Evaluation of cardiothoracic ratios in clinically healthy cats using planimetric analysis of standard radiographic projections.

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.