M-mode echocardiography measurements of healthy Brazilian random bred cats

Bruno Ricardo Soares Alberigi da Silva1* , Alexandre José Rodrigues Bendas2 , Denise do Vale Soares3 , Carolina Aben Athar1 , Fabiana Batalha Knackfuss4 , Renan Salhab Demo5 , Mateus Fernando Senra Almeida5  & Jonimar Pereira Paiva6†  1Veterinary, doctorate, MSc. Programa de Pós-graduação em Medicina Veterinária – PPGMV, Universidade Federal Rural do Rio de Janeiro – UFRRJ, Seropédica, RJ, Brasil 2Veterinary, doctorate, MSc. Programa de Pós-graduação em Medicina Veterinária (Clínica e Reprodução Animal), Universidade Federal Fluminense – UFF, Niterói, RJ, Brasil 3Veterinary. Instituto de Especialidades em Medicina Veterinária – IEMEV, Rio de Janeiro, RJ, Brasil 4Zootechnist, MSc, DSc. Universidade do Grande Rio – UNIGRANRIO, Duque de Caxias, RJ, Brasil 5Undergraduate students of Veterinary Medicine. Universidade Federal Rural do Rio de Janeiro – UFRRJ, Seropédica, RJ, Brasil 6Veterinary, Dr. Departamento de Medicina Veterinária e Cirurgia, Universidade Federal Rural do Rio de Janeiro – UFRRJ, Seropédica, RJ, Brasil †Deceased


Introduction
Echocardiography is a noninvasive exam that allows a safe morphofunctional evaluation of the heart (Boon, 2011). The M-mode is especially helpful for evaluating echogenic tissues, allowing objective measurements of the cardiac chambers, the wall thickness, and valvar movements, as well as functional index calculations, such as the shortening fraction. The Doppler analysis evaluates the velocity and pressure gradient of the blood flow inside the cardiac chambers and large vessels, as well as diastolic function (Chetboul et al., 2012;Bright, 2006). Therefore, M-mode echocardiography is the exam to detect the silent life-threatening myocardiopathies that represent the most frequent feline cardiopathy (Côté et al., 2011;Schwartz, 2003), (Tilley et al., 1977;Fox, 1999;Gundler et al., 2008).
M-mode echocardiography measurements for domestic cats have been standardized for the large breed Maine Coon (mean weight of 7.7 kg) (Gundler et al., 2008). For this breed, left ventricular wall in diastole (LVWd) and ventricular septum in diastole (VSd) values above 6 mm are diagnostic of cardiac hypertrophy (Moise et al., 1986;Schwartz, 2003;Fox et al., 2009;Ferasin, 2009;Côté et al., 2011). For smaller cats, such as the American domestic short hair (mean weight 4.76) (include reference), the normal measurements are an LVWd of 4.6 mm and a VSd of 4 mm (Moise et al., 1986) or an LVWd of 5.5 mm and a VSd of 5 mm (Ware, 2011).
Therefore, considering the varying measurements according to the size of the animal and that Brazilian random bred cats are small and have no weight or echocardiography standards, the aim of this study was to determine M-mode echocardiography standard measurements and correlate them with the BSC in order to allow early detection of silent, life-threatening myocardiopathies.

Material and methods
This study was approved by the animal use committee CEUA-FAPUR protocol number 01-05-13. After the owners' consent, 135 cats independent of sex or age, with no history of cardiopathy, presented for routine care at the Universidade Federal Rural do Rio de Janeiro or at the Instituto de Especialidades em Medicina Veterinária, a private hospital in Rio de Janeiro, were subjected to a physical exam and echocardiography. Cats presenting with clinical signs of disease or measurements of left ventricular wall and ventricular septum, in diastole, over 5 mm (Ware, 2011;Häggström et al., 2016) were not included.
A total of 125 cats were included, weighed and classified according to their BSC from 1 to 5 (Edney & Smith, 1986) .
The history data capture and physical exam were performed by one operator, and the echocardiography was performed by a separate operator to maintain a blind study. The echocardiography equipment used was Esaote, model MyLab 50 Xvision cardiovascular  , using two transducers (1-4 MHZ and 4-11 MHZ). The echocardiographic examination was performed without anesthesia or a tranquilizer. The cats were manually restrained by their owners and positioned on the left lateral recumbence. The echocardiogram exam was performed as proposed by Madron (2016) using the following views: left parasternal long axis, left cranial paraesternal, right ventricular outflow tract, left paraesternal apical chambers 4 and 5, and right paraesternal short axis left ventricular. The left atrium (LA), left atrium/aorta ratio (LA/AO), ventricular septum in diastole (VSd), ventricular septum in systole (VSs), left ventricular wall in diastole (LVWd), left ventricular wall in systole (LVWs), left ventricle in diastole (LVd), left ventricle in systole (LVs), shortening fraction (SF) and ejection fraction (EF) were recorded.
Based on the resampling technique by Bootstrap with 1000000 replicas, the confidence intervals (CIs) of the analyzed variables were calculated to generate reference values according to the results. Analysis of variance was performed to test the significance of each measure, and when significant, Tukey's test was used to determine the difference between the means. To test correlation, the Pearson correlation coefficient was used.

Results
Of the 125 cats included, 72 were female and 53 were male. The mean weight was 4.7+1.5 kg (CV 31%), the BSC mode was 3 for 69 cats, 46 cats presented with a BSC of 4, 8 cats presented with a BSC of 5, and 2 cats presented with a BSC of 2. The fact that 92% of the animals presented with a BSC of 3 or 4 indicates homogeneity of the body size of the cats.
The correlation of BSC with the echocardiographic measurements was positive when LVWs (p = 0.0013) or LVWd (p = 0.0009) was considered (Table 1). When the measurements of LVWd were compared among different BSC cats, LVWd was larger in BSC 4 cats than that in BSC 3 cats

Discussion
The demonstrated positive correlation of systolic and diastolic left ventricle wall (LVW) measurements with BSC shows that cats with a higher BSC must be examined with caution, as their measurement values will tend to be larger. This suggests that the LVW tends to develop an (Q = 5.05; p <0.01) and was larger in BSC 5 cats than that in BSC 3 cats (Q = 3.50; p<0.05). A difference in the LVWs measurements was observed only between BSC 3 and 5 cats (Q = 5.00; p = <0.01).
The systolic function of the myocardium evaluation measurements showed an LVd mean of 1.39 mm ± 0.17 and an LVs mean of 0.65 ± 0.13. The SF mean was 52.87 ± 7.46, and the ejection fraction mean was 85.74 ± 7.25%. These results show significant sample homogeneity with all the coefficients of variation below 20% (Table 1) The reference values of the M-mode echocardiography measurements are shown in Table 2.  adaptive concentric hypertrophy, probably because the heart needs to enhance its contraction force to maintain blood flow in a large body mass. Different from the results reported here, a previous study using allometry observed that echocardiographic measurements were influenced by body weight, mainly VSd (Karsten et al., 2017); the discrepancy between results can be due to the different methods used. Another study observed the influence of body weight on cardiac measurements, mainly in the left atrium/aorta ratio, but they did not use BSC (Häggström et al., 2016). The mean of the left atrium diameter measurements showed no difference compared with that found in other studies (Moise et al., 1986;Sisson et al., 1991). The LA/AO ratio is the most used measurement to evaluate the left atrium size, and the normal reference value for cats is 1:7 (Boon, 2011). In the present study, the mean LA/AO ratio was 1.24 ± 0.13, (c. v of 11%) with high sample homogeneity and a somewhat smaller reference value.
Measurements were compared with those registered before (Table 3), and the differences in LA/AO, VSD, LVWd, LVWs, LVd and LVs were assigned to the studied animals' size and breed (Tilley et al., 1977;Fox, 1999Fox, , 2014Boon, 2011;Ware, 2011;Chetboul et al., 2012).  Studies that included American domestic short hair cats registered cardiac measurements similar to those presented in this study (Table 3) (Fox et al., 1985;Moise et al., 1986;Sisson et al., 1991;Häggström et al., 2016), indicating that breed and therefore size range plays an important role in reference values. When analyzing different studies measurements it must be considered that many studies included sedated cats (Fox et al., 1985) or sampled only a small number if cats (Fox et al., 1985;Moise et al., 1986;Sisson et al., 1991). A large sampling was performed, but the authors did not list breeds or BSC they included (Häggström et al., 2016) . Reference values of VSd and LVWd were stablished using Maine Coon and Ragdoll, two large breeds, with no attention to the body mass that impacts on the size of the heart, therefore the reference was settled below 6 mm (Côté et al., 2011;Schwartz, 2003;Fox et al., 2009;Ferasin, 2009). Since the Brazilian random bred cats included in this study were smaller, the LVWd and VSd measurements were also smaller (Table 2).
In the present study, the mean EF was 85.74% ± 7.25 (c. v 8%). In most reports, the EF was not determined (Fox et al., 1985;Moise et al., 1986;Sisson et al., 1991), and there is no reference value established for adult cats (Moise et al., 1986;Sisson et al., 1991;Branquinho et al., 2010;Boon, 2011;Côté et al., 2011;Häggström et al., 2016); however, for growing kittens from two to 11 weeks of age, the reference EF measurement ranged from 61% to 87% (Schille & Skrodzki, 1999). According to Morcef (2001), a subjective analysis performed by an experienced echocardiographer is the best EF evaluation; however, an objective parameter can avoid biases and help less experienced echocardiographers to properly evaluate EF.

Conclusions
The M-mode echocardiography evaluation of cats is impacted by the animal's body mass; therefore, the BSC must be taken into consideration when interpreting echocardiography measurements. Since Brazilian random bred cats and breeds or local varieties in other countries are smaller than most breeds used for establishing reference measurements, further studies must be conducted to determine the reference measurements for different breeds.