ABSTRACT. Right ventricular (RV) Tei index (index of myocardial performance) has been demonstrated to be clinically useful in estimating RV function in various human cardiac diseases. The purposes of this study were to validate the correlation between RV Tei index and RV function obtained by cardiac catheterization in healthy dogs, and to evaluate the RV Tei index in dogs with tricuspid regurgitation (TR). In healthy dogs, the RV Tei index significantly correlated with the RV peak +dP/dt (r=-0.80, p<0.0001) and -dP/dt (r=0.69, p=0.0001). In normal dogs, the RV Tei index was not significantly correlated with heart rate, body weight, and age. The RV Tei index significantly increased in dogs with moderate to severe TR (0.39 ± 0.35, p=0.0015), filariasis (0.46 ± 0.16, p=0.0131), and trivial to mild TR and severe mitral regurgitation (MR; 0.61 ± 0.14, p=0.0017) when compared with the normal dogs (0.17 ± 0.10). In addition, the RV Tei index in dogs with TR significantly increased in association with pulmonary hypertension [PH(-), 0.19 ± 0.09; PH(+), 0.65 ± 0.14; respectively p<0.0001]. Our study has demonstrated that RV Tei index is a feasible approach to estimate RV function in dogs and is not influenced by heart rate, body weight, and aging. Right ventricular (RV) function plays an important role in determining cardiac output and functional capacity in humans [1,28]. RV dysfunction has been observed in various cardiac disorders such as pulmonary hypertension (PH) [17,37], idiopathic dilated cardiomyopathy (DCM) [22], and heart failure [7,8,27].RV function can be evaluated by cardiac catheterization with RV peak +/-dP/dt, Tau and RV ejection fraction. However, this method is invasive, and it is difficult to evaluate the RV function due to the complex structural geometry, mechanical properties, and contraction pattern of the right ventricle [12,16].A decade ago, the Tei index (index of myocardial performance) derived from pulsed Doppler echocardiography was proposed in human medicine [36,39]. The Tei index is defined as the sum of isovolumetric contraction time (ICT) and isovolumetric relaxation time (IRT) divided by ejection time (ET), and it has been reported to reflect comprehensive cardiac function, including systolic and diastolic performances, in humans [36,39]. The Tei index is a simple and reproducible technique, and is relatively independent of heart rate [4,39] and age [19]. In addition, the Tei index has been demonstrated to be clinically useful in estimating the prognosis in human patients with primary PH [37,44], DCM [9], cardiac amyloidosis [38], and myocardial infarction [32]. It could be used for evaluating the RV function as well as the left ventricular function that is unrelated to the complex RV geometric structure [10, 11, 19, 24, 33-35, 37, 44].In small animal medicine, RV dysfunction is suspected in dogs with various cardiac diseases such as left-sided heart failure, pulmonary stenosis, tricuspid dysplasia, dirofilariasis, idiopathic cardiomyopathy, cor pulmonale, and idiopathic or secondary PH...