Understanding normal cardiac anatomy and physiology is the foundation for flagging heart defects in young dogs, according to Kelly Wiggen, DVM, a cardiology faculty member at the University of Missouri Veterinary Health Center in Columbia. During a recent Fetch dvm360® virtual conference session, Wiggen reviewed the normal structure and function of the canine heart, described the clinical fallout of mishaps in its formation or maturation, and addressed treatment plans for afflicted dogs.
The pumping mechanism
The left and right sides of the heart each have 2 chambers—the atria and the ventricles. The left atrium collects oxygenated blood from the pulmonary veins and directs it into the left ventricle through the mitral valve. The left ventricle pumps this fresh blood through the aortic valve into the aorta and out into general circulation.
The right heart handles deoxygenated blood. The cranial and caudal vena cavae return used blood to the right atrium, which then directs it through the tricuspid valve into the right ventricle. From here, it is pumped into the pulmonary artery through the pulmonary valve and then into the pulmonary vasculature, where it picks up more oxygen.
Normal systemic blood pressure is 120/80 mm Hg. The systolic number, 120, represents the pressure generated by the left ventricle to deliver fresh blood out to the body. What enable the cardiac pump to function are pressure differentials between the heart and great vessels, as well as between the cardiac chambers themselves: Blood is pumped down pressure gradients through valves that open and snap shut, correlated with the heart sounds.
The pulmonic and aortic valves are atop the heart, near its base. The tricuspid and mitral valves reside closer to the sternum, at the apex. To listen for their normal sounds, as well as murmurs associated with blood regurgitation or turbulence from inborn heart defects or damaged valves, Wiggen recommends auscultating 5 spots on the dog:
Left axilla: Continuous murmurs associated with patent ductus arteriosus (PDA)
Left heart base: Normal sounds from pulmonary and aortic valves; systolic murmurs associated with pulmonic stenosis (PS) and subaortic stenosis (SAS), tetralogy of Fallot, and physiologic variations from fever, anemia, or excitation; diastolic murmurs, usually due to a leaky valve
Left cardiac apex: Normal sounds from mitral valve activity; systolic murmurs associated with mitral valve dysplasia (puppies) and degenerative mitral valve disease, endocarditis, or dilated cardiomyopathy (older dogs); diastolic murmurs connected to mitral valve stenosis
Right heart base: Systolic murmurs from ventricular septal defect (VSD)
Right cardiac apex: Normal sounds from tricuspid valve activity; systolic murmurs associated with tricuspid valve dysplasia (puppies) and degenerative mitral valve disease, endocarditis, or dilated cardiomyopathy (older dogs); diastolic murmurs connected to tricuspid valve stenosis
Murmurs, if found, are graded from 1 to 6 (see “Heart murmur grades”).
To match a murmur to the pathology, investigate the location where it’s loudest, its cardiac cycle phase, and related femoral pulse quality, Wiggens said. For instance, PDA advertises itself with a continuous murmur that is loudest at the left axilla, and bounding pulses. SAS and PS are linked to systolic murmurs loudest at the left heart base. PS typically has normal pulses, whereas pulses can be weak in patients with SAS. A VSD would be loudest at the right heart base, and congenital defects of the mitral and tricuspid valve would be heard at the left and right apex, respectively.
A murmur does not always mean a heart defect; some murmurs are innocent. How do you tell them apart? Benign murmurs, Wiggen said, typically can be heard only at 1 small spot, are usually limited to early or mid-systole, and can change with excitement. Likewise, she added, a heart defect doesn’t always mean a murmur.
The physical exam combined with the signalment can steer the practitioner toward the disease process. For instance, a 6-month-old golden retriever with a murmur likely has SAS. A 6-month-old Chihuahua, on the other hand, is a strong candidate for PDA.
The top heart defects
The most common congenital heart defect in dogs is a PDA. Here, the ductus arteriosus—a vessel that connects the pulmonary artery and aorta in the fetus, enabling blood to bypass the nonfunctional lungs—fails to close at birth. As a result, some of the oxygenated blood gets dumped back into the pulmonary artery rather than into the systemic circulation. This left-to-right shunting causes left-sided volume overload and subsequent left-sided congestive heart failure (CHF).
PDA often affects smaller breeds, particularly Havanese, toy poodles, Maltese, Yorkshire terriers, and Chihuahas. However, it can also be seen in larger breeds, such as shelties, collies, and German shepherds. It also occurs more commonly in females than males and more in dogs than cats.
Puppies with PDA are often asymptomatic, although they may be smaller and quieter than their littermates. They typically have low diastolic pressures, causing bounding femoral pulses, and a continuous murmur. Electrocardiography can reveals evidence of left-sided chamber enlargement. X-rays often show left atrial and ventricular expansion, as well as dilated pulmonary arteries and veins, and bulging aorta, main pulmonary artery, and auricles (Figure 1).
Figure 1. Patent ductus arteriosus.
Suspected PDA cases are best referred to a cardiologist for confirmation of the diagnosis, Wiggen said. Other potential heart defects must be ruled out because they may change the prognosis or the ability for the PDA to be fixed.
Two treatment options exist for PDA. Option 1 is to close off the vessel with an occluder device, which carries a lower risk for embolization and infection. Option 2 is surgical correction via thoracotomy, which risks fatal hemorrhage and persistent flow through the PDA. Wiggen prefers the occluder device but noted that it is only available for patients with an average weight of about 6 lb (2.7 to 3 kg), and for PDAs of certain structure.
Whatever method is chosen, early closure carries an excellent prognosis, Wiggen said. “This is one of those congenital diseases where we can actually fix these dogs rather than just palliating their disease,” she said.
Later diagnosis produces lesser results, however. By age 1, two-thirds of affected dogs have developed left-sided CHF. Nevertheless, Wiggen recommends correcting PDAs in these patients because doing so still often extends their life by years.
Subaortic stenosis and pulmonic stenosis.
The next 2 inborn cardiac flaws, SAS and PS, are tied for second most common. SAS occurs at the left side of the heart, below the level of the (aortic) valve. PS happens on the right, at the level of the (pulmonary) valve. Despite their different locations, the pathologies are similar, Wiggen said.
“It’s the same phenomenon as if you take a garden hose and started moving your finger over the end,” she explained. “The more you move your thumb over it, the faster that water is going to have to flow. That’s what’s happening in the heart.”
The greater the narrowing, the worse the obstruction, and the harder the heart must pump. The chambers thicken in response, yet the myocytes outgrow their vascular supply, leading to myocardial ischemia and fibrosis, conduction disturbances, and eventual CHF. With pulmonic stenosis, the valves themselves can be dysplastic as well.
SAS is more common in large breeds. Because it is progressive up to when a dog is full grown, a murmur may not be present at the first puppy visit. PS usually affects small breeds, is not progressive, and is more likely to manifest at the initial puppy exam. Both conditions are usually asymptomatic, but dogs with SAS may display left-heart failure signs, such as coughing and respiratory distress. Dogs with PS show right-heart sequelae, such as abdominal distention from ascites.
A basilar systolic murmur accompanies both diseases. Femoral pulses are generally weak in dogs with SAS but normal in dogs with PS. X-rays for the former show left heart enlargement and a prominent aorta (Figure 2) and, for the latter, right heart enlargement and a dilated main pulmonary artery (Figure 3). On electrocardiography, dogs with SAS can have wide P waves and tall R waves, indicative of left atrial and left ventricular enlargement, respectively. Patients with PS can have tall P waves and deep S waves, reflecting enlargement of the right heart chambers. For both conditions, echocardiograms may reveal chamber thickening, blood flow obstruction, increased flow velocity leaving the heart, and valves that are leaky. A narrowing is often visible below the level of the aortic valve in the case of SAS, and poorly mobile or abnormal leaflets are visible with PS.
Figure 2. Subaortic stenosis.
Both diseases are graded based on pressure generated by the affected ventricle. Mild and moderate cases are often clinically silent, do not require treatment (except beta blockers in some cases of moderate PS), and carry an excellent prognosis.
Dogs with severe SAS are at risk for left-sided CHF and sudden death before age 5. They may be prescribed b-blockers (eg, atenolol), which decrease the rate and force of contractions. Balloon valvuloplasty may improve clinical signs, but neither this nor medication has been demonstrated to improve survival time.
Dogs with severe PS are at risk for right-sided CHF and sudden death. These patients benefit from b-blockers and balloon valvuloplasty.
Figure 3. Pulmonic stenosis.
Other birth defects
Other heart malformations occur less commonly. Tricuspid valve dysplasia, to which Labrador retrievers are predisposed, boosts right atrial pressure, resulting in jugular vein distention and pulsation, hepatomegaly, and ascites. A right apical systolic murmur may be heard on puppy exam. This condition is usually managed medically, although surgical intervention is available at Colorado State University.
Mitral valve dysplasia, an uncommon anomaly that strikes bull terriers preferentially, causes left heart failure. Its attendant signs are respiratory distress, cough, tachycardia, and hypothermia. A left apical systolic murmur accompanies this condition, which also is treated medically.
Puppies born with a VSD, a hole in the septum between the ventricles that allows blood to pass from the left to right side of the heart, experience left-sided volume overload. Auscultation reveals a left basilar systolic murmur. The prognosis depends on VSD location and volume shunted. The smaller the VSD, the less chance of volume overload and the better the prognosis. Small VSDs, which ironically produce loud murmurs, may close over time. Large VSDs cause higher volume overload and carry a poor prognosis. If the defect is not too big and is in an adequate location, it can be managed by surgery or device occlusion.
Silent heart disease
Although the aforementioned cardiac malformations produce murmurs, Wiggen said, there also are heart diseases that don’t produce murmurs. The most common “murmurless” heart defect is the reverse PDA, which occurs when the left-to-right shunting of a PDA switches direction. As a result, deoxygenated blood is pushed into circulation via the aorta, and the dog becomes hypoxemic, particularly in the caudal half of the body. The kidney boosts erythropoietin production in response, and more red blood cells are churned out. Affected dogs experience multiple clinical signs, such as lethargy, exercise intolerance, respiratory distress, pelvic limb weakness, and differential cyanosis (pink up front, blue at rear).
Blood work shows polycythemia with a regenerative response. X-rays and echocardiography reveal an enlarged right heart and prominent main pulmonary artery as well as peripheral pulmonary artery dilation (Figure 4). Phosphodiesterase inhibitors, such as sildenafil and tadalafil, are used to treat pulmonary hypertension, and therapeutic phlebotomy may alleviate blood hyperviscosity and prevent associated seizures. Some cardiologists will also recommend hydroxyurea to also help manage the secondary polycythemia. The average lifespan of affected dogs is 5 years.
Figure 4. Reverse patent ductus arteriosus.
When you hear a heart murmur in a puppy, Wiggen said, blend the signalment, history, and physical exam findings to determine what anomaly is most likely present. It’s important to remember that a pet may have multiple cardiac defects.
Joan Capuzzi, VMD, is a small animal veterinarian and journalist based in the Philadelphia area.