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Abbreviations:
CDMD chronic degenerative mitral valvular disease
HWD heartworm disease
NYHA modified New York heart association functional classification
PH pulmonary hypertension
TBC tracheobronchial collapse
TD tracheobronchial diameter
TI thoracic inlet distance
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The respiratory and cardiovascular systems function as a coupled unit. The left heart receives blood from the pulmonary circulation and ejects blood into the systemic circulation; the right heart receives blood from the systemic circulation and ejects blood into the pulmonary circulation.1 As a result, alterations of the pulmonary vascular resistance and compliance resulted from several pathogenic conditions may change the afterload and preload of the right and left heart.1-3 In addition to the hemodynamic changes of pulmonary circulation, the other possible pathogenesis of cardiac remodeling in respiratory system disorders include systemic inflammation, oxidative stress, hypoxia, activation of the sympathetic nervous system.4,5
Chronic left heart diseases may lead to the increased left atrial and pulmonary wedge capillary pressure.6 The enlarged left atrium and the elevated pressure in pulmonary vascular bed may result in the anatomic compression of respiratory tract, pulmonary edema, or pulmonary hypertension.6-9 The pulmonary vascular structural and functional changes resulted from chronic hypoxia, inflammation, direct
occlusions or damages caused by chronic pulmonary edema, respiratory tract disorders, or parasitic infestation may lead to the elevated pulmonary vascular resistance, pulmonary artery hypertension, and then right heart failure.10-14 The left heart may also be affected by the uncompensated right heart due to the phenomenon of ventricular interdependence.15,16
Fifty-four clinically healthy small breed dogs and 133 dogs with Chronic
degenerative mitral valvular disease (CDMD) in different condition of modified New York heart association (NYHA) functional classification joined in the first study to determine the relationship between tracheobronchial collapse (TBC) and CDMD. In this study, the ratio of tracheobronchial diameters to thoracic inlet distance (TD/TI), diameters at six locations of trachea and bronchi and the angle of tracheal bifurcation were measured on thoracic radiographs. The result presented that the enlarged left
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atrium secondary to CDMD might result in the anatomical compression of the trachea and bronchus; however, the study failed to identify that trachea collapse is one of the risk factors for the development of CDMD and left heart failure. In the second study, several right heart echocardiographic indices were measured from 40 clinically healthy dogs and 169 dogs with elevated pulmonary artery pressure resulted
respectively from respiratory disorders, heartworm disease (HWD) and CDMD. The chronic respiratory disorders, HWD and CDMD leading to the pulmonary vascular resistant and hemodynamic changes are three of the well recognized etiologies of pulmonary hypertension (PH) in dogs.13,17-19 As the pulmonary pressure overload persists, the inability of the right ventricle to cope with the elevated resistant results in a vicious cycle of right ventricular dilation and deformation.13,17,20 The results of this study indicated right ventricular remodeling were affected by the chronic respiratory disorders, HWD, and moderated to severe CDMD. Although the CDVD was the most common cause among these three causes of PH in this study, the pulmonary vascular and right ventricular adaptation were more obvious in dogs with heartworm
infestation and chronic respiratory disorders. The different ventricular
interdependence among the three causes of PH might be attributed to the interaction between the dilated right ventricle and the different conditions of left ventricle.15,21
The respiratory system diseases of dogs might result in different cardiac manifestations. Based on the findings of our studies, the pulmonary vascular remodeling due to the respiratory tract disorders, HWD, and CDMD leaded to the right ventricular adaptation and altered of the patterns of ventricular interdependence.
The respiratory tracts were anatomically compressed by the enlarged left atrium caused by CDMD. However, the left ventricle adaptation in respiratory system diseases of dogs still needed to investigate in further study.
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