Murmurs in puppies and kittens (Proceedings)
Congenital heart disease (CHD) is a morphologic defect of the heart or great vessels which is present at birth. "Congenital" does not imply the defect was inherited, although many CHD are heritable. CHD may also be caused by de novo spontaneous mutations, which have the potential to be transmitted to progeny. Diagnosis of CHD is important for the health of the patient and also to eliminate affected individuals from the breeding pool.
Cardiac auscultation often helps narrow the differential list and can be pathognomic for particular congenital defects such as patent ductus arteriosus (PDA). Evaluation of femoral arterial pulse strength, jugular veins, and mucous membrane color are also important aspects of the physical examination. Diagnostic approach for the CHD patient includes thoracic radiographs, echocardiography, electrocardiography, and complete blood count and arterial blood gas in cyanotic heart diseases.
Innocent murmursInnocent physiologic murmurs are not uncommon in healthy puppies, but are impossible to distinguish from pathologic murmurs due to structural cardiac disease. Therefore, an echocardiogram is needed to evaluate whether the murmur is innocent versus secondary to congenital heart disease. In a large 6-year study of over 1200 Boxer dogs, 52% of healthy Boxer dogs had left basilar systolic murmurs and normal echocardiograms with no evidence of congenital heart disease. A majority (97%) of these physiologic murmurs were soft (I-II/VI). Likewise, of the Boxers diagnosed with congenital heart disease, 85% had soft I-II/VI systolic murmurs, making it impossible to differentiate physiologic innocent murmurs from pathologic murmurs due to congenital heart disease. The cause of innocent physiologic murmurs in healthy animals is unknown. A possible explanation is that the stroke volume is greater in proportion to the size of the great vessels in puppies compared to adult dogs. Soft innocent murmurs in Boxer puppies have been shown to be highly variable in presence and intensity both within days and between days. Although often innocent murmurs are stated to commonly resolve by 6 months of age, a prospective study of growing Boxer puppies showed that these innocent murmurs commonly persisted into adulthood. In contrast, another study found that murmurs disappeared in half of adult Boxers who had soft murmurs as puppies.
Congenital heart diseases are divided into 4 main categories: ventricular outflow obstructions, left to right shunts, right to left shunts, and atrioventricular valve dysplasia or stenosis, along with other less common defects.
Ventricular outflow obstructions
Subaortic stenosis is the most common CHD of large breed dogs. It is caused by a fibrotic ridge, tunnel, or ring arising in the left ventricular outflow tract just below the level of the aortic cusps. SAS develops over the first few months of life, and may progress up to 12 months of life. SAS is heritable in Newfoundlands, likely as an autosominal dominant single trait, and is also common in Golden Retrievers, Rottweilers, Boxers, German Shepherd dogs, and Great Danes. Increased resistance to left ventricular ejection results in a high left ventricular systolic pressure, which causes concentric left ventricular hypertrophy and subendocardial fibrosis. Myocardial ischemia may result from reduced capillary density and reduced coronary blood flow. Exertional syncope or sudden death is common in severe SAS. Ventricular arrhythmias are the probable cause of sudden death. Physical examination abnormalities include a left basilar holosystolic murmur which often radiates to the right and to the carotid arteries. In moderate to severe SAS, femoral pulses are dampened. Thoracic radiographs are often unremarkable or may depict a post-stenotic dilation of the ascending aorta. Echocardiography is necessary to determine the diagnosis and severity of obstruction. Measurement of peak aortic blood flow velocity (and calculated pressure gradient by the modified Bernouilli equation of PG = 4V2) using the left apical 5-chamber view is necessary to determine the severity of obstruction: 20-50 mmHg (mild), 50-80 mmHg (moderate), > 80 mmHg (severe). Recent studies have identified that neither balloon valvuloplasty nor open heart surgical correction of the fibrotic ring improve survival over beta blockade alone. Patients with moderate or severe SAS should be placed on a beta blocker in efforts to reduce risk of sudden death. Patients with mild SAS have greatly diminished risk of sudden death, and it is debatable whether they should be placed on a beta blocker. Prophylactic antibiotics should be given prior to any surgical or dental procedure to prevent development of aortic valve infective endocarditis.
Valvular pulmonic stenosis is the third most common congenital heart disease in dogs, and occurs most frequently in small breeds. Bulldogs and Boxer dogs are predisposed to develop either valvular PS or subvalvular extramural PS due to an anomalous left coronary artery that originates from the right coronary artery and encircles the pulmonary artery. PS causes a left basilar holosystolic murmur and normal femoral pulses. Thoracic radiographs typically show right heart enlargement and post-stenotic dilation of the pulmonary artery. Echocardiography is useful to determine the diagnosis, the severity, and whether the obstruction is subvalvular or valvular. Other concurrent defects such as tricuspid valve dysplasia (TVD) or right to left shunting patent foramen ovale should also be evaluated with echocardiography. The combination of TVD and PS commonly result in severe right sided congestive heart failure (i.e. ascites, hepatomegaly, jugular distension, +/- pleural effusion). Transesophageal echocardiography or aortic root angiography may be necessary to diagnose the anomalous left coronary artery. Dogs with severe PS are at risk for sudden death, syncope, or development of right sided CHF due to either diastolic dysfunction or worsening tricuspid regurgitation. Balloon valvuloplasty is the treatment of choice of severe valvular PS, and is associated with a 53% reduction in risk of sudden death as well as improved quality of life.