the naturally undulating spine and sternum.8 The radiographer initiated the exposure by pressing the handheld push button while closely observing the cat’s abdominal breathing pattern through the lead glass window. For each cat, 8 thoracic radiographs were acquired manually. These included both end-inspiratory and end-expiratory right and left lateral, ventrodorsal, and dorsoventral projections, without factoring in cardiac cycle. The selection of radiographs was contingent upon their precise positioning, high-quality detail, appropriate density, and optimal contrast, prompting retakes of the images that did not meet these specifications.
The diaphragmatic excursion was maximal on end-inspiratory radiographs and minimal on end-expiratory radiographs. End-inspiratory lateral radiographs depicted an increased craniocaudal distance between the cardiac apex and the diaphragmatic cupula, centered at 8th thoracic vertebra, and a lumbodiaphragmatic angle positioned cranially to the 13th thoracic vertebra.9 Conversely, the end-expiratory lateral radiographs showed direct apposition of the cardiac apex with the diaphragmatic cupula, centered at the 7th thoracic vertebra, and a lumbodiaphragmatic angle positioned cranially to the 12th thoracic vertebra.9 Similarly, end-inspiratory dorsoventral and ventrodorsal radiographs were characterized by an increased craniocaudal distance between the cardiac apex and diaphragmatic cupula, centered at the 11th thoracic vertebra, and the costodiaphragmatic/costophrenic angles positioned between the 11th and 10th intercostal spaces.9 Conversely, end-expiratory dorsoventral and ventrodorsal radiographs revealed the cardiac apex juxtaposed to the diaphragmatic cupula, centered at 10th thoracic vertebra, and the costodiaphragmatic angles situated between the 9th and 10th intercostal spaces.9
Upon acquiring the radiographic images, the digital radiographs were archived in the DICOM image format. The archived DICOM images were subsequently analyzed and processed through MicroDicom DICOM viewer, version 2023.3 (MicroDicom Ltd), an open-access DICOM image-viewing software compatible with Windows (Microsoft Corp).
Radiographic mensuration
The radiographic projection area measurements of the cardiac and thoracic cavity silhouettes were expressed in square centimeters. The raw data were transferred to a spreadsheet (Sheets; Google LLC) for analysis. To improve the intraobserver reliability, each radiographic parameter (cardiac and thoracic cavity silhouette areas) was measured in triplicate in tandem for each cat, and the mean value of these triplicate measurements was used as the final value.
Radiographic magnification
To account for the radiographic magnification, cardiothoracic silhouette areas were corrected by dividing by the square of the magnification factor. The magnification factor was derived by dividing the focus-to-film distance (remaining constant at 80 cm in all images) by the focus-to-target object distance.9 The focus-to-target object distance was calculated by subtracting the target object–to-film distance from the focus-to-film distance.9 The target object–to-film distance was measured on the radiographic images between the radiation side of the physical center of mass of the target object—the point of intersection of the long and short axes of the heart and the centroid of the approximately triangular thoracic cavity silhouette—and the center of the radiographic film along the central axis of radiation.9
Calculation of the planimetric cardiothoracic ratio
The cardiac silhouette area (A) was measured in a closed loop on right lateral, left lateral, ventrodorsal, and dorsoventral radiographic views (Figures 1 and 2).10 The area measurement tool of the DICOM image-viewing software was used to trace the contour of the cardiac silhouette along the cranial cardiac border, cranial cardiac waist (craniodorsal region composed of 3 overlapping structures—the right atrium, pulmonary artery, and aorta), cardiac apex, caudal cardiac border, caudal cardiac waist (caudodorsal region composed only of the left atrium), cardiac base, and, again, the cranial cardiac border. On lateral radiographic projections, the cardiac silhouette was delineated, commencing at the ventral edge of the tracheal bifurcation and continuing along the left side of the heart to the cranial zone of the descending aorta that contacts the ventral part of the trachea and ending with the junction of this last point with the initial point.11 Likewise, on the dorsoventral and ventrodorsal projections, 9 points were placed as follows: the first point was marked on the left cranial edge at 1 o’clock, followed by 3 points at approximately the same distance (at 3, 4, and 5 o’clock); the fifth point was placed on the cardiac apex (6 o’clock); and the next 4 points, which delineated the right cardiac silhouette, were made parallel to the left points (7, 8, 9, and 11 o’clock) before returning to the starting point.11 The overlapping right-sided cardiac structures on the lateral views and the mediastinal structures on the dorsoventral and ventrodorsal views distorted the accurate delineation of the cardiac silhouette. These difficulties were overcome by careful region-of-interest selection and utilizing the DICOM viewer’s automated curve construction tool to minimize the distortion artifacts.11 Similarly, the thoracic cavity silhouette area (B) was measured on the lateral radiographs by tracing the contour of the thoracic cavity in a closed path along the ventral border of the thoracic vertebrae, lumbodiaphragmatic angle, crura of the diaphragm, sternodiaphragmatic angle, dorsal border of the sternum, thoracic inlet, and back to the starting point at the ventral border of the thoracic vertebrae.10 Furthermore, on the dorsoventral and ventrodorsal radiographs, the contour of the thoracic cavity was traced in a closed loop beginning from the thoracic inlet, lateral thoracic wall, costodiaphragmatic angle, diaphragmatic dome, contralateral costodiaphragmatic angle, contralateral thoracic wall, and back to the starting point at the thoracic inlet.10 The planimetric cardiothoracic ratio (R) was calculated as a percentage using the following equation: R (%) = {(A/B)×100}.
An overlay of a feline thoracic radiograph depicting cardiac and thoracic cavity silhouettes on an end-expiratory left lateral recumbent projection. A—Cardiac silhouette area. B—Thoracic cavity silhouette area. Numbers 1 through 6 are the boundaries of the thoracic cavity silhouette: 1, thoracic inlet; 2, sternal border; 3, sternodiaphragmatic angle; 4, diaphragmatic silhouette; 5, lumbophrenic angle; and 6, thoracic spinal border. Numbers 7 through 11 are the boundaries of the cardiac silhouette: 7, aortic arch—the proximal curved portion of the thoracic aorta as it leaves the base of the heart, passing craniodorsally to the pulmonary artery, is obscured by the superimposition of trachea; 8, left atrium; 9, left ventricle; 10, right ventricle; and 11, right auricle.
Citation: American Journal of Veterinary Research 86, 6; 10.2460/ajvr.24.11.0351
An overlay of a feline thoracic radiograph depicting cardiac and thoracic cavity silhouettes on an end-inspiratory dorsoventral recumbent projection. A—Cardiac silhouette area. B—Thoracic cavity silhouette area. Numbers 1 through 6 are the boundaries of the thoracic cavity silhouette: 1, thoracic inlet; 2, right thoracic wall; 3, right costophrenic angle; 4, diaphragmatic silhouette; 5, left costophrenic angle; and 6, left thoracic wall. Numbers 7 through 11 are the boundaries of the cardiac silhouette: 7, left auricle; 8, left ventricle; 9, right ventricle; 10, right atrium; and 11, cardiac base, where the aortic arch, main pulmonary artery, and cranial vena cava attach, is radiographically indistinct within the cranial mediastinum’s soft tissue opacity and obscured by spinal superimposition. L = Left. R = Right.
Citation: American Journal of Veterinary Research 86, 6; 10.2460/ajvr.24.11.0351
Statistical analysis
Statistical analyses were performed using freely available online tools (Statistics Kingdom [https://www.statskingdom.com/index.html] and Online Web Statistical Calculators for Categorical Data Analysis [https://astatsa.com]). Descriptive statistics, including mean, SD, 95% CIs, and minimum and maximum values were calculated. The Shapiro-Wilk test was used to evaluate the normality of the data distribution. To account for the simultaneous comparison of various combinations of radiographic views and respiratory phases, a balanced 1-way, repeated-measures ANOVA was utilized, ensuring an equal number of observations for each combination in the study design. Furthermore, to pinpoint specific differences in planimetric cardiothoracic ratios across multiple combinations of radiographic views and respiratory phases, the Tukey honestly significant difference test, a post hoc multiple-comparisons analysis, was applied. A significance level (α) of 0.05 was set, with P values less than .05 considered statistically significant.
Results
Among the select 50 domestic shorthair cats that underwent radiographic imaging, 24 were male (48%), and 26 were female (52%). The average age of the participants was 2.40 ± 0.61 years, with a range of 1.25 to 4.5 years. The average weight of the cats was 3.68 ± 0.72 kg, ranging from 3.25 to 5.92 kg.
The Shapiro-Wilk test indicated that all radiographic measurements were normally distributed. Descriptive statistics for cardiothoracic parameters are provided in Table 1. The P value corresponding to the F statistic of the 1-way analysis of variance was lower than .05, which suggested that 1 or more pairs of a combination of radiographic view and respiratory phase were significantly different. Table 2 presents the results of post hoc comparisons of planimetric cardiothoracic ratios.
The mean ± SD, mean 95% CI, and minimum to maximum of the planimetric cardiothoracic measurements obtained from 50 clinically normal domestic shorthair cats.
| Radiographic view | End-inspiratory values | End-expiratory values | ||||
|---|---|---|---|---|---|---|
| A (cm2) | B (cm2) | R (%) | A (cm2) | B (cm2) | R (%) | |
| Right lateral | ||||||
| Mean ± SD | 15.65 ± 0.98 | 71.12 ± 6.76 | 22.17 ± 2.17 | 15.88 ± 0.97 | 62.56 ± 4.96 | 25.51 ± 2.56 |
| Mean 95% CI | 15.38–15.93 | 69.20–73.04 | 21.55–22.79 | 15.61–16.16 | 61.15–63.97 | 24.78–26.24 |
| Minimum−maximum | 12.81–18.19 | 56.32–85.63 | 17.42–27.02 | 12.85–18.53 | 49.24–73.19 | 20.47–32.6 |
| Left lateral | ||||||
| Mean ± SD | 15.31 ± 1.29 | 72.53 ± 7.01 | 21.15 ± 1.73 | 15.55 ± 1.26 | 64.34 ± 5.87 | 24.28 ± 2.13 |
| Mean 95% CI | 14.94–15.68 | 70.54–74.53 | 20.66–21.64 | 15.19–15.91 | 62.65–66.02 | 23.68–24.88 |
| Minimum−maximum | 12.4–19.29 | 58.09–94.31 | 18.07–25.5 | 12.59–19.38 | 50.21–83.55 | 20.34–29.19 |
| Dorsoventral | ||||||
| Mean ± SD | 16.64 ± 1.79 | 59.01 ± 6.32 | 28.31 ± 2.74 | 16.89 ± 1.74 | 51.81 ± 4.87 | 32.7 ± 2.97 |
| Mean 95% CI | 16.13–17.15 | 57.22–60.81 | 27.53–29.10 | 16.4–17.4 | 50.43–53.20 | 31.86–33.54 |
| Minimum−maximum | 12.82–20.84 | 46.76–69.84 | 24.43–38.85 | 13.14–20.4 | 41.98–59.33 | 27.91–42.92 |
| Ventrodorsal | ||||||
| Mean ± SD | 15.61 ± 1.48 | 55.93 ± 3.82 | 27.96 ± 2.72 | 15.75 ± 1.56 | 50.22 ± 4.38 | 31.56 ± 3.96 |
| Mean 95% CI | 15.18–16.02 | 54.84–57.02 | 27.19–28.73 | 15.31–16.20 | 48.97–51.47 | 30.43–32.68 |
| Minimum−maximum | 12.39–19.17 | 48.37–65.87 | 22.96–33.57 | 12.41–20.06 | 41.48–60.91 | 24.71–45.24 |
A = Cardiac silhouette area. B = Thoracic cavity silhouette area. R = Planimetric cardiothoracic ratio.
Results of the post hoc analysis of the planimetric cardiothoracic ratios obtained from various radiographic views during end-inspiratory and end-expiratory phases to evaluate whether the observed Tukey-Kramer honestly significant difference (HSD) Q statistic (Qi,j) was greater than the critical value of Qi,j for all relevant 28 pairs considered for simultaneous comparison.
| Pair No. | Pairs (combinations of radiographic views and respiratory phases) being simultaneously compared | Observed Qi,j | Tukey HSD P value | Tukey HSD inference |
|---|---|---|---|---|
| 1 | End-inspiratory right lateral vs end-expiratory right lateral | 8.76 | .001 | Significant (P < .05) |
| 2 | End-inspiratory right lateral vs end-inspiratory left lateral | 2.67 | .55 | Insignificant (P > .05) |
| 3 | End-inspiratory right lateral vs end-expiratory left lateral | 5.54 | .003 | Significant (P < .05) |
| 4 | End-inspiratory right lateral vs end-inspiratory dorsoventral | 16.10 | .001 | Significant (P < .05) |
| 5 | End-inspiratory right lateral vs end-expiratory dorsoventral | 27.62 | .001 | Significant (P < .05) |
| 6 | End-inspiratory right lateral vs end-inspiratory ventrodorsal | 15.19 | .001 | Significant (P < .05) |
| 7 | End-inspiratory right lateral vs end-expiratory ventrodorsal | 24.52 | .001 | Significant (P < .05) |
| 8 | End-expiratory right lateral vs end-inspiratory left lateral | 11.44 | .001 | Significant (P < .05) |
| 9 | End-expiratory right lateral vs end-expiratory left lateral | 3.23 | .31 | Insignificant (P > .05) |
| 10 | End-expiratory right lateral vs end-inspiratory dorsoventral | 7.34 | .001 | Significant (P < .05) |
| 11 | End-expiratory right lateral vs end-expiratory dorsoventral | 18.86 | .001 | Significant (P < .05) |
| 12 | End-expiratory right lateral vs end-inspiratory ventrodorsal | 6.43 | .001 | Significant (P < .05) |
| 13 | End-expiratory right lateral vs end-expiratory ventrodorsal | 15.86 | .001 | Significant (P < .05) |
| 14 | End-inspiratory left lateral vs end-expiratory left lateral | 8.21 | .001 | Significant (P < .05) |
| 15 | End-inspiratory left lateral vs end-inspiratory dorsoventral | 18.78 | .001 | Significant (P < .05) |
| 16 | End-inspiratory left lateral vs end-expiratory dorsoventral | 30.29 | .001 | Significant (P < .05) |
| 17 | End-inspiratory left lateral vs end-inspiratory ventrodorsal | 17.87 | .001 | Significant (P < .05) |
| 18 | End-inspiratory left lateral vs end-expiratory ventrodorsal | 27.30 | .001 | Significant (P < .05) |
| 19 | End-expiratory left lateral vs end-inspiratory dorsoventral | 10.57 | .001 | Significant (P < .05) |
| 20 | End-expiratory left lateral vs end-expiratory dorsoventral | 22.08 | .001 | Significant (P < .05) |
| 21 | End-expiratory left lateral vs end-inspiratory ventrodorsal | 9.66 | .001 | Significant (P < .05) |
| 22 | End-expiratory left lateral vs end-expiratory ventrodorsal | 19.09 | .001 | Significant (P < .05) |
| 23 | End-inspiratory dorsoventral vs end-expiratory dorsoventral | 11.51 | .001 | Significant (P < .05) |
| 24 | End-inspiratory dorsoventral vs end-inspiratory ventrodorsal | 0.91 | .9 | Insignificant (P > .05) |
| 25 | End-inspiratory dorsoventral vs end-expiratory ventrodorsal | 8.52 | .001 | Significant (P < .05) |
| 26 | End-expiratory dorsoventral vs end-inspiratory ventrodorsal | 12.43 | .001 | Significant (P < .05) |
| 27 | End-expiratory dorsoventral vs end-expiratory ventrodorsal | 2.99 | .41 | Insignificant (P > .05) |
| 28 | End-inspiratory ventrodorsal vs end-expiratory ventrodorsal | 9.43 | .001 | Significant (P < .05) |
The Tukey HSD is often a follow-up test to 1-way ANOVA when the F test has revealed the existence of a significant difference between some of the tested groups. The Tukey HSD test is a post hoc test commonly used to assess the significance of differences between pairs of group means. The P value corresponding to the F statistic of 1-way ANOVA was lower than .05, strongly suggesting that 1 or more pairs of treatments (combinations of radiographic views and respiratory stages) were significantly different. The critical value of Qi,j {(Qα=05,k=8,ν=392)critical} was 5.03 in the studentized range distribution, where α denoted significance level, k represented treatment groups (combinations of radiographic views and respiratory phases), and ν indicated degrees of freedom for the error term.
Discussion
The radioanatomic topography, which includes the spatial relationships between the heart and thoracic cavity landmarks, as well as the relative sizes of the cardiac and thoracic cavity silhouettes, demonstrated a near–mirror-image relationship within a given respiratory phase for right and left lateral and dorsoventral and ventrodorsal radiographic projections, respectively. Consequently, no statistically significant differences were found in the following 4 out of 28 comparisons: end-inspiratory right lateral versus end-inspiratory left lateral, end-expiratory right lateral versus end-expiratory left lateral, end-inspiratory dorsoventral versus end-inspiratory ventrodorsal, and end-expiratory dorsoventral versus end-expiratory ventrodorsal radiographic projections. However, planimetric cardiothoracic ratios measured from dorsoventral and ventrodorsal projections were consistently higher than those from right and left lateral projections, regardless of respiratory phase. This discrepancy might have stemmed from the natural variation in cardiac silhouette size across different radiographic projections as dorsoventral and ventrodorsal views typically produce larger cardiac silhouettes compared to lateral views. On the contrary, the significant differences identified in 24 out of 28 planimetric cardiothoracic ratios across various radiographic views and respiratory phases were likely due to the expansion and contraction of the thoracic cavity silhouette area during inspiration and expiration. These variations were accentuated by the inherent differences in the silhouette areas of the heart and thoracic cavity between lateral and dorsoventral/ventrodorsal projections.
Radiographic evaluation of the cardiac disease primarily focuses on assessing the size and shape of the cardiac silhouette.10 On lateral radiographic projections, the cardiac long axis reflects the combined size of the left atrium and left ventricle, whereas the cardiac short axis includes portions of both the left and right atria, most likely at the level of the atrioventricular sulcus and atrioventricular valves.11 On the dorsoventral and ventrodorsal radiographic projections, the cardiac long axis reflects the combined size of the right atrium and left ventricle, whereas the cardiac short axis includes the right ventricle and left atrium.11 The estimation of the size of a cardiac silhouette based on the measurements of cardiac long and short axes may be less accurate as such measurements do not include the area of all cardiac structures.11 When considering lateral radiographic views, the long axis of the heart indicates the combined size of the left atrium and left ventricle. In contrast, the short axis includes parts of both the left and right atria, often at the level of the atrioventricular sulcus and valves. However, in dorsoventral and ventrodorsal radiographic views, the long axis demonstrates the combined dimensions of the right atrium and left ventricle, and the short axis includes the right ventricle and left atrium. Therefore, using long and short axis measurements to estimate cardiac silhouette size may be less precise as these measurements do not account for the entirety of all cardiac structures. As a result, the diagnostic accuracy of the vertebral heart score and other related linear radiographic indices in distinguishing between cats with cardiac disorders and healthy cats and in predicting left atrial enlargement in cats with left-sided cardiac disorders is only moderate.3,12 Moreover, linear cardiothoracic ratios have low sensitivity and specificity and are generally unreliable in cats unless cardiac remodeling is advanced.13 The feline cardiac diseases are associated with an increase in the overall cardiac silhouette, reflecting equivalent increases in the measurements of its long and short axes.3,12 Anatomical cross-sections have shown a strong correlation between cardiac and thoracic circumferences.14 In addition, planimetric cardiothoracic ratio has demonstrated a stronger association with cardiac function compared to unidimensional cardiothoracic ratio.15 Thus, comparing the silhouette areas of the heart and thoracic cavity on a thoracic radiograph may offer a more objective measurement of cardiac size than traditional methods, such as the vertebral heart score and unidimensional cardiothoracic ratio.
It is acknowledged that this study did not explicitly account for the potential confounding effects of general anesthesia and the specific phase of the cardiac cycle on cardiac silhouette measurements. Zwicker et al16 reported mild drug-induced cardiomegaly in healthy adult cats following IM administration of dexmedetomidine at a dosage of 40 μg/kg as evidenced by a small but significant increase in both the vertebral heart score on the right and left lateral views and the linear cardiothoracic ratio on the ventrodorsal and dorsoventral views, exceeding established reference intervals. On the contrary, in feline subjects sedated with a 5-mg/kg IM dose of alfaxalone, no statistically significant differences were identified between presedation and postsedation vertebral heart scores on right lateral, left lateral, and ventrodorsal radiographic projections or echocardiographic measurements of long-axis and short-axis left atrial diameter.17 Similarly, xylazine (1 mg/kg, IM) and tiletamine and zolazepam combination (5 mg/kg, IV) anesthesia in cats did not have any significant impact on the echocardiographically measured diameters of the left atrium and left ventricle.18 Regarding the changes in heart size during the cardiac cycle, the phase of the cycle (systole or diastole) affects the radiographic appearance of the cardiac silhouette, and these effects vary depending on the imaging position.19 However, in no radiographic view are the anatomical differences between systole and diastole substantial enough to be mistaken for signs of cardiac disease.19 Moreover, the reciprocal emptying and filling of the atria and ventricles in felines and canines results in minor fluctuations in the shape and size of the heart throughout the cardiac cycle, having a negligible effect on the estimation of the heart volume.20
As this study focused on clinically healthy cats with near-ideal body condition scores, the cardiothoracic ratios reported may not be directly generalizable to cats with extreme body conditions, such as obesity or emaciation. Moreover, despite careful attention to radiographic positioning, minor misalignments or rotational errors could still affect the precision of cardiothoracic measurements. Additionally, further research is needed to evaluate interobserver variability in planimetric cardiothoracic ratios. Lastly, the ability of planimetric cardiothoracic ratios to accurately detect changes in cardiac size in cats with heart disease has yet to be established.
This study establishes reference values for assessing the cardiac size in clinically healthy cats using the planimetric cardiothoracic ratio. The distinct contrast between the radiopaque cardiac silhouette and the radiolucent thoracic background, along with the relative simplicity of its calculation, suggests that the planimetric cardiothoracic ratio may serve as a valuable tool for a comprehensive evaluation of the feline cardiac size.
Acknowledgments
The author extends sincere gratitude to the anonymous peer reviewers for their insightful critiques and invaluable contributions to this paper. The time and effort dedicated by the reviewers to providing such thorough feedback is deeply appreciated.
Disclosures
The author has nothing to disclose. No AI-assisted technologies were used in the composition of this manuscript.
Funding
The author has nothing to disclose.
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