**1. Introduction**

Myxomatous mitral valve disease (MMVD) accounts for 75–80% of heart diseases in dogs and is more prevalent in small and elderly dogs [1, 2]. Due to similarities to human mitral valve prolapse (MVP), it gains huge interest in veterinary and human cardiologists. MMVD and MVP are characterized by progressive myxomatous degeneration of atrioventricular valves and subsequent mitral regurgitation (MR) [3], causing left atrial (LA) and left ventricular (LV) volume overload and left-sided congestive heart failure (CHF) [4]. Although there are many similarities in both diseases (e.g. macroscopic and microscopic pathology, strong genetic background, marked effect of age on prevalence and severity, slow progression), discrepancies in these two diseases (e.g. much more prevalent in dogs than in humans, less prone to develop endocarditis in dogs, less prominent systolic clicks in dogs) exist [5].

Although there are many diagnostic methods for MMVD, the standard transthoracic echocardiographic examination is a gold standard test for diagnosis and prognosis of MMVD and MVP in dogs and humans, respectively. The echocardiography is non-invasive and enables the clinicians to detect the mitral valve (MV) lesions, to evaluate MR severity, and to assess its impact on cardiac remodeling, myocardial function, left ventricular filling pressures, as well as pulmonary arterial pressure [6–11]. However, conventional ultrasound imaging modalities such as two-dimensional (2D), M-mode, color Doppler, pulse-wave (PW), and continuouswave (CW) Doppler echocardiography may not be enough to precisely evaluate the severity of mitral valve diseases and to monitor disease progression. Advanced ultrasound technologies such as tissue Doppler imaging (TDI), strain and strain rate imaging, and two-dimensional speckle tracking echocardiography (STE) gain popularity in veterinary and human cardiologists because these technologies can assess and monitor global and regional myocardial function more precisely [12], although those require more expensive ultrasound machine and training.

Human MVP causing MR is divided into two groups: primary (i.e. structural intrinsic valvular disease) and secondary (i.e. nonstructural functional MR caused by non-mitral valve diseases). Like dogs, causes of primary MR are degenerative diseases on the mitral valve, such as Barlow, fibroelastic degeneration, Marfan, Ehlers-Danlos, and annular calcification, although rheumatic disease and toxic valvulopathy can also cause MR in humans [13]. Severity of MR in humans is graded qualitatively (e.g. mitral valve morphology, color flow MR jet, flow convergence zone, CW signal of MR jets), semi-quantitatively (e.g. vena contracta [VC] width, pulmonary vein flow, mitral inflow, TVI mit/TVI Ao), and quantitatively (effective regurgitant orifice area [EROA], regurgitant volume [R Vol]) [13]. LV and LA size and the systolic pulmonary arterial pressure are also used to determine the severity of CHF caused by MR.

Although the severity MR and CHF in dogs with MMVD are being evaluated as similar to human cardiology, the veterinary cardiologists are more focused on the severity of cardiac remodeling (e.g. LA and LV dilation) and cardiac dysfunction caused by MR, because surgical restoration of defected mitral apparatus is rarely done in dogs. Therefore, the echocardiographic indices related to MR severity (e.g. MV morphology, flow convergence zone) are not routinely assessed in dogs with MMVD.

The chapter reviews conventional echocardiographic indices being used for diagnosis and prognosis of canine MMVD to provide a better understanding of the similarities and discrepancies between canine MMVD and human MVP to veterinary and human cardiologists and researchers.

### **2. Assessment of MR severity**

Echocardiography is a gold standard test in the assessment and management of humans and dogs with MR. Although color flow MR jets and CW signal of MR jets are useful for detecting MR, a more quantitative approach is required for determining the severity of MR in humans and dogs.

#### **2.1 Semi-quantification of MR**

Although quantitative assessment of MR (e.g. EROA and R Vol) is being used for grading the severity of MR in humans [13], it is rarely used in canine practice,

**33**

**Figure 1.**

*LAA (2.12 cm2*

*Echocardiographic Features in Canine Myxomatous Mitral Valve Disease: An Animal Model…*

because it requires more time-consuming interrogation and is often hard to define the location of EROA and flow convergence shape in dogs with MR [14]. Therefore, canine study, especially in practice, focuses more on the semi-quantification of MR (e.g. the maximal ratio of the regurgitant jet area signal to LA area [ARJ/LAA ratio]

The ARJ/LAA ratio can be easily obtained with the color Doppler imaging (CDI) method and showed good repeatability and reproducibility in dogs [15, 16]. MR can be graded as mild (<30%), moderate (30–70%), or severe (>70%), based on the ARJ/LAA ratio on the CDI study in dogs with MMVD (**Figure 1A**) [15, 16]. Unfortunately, the ARJ/LAA ratio can be affected by many intrinsic factors (e.g. SAP, LA pressure, the spatial orientation of MR jet) and extrinsic factors (e.g. pulse repetition frequency and gain settings) [17]. Several studies found the ARJ/LAA ratio was not closely correlated with the severity of MMVD in dogs, especially in dogs with American College of Veterinary Internal Medicine (ACVIM) B2 and C [18, 19]. The vena contracta is the narrowest width of the MR jet that occurs at or just downstream from the regurgitant orifice and can reflect the severity of MR (obtained from measuring the regurgitant orifice size by the CDI study) (**Figure 1B**) [17]. Although it requires less time-consuming interrogation and is technically easier to obtain, this method is prone to errors, especially in dogs having dynamic regurgitant orifices [14]. Several studies found the VC and VC-derived variables were closely correlated with the severity of MR and survival time in dogs with MMVD [20–22]. One study found the mean (±SD) diameter of the mitral regurgitation vena contracta in a right parasternal long-axis (RPLx) view (VCR:Ao) and a left apical four-chamber view (VCL:Ao) indexed to aortic diameter were 0.21 ± 0.14 and 0.24 ± 0.12, respectively, in dogs with MMVD [21]. Furthermore, the VCL:Ao > 0.24 was closely correlated with survival time (hazard ratio 4.87) [21].

The measurement of the flow convergence area by proximal isovelocity surface

*(A) Semi-quantification of mitral regurgitation (MR). The maximal ratio of the regurgitant jet area signal to the left atrial area (ARJ/LAA ratio) was obtained from a left apical four-chamber view. The ARJ (1.12 cm2*

*left apical four-chamber view (VCL) was measured as the diameter of the narrowest point of the mitral* 

*regurgitation jet, downstream of the region of proximal flow convergence.*

*) of this case was 53%, indicating moderate MR. (B) the vena contracta diameter from a* 

*)/*

area (PISA) is a gold standard method to quantify the MR jet in humans [13]. Although one study found the PISA method was repeatable and reproducible in awake dogs [16], it is more time-consuming and requires several precautions to obtain the optimal acquisition of the flow convergence images in dogs with MMVD [14]. The regurgitation fraction (RF) obtained by the PISA can be used for

*DOI: http://dx.doi.org/10.5772/intechopen.91819*

and vena contracta).

**2.2 Quantification of MR**

*Echocardiographic Features in Canine Myxomatous Mitral Valve Disease: An Animal Model… DOI: http://dx.doi.org/10.5772/intechopen.91819*

because it requires more time-consuming interrogation and is often hard to define the location of EROA and flow convergence shape in dogs with MR [14]. Therefore, canine study, especially in practice, focuses more on the semi-quantification of MR (e.g. the maximal ratio of the regurgitant jet area signal to LA area [ARJ/LAA ratio] and vena contracta).

The ARJ/LAA ratio can be easily obtained with the color Doppler imaging (CDI) method and showed good repeatability and reproducibility in dogs [15, 16]. MR can be graded as mild (<30%), moderate (30–70%), or severe (>70%), based on the ARJ/LAA ratio on the CDI study in dogs with MMVD (**Figure 1A**) [15, 16]. Unfortunately, the ARJ/LAA ratio can be affected by many intrinsic factors (e.g. SAP, LA pressure, the spatial orientation of MR jet) and extrinsic factors (e.g. pulse repetition frequency and gain settings) [17]. Several studies found the ARJ/LAA ratio was not closely correlated with the severity of MMVD in dogs, especially in dogs with American College of Veterinary Internal Medicine (ACVIM) B2 and C [18, 19].

The vena contracta is the narrowest width of the MR jet that occurs at or just downstream from the regurgitant orifice and can reflect the severity of MR (obtained from measuring the regurgitant orifice size by the CDI study) (**Figure 1B**) [17]. Although it requires less time-consuming interrogation and is technically easier to obtain, this method is prone to errors, especially in dogs having dynamic regurgitant orifices [14]. Several studies found the VC and VC-derived variables were closely correlated with the severity of MR and survival time in dogs with MMVD [20–22]. One study found the mean (±SD) diameter of the mitral regurgitation vena contracta in a right parasternal long-axis (RPLx) view (VCR:Ao) and a left apical four-chamber view (VCL:Ao) indexed to aortic diameter were 0.21 ± 0.14 and 0.24 ± 0.12, respectively, in dogs with MMVD [21]. Furthermore, the VCL:Ao > 0.24 was closely correlated with survival time (hazard ratio 4.87) [21].

#### **2.2 Quantification of MR**

*Advanced Concepts in Endocarditis - 2021*

clicks in dogs) exist [5].

of CHF caused by MR.

nary and human cardiologists and researchers.

ing the severity of MR in humans and dogs.

**2. Assessment of MR severity**

**2.1 Semi-quantification of MR**

progression), discrepancies in these two diseases (e.g. much more prevalent in dogs than in humans, less prone to develop endocarditis in dogs, less prominent systolic

Although there are many diagnostic methods for MMVD, the standard transthoracic echocardiographic examination is a gold standard test for diagnosis and prognosis of MMVD and MVP in dogs and humans, respectively. The echocardiography is non-invasive and enables the clinicians to detect the mitral valve (MV) lesions, to evaluate MR severity, and to assess its impact on cardiac remodeling, myocardial function, left ventricular filling pressures, as well as pulmonary arterial pressure [6–11]. However, conventional ultrasound imaging modalities such as two-dimensional (2D), M-mode, color Doppler, pulse-wave (PW), and continuouswave (CW) Doppler echocardiography may not be enough to precisely evaluate the severity of mitral valve diseases and to monitor disease progression. Advanced ultrasound technologies such as tissue Doppler imaging (TDI), strain and strain rate imaging, and two-dimensional speckle tracking echocardiography (STE) gain popularity in veterinary and human cardiologists because these technologies can assess and monitor global and regional myocardial function more precisely [12],

although those require more expensive ultrasound machine and training.

Human MVP causing MR is divided into two groups: primary (i.e. structural intrinsic valvular disease) and secondary (i.e. nonstructural functional MR caused by non-mitral valve diseases). Like dogs, causes of primary MR are degenerative diseases on the mitral valve, such as Barlow, fibroelastic degeneration, Marfan, Ehlers-Danlos, and annular calcification, although rheumatic disease and toxic valvulopathy can also cause MR in humans [13]. Severity of MR in humans is graded qualitatively (e.g. mitral valve morphology, color flow MR jet, flow convergence zone, CW signal of MR jets), semi-quantitatively (e.g. vena contracta [VC] width, pulmonary vein flow, mitral inflow, TVI mit/TVI Ao), and quantitatively (effective regurgitant orifice area [EROA], regurgitant volume [R Vol]) [13]. LV and LA size and the systolic pulmonary arterial pressure are also used to determine the severity

Although the severity MR and CHF in dogs with MMVD are being evaluated as similar to human cardiology, the veterinary cardiologists are more focused on the severity of cardiac remodeling (e.g. LA and LV dilation) and cardiac dysfunction caused by MR, because surgical restoration of defected mitral apparatus is rarely done in dogs. Therefore, the echocardiographic indices related to MR severity (e.g. MV morphology, flow convergence zone) are not routinely assessed in dogs with MMVD. The chapter reviews conventional echocardiographic indices being used for diagnosis and prognosis of canine MMVD to provide a better understanding of the similarities and discrepancies between canine MMVD and human MVP to veteri-

Echocardiography is a gold standard test in the assessment and management of humans and dogs with MR. Although color flow MR jets and CW signal of MR jets are useful for detecting MR, a more quantitative approach is required for determin-

Although quantitative assessment of MR (e.g. EROA and R Vol) is being used for grading the severity of MR in humans [13], it is rarely used in canine practice,

**32**

The measurement of the flow convergence area by proximal isovelocity surface area (PISA) is a gold standard method to quantify the MR jet in humans [13]. Although one study found the PISA method was repeatable and reproducible in awake dogs [16], it is more time-consuming and requires several precautions to obtain the optimal acquisition of the flow convergence images in dogs with MMVD [14]. The regurgitation fraction (RF) obtained by the PISA can be used for

#### **Figure 1.**

*(A) Semi-quantification of mitral regurgitation (MR). The maximal ratio of the regurgitant jet area signal to the left atrial area (ARJ/LAA ratio) was obtained from a left apical four-chamber view. The ARJ (1.12 cm2 )/ LAA (2.12 cm2 ) of this case was 53%, indicating moderate MR. (B) the vena contracta diameter from a left apical four-chamber view (VCL) was measured as the diameter of the narrowest point of the mitral regurgitation jet, downstream of the region of proximal flow convergence.*

determining the severity of MR in dogs (e.g. mild if <30%, moderate if 30–75%, and severe if >75% of RF). Several studies found the RF was closely correlated with the severity of MMVD and other echocardiographic variables (e.g. LA/Ao and systolic PA pressure) [16, 23, 24]. However, one study found the MR quantification of the PISA method showed a wide range of RF (~33% asymptomatic MMVD dogs had moderate to severe RF) and thus is not routinely done in canine practice [16].
