div epub:type=”chapter” role=”doc-chapter”>
13. Valvular Insufficiency
13.1 Chronic Mitral Insufficiency (CMI)
Chronic mitral valve insufficiency (CMI) is the most common cause of congestive heart failure in small breeds of dogs. It may lead to mitral regurgitation (MR) in long-standing cases. CMI is characterized by back flow of blood from the left ventricle to left atrium during ventricular systole leading to forward failure, backward failure, and volume overload. Its clinical severity is classified as functional heart failure class I, II, III, and IV. Clinically CMI is characterized by various grades of cardiac murmurs, fatigue, polydipsia, tachycardia, orthopnea, tachypnea, infrequent cyanosis, jugular pulse, hepatojugular reflex, arrhythmias, and dyspnea.
Etiology of CMI varies from genetic predisposition (especially in small and medium breeds of dog), arrhythmias, dilated cardiomyopathy, chest trauma to tumors. Diagnosis of CMI poses challenge in routine practice and requires a systematic approach employing detailed clinical examination, radiological examination, electrocardiographic examination, and echocardiographic examination.
13.1.1 Diagnostic Profile
Murmurs of various grades are detected on chest auscultation. Soft, blowing and an early systolic murmurs over mitral valve in the left chest may arise suspicion of mitral valve insufficiency.
Clinical manifestations are not very much specific. These are of general nature such as marked exertion, weakness, polydipsia, tachypnea, orthopnea, tachycardia, or cyanosis depending on the class of functional heart failure. In many cases there is a chronic productive or nonproductive cough. Cough is easily elucidated by tracheal palpation if tracheal collapse is coexisting. Therefore clinical signs may arise clinical suspicion especially in aged dogs of small breeds. The dogs with clinical suspicion of CMI should be thoroughly investigated.
Echocardiography is of vital importance in the diagnosis of CMI as status, and functioning of the heart valves can only be evaluated by echocardiography.
The echocardiographic changes in the mitral valve, left atrium, or left ventricle depend on the class of functional heart failure. Echocardiography in dogs with CMI may reveal mitral value thickening (Fig. 13.1) and change in its shape, increase in chamber size (atrium, ventricle) and/or contractility force of ventricles, etc.
Doppler echocardiography can detect retrograde flow of blood (from the left ventricle to left atrium during systole). Echo- and Doppler echocardiography are the only means to confirm chronic mitral insufficiency.
Electrocardiography is not of diagnostic value in confirming CMI. But it may detect some changes associated with different class of heart failure (class II, III, and IV functional heart failure). The following changes have generally been observed in cases of CMI:
P wave in lead II may be broad (duration more than 0.04 s) and notched, and its amplitude may remain between 0.35 and 0.4 mV suggesting left atrial enlargement.
Amplitude of R wave may increase in different leads. It may be more than 2.5 mV in lead II, more than 3.0 mV in lead CV6LU, and 1.0 mV in lead CV5RL.
Amplitude of S wave may be more than 0.5 mV in standard bipolar leads (lead I, II, III).
QRS may be broad (more than 0.06 s).
There may be frequent depression of T wave.
There may be changes in S-T segment (depression or elevation).
Atrial fibrillation, supraventricular premature beats, ventricular premature complexes, and ventricular tachyarrhythmia may be detected in electrocardiogram.
In class IV heart failure, low-voltage complexes may be evident.
Radiography may be helpful in detecting the changes in the size of left atrium. Enlargement of the left atrium (Fig.13.2) and/or left ventricle is a common radiographic finding in cases of CMI. In advanced cases pulmonary edema in dorso-caudal hilar region, enlargement of pulmonary vasculature, and dorsal displacement of trachea (suggesting left heart failure) are also evident. When mitral valve insufficiency is complicated with pulmonary hypertension, hepatomegaly and ascites (suggesting right heart failure) can also be visualized in radiographs.
Cardiac biomarkers- Levels of cardiac biomarkers (Cardiac troponin-I, NT pro-BNP) have been found elevated in cases of chronic mitral valve insufficiency.
No medical treatment provides complete cure. Nevertheless, treatment of chronic mitral valve insufficiency is directed as per degree of heart failure. It is outlined below:
Dogs with CMI showing class I heart failure generally do not require any treatment. But their excitability can be controlled with phenobarbital (@ 1–2 mg/kg given orally twice or thrice daily) and diazepam (@ .5–2.2 mg/kg orally). Isosorbide dinitrate can be given @1–2 mg/kg twice daily orally for reducing heart load and improving coronary circulation.
Dogs with CMI showing class II heart failure should be treated to prevent deterioration. The purpose of using phenobarbital, diazepam, and isosorbide is same as described under class I heart failure. Use of propranolol (@ 0.3–1.0 mg/kg orally twice or thrice daily) is to control associated tachycardia. To prevent or delay the onset of congestion, use of captopril (@ 0.25–0.5 mg/kg orally twice or thrice daily) is recommended. Enalapril (@ 0.25–0.5 mg/kg orally twice or thrice daily) can also be used in place of captopril. Furosemide (@ 2–4 mg/kg orally twice daily) is recommended as a diuretic to control congestion. Its dose should be reduced/discontinued when the dog is stabilized.
Dogs with CMI showing class III heart failure can be managed with the use of furosemide, nitroglycerine, isosorbide dinitrate, hydralazine, enalapril/captopril/prazosin, and digoxin as per clinical need. Furosemide (@ 4–5 mg/kg orally, IV, IM, twice or thrice daily until pulmonary edema is reduced followed by once daily dose) and spironolactone (4–5 mg/kg orally twice daily) are recommended for reducing the pulmonary edema. Nitroglycerine ointment (2%) is applied on skin area (0.5–3.0 cm) in groin region or on inner ear flap of dogs having problem in oral administration of diuretics. Isosorbide dinitrate can be given @1–2 mg/kg twice daily orally for reducing heart load and improving coronary circulation. If furosemide is less effective, its efficacy may be improved by using hydralazine (@ 1–3 mg/kg orally twice daily). Enalapril (0.25–0.5 mg/kg orally twice daily) or prazosin (1.0 mg/15 kg orally twice or thrice daily) is used to delay congestion and manage arrhythmias. ACE inhibitors like enalapril and diuretics like spironolactones should not be used together as electrolyte imbalance may occur. Digoxin (@ 0.005–0.008 mg/kg PO BID) is recommended in the management of supraventricular tachycardia.