Echocardiography has become the gold standard for the diagnosis of cardiac diseases. It has changed the way in which the anatomy of the heart is clinically evaluated, allowing to non-invasively assessing cardiac function. In cats, the small size of the heart sometimes makes it difficult to establish a cut-off threshold between a sick or a normal heart, since a few millimeters can make the difference in diagnosing an abnormal condition. Until today, no study had been done to determine normal values for M-mode echocardiography in Brazilian Shorthair Cats (BSC). Therefore, this study was conceived to determine the normal reference range of M-mode parameters in healthy non-anesthetized BSC.

For such, 32 cats (mean weight 4.33 kg) were divided into two groups: G1 had 16 female BSC, and G2 had 16 male BSC. Cats were placed on right lateral recumbency and the exam was performed at the right parasternal location, where the following parameters were measured using the long-axis four-chamber view: diameter of aortic root (AO), and diameter of left atrium (LA), allowing the calculation of the relation between LA and AO; and using the short-axis view: internal diameter of right ventricle (IDRV), internal diameter of left ventricle in systole (IDLVs) and diastole (IDLVd), interventricular septal thickness in systole (IVSs) and diastole (IVSd), left ventricle free wall in systole (LVFWs) and diastole (LVFWd), E-point-to-septal-separation (EPSS), shortening fraction (SF), and ejection fraction (EF). The data was compared between groups using Student's test at a significance level of 5%. To determine the 95% confidence interval (CI), it was used the average of both groups when statistical differences were not present. However, when significant differences were observed, an individual 95% CI for males and females was calculated.

Statistical analysis of data showed differences between G1 and G2 only for IDRV (P=0.0009) and IVSs (P=0.0406). The following means and 95% CI resulted for the studied parameters (showed as mean±standard deviation; lower and upper 95% CI): AO (0.82±0.10; 0.78-0.85 cm), LA (1.07±0.13; 1.02-1.12 cm), LA/AO (1.32±0.19; 1.25-1.39), IDRV females (0.33±0.11; 0.28-0.39 cm), IDRV males (0.49±0.13; 0.42-0.55 cm), IDLVs (0.70±0.16; 0.65-0.75 cm), IDLVd (1.42±0.17; 1.36-1.47 cm), IVSs females (0.67±0.11; 0.61-0.72 cm), IVSs males (0.75±0.10; 0.70-0.79 cm), IVSd (0.44±0.07; 0.41-0.46 cm), LVFWs (0.69±0.12; 0.65-0.73 cm), LVFWd (0.45±0.09; 0.42-0.48 cm), EPSS (0.10±0.03; 0.08-0.11 cm), SF (50.9±8.1; 48.1-53.7 %), and EF (84.5±6.7; 82.2-86.9 %).

Results allowed standardizing M-mode parameters for BSC, which will facilitate an accurate evaluation and interpretation of echocardiograms in such cats.