## 1 Introduction  
Mitral valve regurgitation is the most common heart disease in dogs. It is suffered primarily by dogs of small and medium-sized breeds. The most common cause of mitral valve regurgitation is myxomatous mitral valve degeneration (MMVD) [1, 2]. Analysis of the population of dogs with MMVD indicates that the development of the disease has a genetic basis, and it is particularly conserved in the population of Cavalier King Charles Spaniels [3, 4, 5, 6]. The disease leads to non-inflammatory degenerative changes in the mitral valve leaflet margins, resulting in thickening. This leads to the formation of pathological mitral regurgitation (MR), as well as the rupture of the tendinous cords [7]. The backflow of blood into the left atrium causes an increase in the pressure of blood pushing against its walls (volume overload), resulting in the secretion of atrial natriuretic peptide (ANP), which lowers blood pressure inside the atrium [8, 9, 10]. ANP also contributes to cardiac remodeling, i.e., atrial dilatation. This process, called eccentric hypertrophy, allows the size of the left atrium to adjust to the blood volume so that the left atrial pressure value returns to normal values. The remodeling process involves the left ventricle in parallel, followed by the right ventricle and the right atrium. Typical of the course of congestive heart failure (CHF) due to MMVD are multiple recurrences of pulmonary edema of varying degrees of severity [11], and pulmonary hypertension (PH) [12; 13]. Right heart enlargement develops in more severe and/or chronic cases of PH [11]. Hypertension is also often accompanied by a chronic cough [14; 15]. Other symptoms commonly associated with PH are syncope and restlessness. Exercise intolerance, weakness, weight loss, and depression are considered non-specific but associated with PH [1; 12; 13; 16; 17; 18; 19]. In symptomatic heart failure (stage C according to the American College of Veterinary Internal Medicine, ACVIM), patients with a high probability of PH have a significantly worse prognosis, with a survival time about 2 months shorter compared to patients with a lower probability of PH [16; 17; 20]. Even after surgical intervention, there are cases of PH recurrence [21]. PH in dogs is found when pulmonary artery blood pressure rises above pulmonary artery pressure (PAP) \(\geq\) 25 mm Hg during echographic examination [22; 23]. Estimation of PH is based on the assessment of systolic PAP. It is calculated by quantifying the peak velocity of tricuspid regurgitation (TRV) and then deriving the pressure gradient (PG) between the RV and right atrium (simplified Bernoulli equations (PG = 4 \(\times\) velocity [m/s]\({}^{2}\)) [23]. It is also important to assess the pulmonary artery wedge pressure (PAWP) and the size of the left atrium (LA) [23]. The range of PAP's value in dogs is higher than that in humans, in whom PH, according to the current World Symposium on Pulmonary Hypertension guidelines, is found at values exceeding 20 mm Hg [24; 25]. In general, increased PAP may be due to six reasons (pulmonary arterial hypertension (PAH), venous pulmonary hypertension (PVH), pulmonary hypertension secondary to respiratory disease/ischemia, pulmonary embolism, due to cardiovascular parasites, and idiopathic pulmonary hypertension) [23]. However, for this review, the first two are the most important: pulmonary arterial hypertension (classified in the group of precapillary PH conditions), which is caused by primary vascular lesions in the pulmonary arteries [26; 27; 28; 29], and pulmonary venous hypertension, which is associated with left ventricular heart failure (classified in the group of postcapillary PH conditions) [23]. The first disease is much rarer and the former is a primary, chronic, and progressive disorder leading to endothelial and smooth muscle proliferation and dysfunction, and is one of the most serious vasculopathy, in addition, the exact causes of the disease's development are still unknown [30; 31].