Myths in clinical cardiology

Ivan Sosa, DVM, DACVIM (Cardiology)
Massachusetts Veterinary Referral Hospital, Woburn, MA
Posted on 2017-05-23 in Cardiology

It is generally accepted that, in order to provide good quality work, we need to keep our knowledge up to date, mainly through continuous education courses, reading articles, etc. However, some of the diagnostic procedures, historical and physical examination findings or laboratory analysis that we have learned during our veterinary training and after graduation have been taught as dogma, with little evidence to support their validity. Sometimes, these misconceptions make it to veterinary medicine as extrapolated information from human medicine. Relying on false assumptions can be dangerous to your patient. This lecture will review a list of myths involving cardiac disease with little evidence to support their validity.

Cough, crackles, and congestive heart failure (CHF)

Degenerative valvular disease is the most common acquired heart disease in dogs leading to CHF. It is important to know that coughing is not a consistent finding of CHF and that this clinical sign should not be taken as a holy grail for the diagnosis of this condition. More often than not, dogs with valvular disease are also predisposed to chronic airway disease, such as tracheal collapse, chronic bronchitis, etc. It may be difficult for some clinicians to know whether coughing is related to cardiac or respiratory disease in a dog with a heart murmur. Although there are clues in the history and physical exam, that will help to differentiate the origin of this cough, it is essential to understand the physiopathology of congestive heart failure for providing an adequate diagnosis.

Congestive heart failure is a clinical syndrome that develops in these patients as a result of a leakage of the mitral valve. With time, atrial pressure rises causing pulmonary vein congestion and an increased filtration of lymphatic fluid in the pulmonary interstitium. When fluid accumulates in the interstitial space dogs will show an increased respiratory rate (tachypnea) and thoracic radiographs will show the typical interstitial pattern located in the perihilar or caudodorsal lung regions. In cases of advanced congestive heart failure, fluid will accumulate within the alveoli, impairing the patient’s oxygenation and consequently leading to difficulty breathing (dyspnea), and the typical auscultation of crackles. However, crackles are not always indicative of congestive heart failure. Diseased alveoli in patients with chronic bronchitis can also cause crackles.

A cough may be present in patients with heart disease due to main stem bronchus compression secondary to atrial enlargement. However, not all patients with atrial enlargement are in congestive heart failure. The combination of coughing and crackles in a patient with a heart murmur seems to be a common source of misdiagnosis of congestive heart failure.

Coughing is a sign of CHF in people. This cough usually appears at night or early morning, as a consequence of a change in body position from vertical to horizontal when sleeping. Dogs in CHF can have a soft cough. However, this cough is acute, since a dog in CHF will deteriorate without our intervention. The chronicity of the cough and/or crackles is key in these patients. A Chronic cough and/or crackles should make us think that there is an underlying airway disease, and must not result in the automatic prescription of a diuretic, especially if there is no tachypnea, as this has been shown to be the best indicator of CHF.

Sometimes, the diagnosis of CHF is not clear cut. If in doubt, a furosemide trial (2mg/Kg PO q12hrs) can be done for 2-3 days. If the patient responds, it is likely that the patient was in CHF and proper standard therapy should be started (ACE-I and pimobendan). Some patient’s cough may respond initially after starting diuretics even if these patients are not in CHF. This may be due to the effect in atrial pressure and decreased airway compression. However, this is not an appropriate way of treating the cough. Adding cough suppressants or bronchodilators may be as effective, and less detrimental for this patient.

Heart rate and dog size

For a long time, we have been taught that the heart rate in large breed dogs is different (lower) than that of small breed dogs. Although it is true that there is a difference among species (an elephant’s heart rate is way lower than that of a mouse) there is little evidence supporting the same within canine breeds.

Most studies in dogs show no correlation between the heart rate and dog size. These studies are based on monitoring the heart rate with 24-hours Holter monitor or electrocardiograms while the patients are at home. There is another study involving a greater number of dogs that showed that larger dogs can have a lower heart rate compared to smaller dogs. However, this difference is only of about 10bpm. Therefore, it is unlikely that this result has clinical relevance.

Thoracic radiographs, inverted D and cardiomegaly

We have been taught that an inverted ‘D’ on a ventrodorsal thoracic radiograph indicates right ventricular enlargement. However, when we perform echocardiograms we can observe a normal heart in many of these dogs. It is important to remember that an inverted ‘D’ can be a normal variant in a lot of patients and should not be always used as an indication of cardiomegaly.

The vertebral heart scale (VHS) was published many years ago as a measure to help clinicians to identify cardiomegaly. However, many practitioners use this index to rule out heart disease. It is important to remember that many cardiac conditions do not result in cardiomegaly, and echocardiography may be more suitable to rule out certain diseases.

Electrocardiogram and arrhythmia monitoring

The electrocardiogram (ECG) is an extremely useful tool for arrhythmia identification. With the development of cardiac ultrasound, as well as the low sensitivity of ECGs for identifying chamber enlargement, the utility of ECG has gone to second plane.

Although ECGs can be used to identify arrhythmias, the absence of these electric disturbances during the ECG cannot rule them out. Therefore, other diagnostic tests (24-hours Holter monitoring) may be necessary when the signalment, history or physical exam suggest that an arrhythmia may be present. A 2-minute ECG will only give 0.15% of the information about cardiac events that occur in a patient. Therefore, in cases of intermittent arrhythmias, such as ventricular premature complexes, or when monitoring antiarrhythmic medication, we should opt for a more appropriate diagnostic test, such as the Holter monitor.

Correlation between dental and cardiac diseases

The association between dental disease and degenerative valvular disease in dogs is likely the biggest myth in veterinary cardiology.

In people, it used to be typical practice for a long time the use of prophylactic antibiotic therapy in oral procedures such as dental cleaning. This approach changed many years ago, and only high-risk patients receive antibiotics to prevent cardiac diseases. Somehow, this information has been partly extrapolated to veterinary medicine. Although human medicine has changed their policies about this topic, we still find many vets recommending dental cleaning to avoid valvular disease in dogs, or thinking that cardiac disease in dogs may be caused by poor oral health. Although regular dental cleanings are necessary for many reasons unrelated to this topic, well controlled studies have failed to find an association between degenerative valvular disease and dental health. An exception would be dogs with congenital subaortic stenosis, in which aortic valvular damage due to turbulent flow may put these patients at a higher risk of developing valvular endocarditis. These patients may benefit from prophylactic antibiotic therapy should they have any surgical procedure, including dental cleaning.

Further reading

• Autran de Morais H, Saretta Schwartz D. Pathophysiology of heart failure. In: Ettinger SJ, Feldman EC, eds. Textbook of Veterinary Internal Medicine: Diseases of the Dog and Cat, 6th ed. Oxford: Elsevier Saunders; 2005:914-40.
• Buchanan JW. Vertebral scale system to measure canine heart size in radiographs. J Am Vet Med Assoc 1995;206(2):194-9.
• Ferasin L. Lack of correlation between canine heart rate and body size in veterinary clinical practice. J Sm Anim Pract 2010;51:412-8.
• Ferasin L. Risk factors for coughing in dogs with naturally acquired myxomatous mitral valve disease. J Vet Intern Med 2013;27:286-92.
• Peddle GD, Drobatz KJ, Harvey CE, Adams A, Sleeper MM. Association of periodontal disease, oral procedures, and other clinical findings with bacterial endocarditis in dogs. J Am Vet Med Assoc 2009;234(1):100-7.
• Peddle GD, Sleeper MM, Ryan MJ, Kittleson MD, Pion P. Questions validity of study on periodontal disease and cardiovascular events in dogs. J Am Vet Med Assoc 2009;234(12):1525-8.