When to take this cough to heart

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One dog, 2 independent lightning strikes—yes, it can happen! It often does, particularly as animals develop myriad problems with age. Case in point: older dogs commonly present for coughing. This population is also prone to left-sided systolic murmurs.

The clinician who listens to the coughing dog and auscults a murmur might jump to “eureka”-like conclusions. Not so fast, warned Lance Visser, DVM, MS, DACVIM, associate professor of cardiology at Colorado State University College of Veterinary Medicine & Biomedical Sciences, at the American Veterinary Medical Association’s (AVMA) Convention in Denver, Colorado.¹

“Just because a dog is coughing and has a loud murmur does not mean the problem is cardiac,” he said.

Concurrent heart disease aside, a specific cough event might be caused by various airway diseases,^2,3 but if truly cardiac, the cough mechanism is up for debate.

“We don’t know exactly why dogs with cardiac disease and cardiomegaly cough,” he conceded, “but we sure do know it’s quite common.”

The primary differentials, traditionally, are left mainstem bronchial compression,^4,5,6 and pulmonary edema, both due to cardiomegaly related to myxomatous mitral valve disease (MMVD). While a cardiac cough should never be allowed to slip through the cracks, that ubiquitous “pesky coughing dog” can be a conundrum, he added, because the devil is often in the details.

Is the “cough” really a cough?

The first bedeviling detail is that mystery noise piping from the patient’s mouth. What the owner is coining a cough might not be so. “Coughing”—in owner-speak—could be gagging, retching, vomiting, reverse sneezing, or even a huff-like, throat-clearing expiratory reflex. “So a lot of things can mimic a cough,” Visser said.

Owners sometimes can provide video of the episodes to substantiate the cough claim and yield further information. With luck, the dog will demonstrate the move during the exam.

Characterizing the cough

Once the cough is verified, it needs description. What does it sound like? Loud, brassy, or honking sounds are often linked to airway inflammation, tracheobronchomalacia, or bronchial compression. A soft cough suggests lower airway and/or pulmonary parenchymal disease, including pulmonary edema.

Also, asking clients questions such as how frequent is the cough, and when did it first begin? Has it stayed the same, or worsened over time? Is the cough productive? Is it brought on or exacerbated by exercise? Excitement? Sleeping? A certain body positions? Or time of day? Can help you get more of the information needed for a diagnosis.

A nocturnal and productive cough may signal the presence of pulmonary edema, but not always. Both collapsing trachea and chronic bronchitis may bring about pooling of secretions during rest, leading to hacking fits.

Many roots of coughing

A cough can be birthed at different sites along the respiratory pathway. In the upper airways, the main cough triggers are laryngeal paralysis, more common in large dogs; obstructive foreign bodies or masses; collapsing trachea, which mostly affects toy breeds; bronchial compression, due to left atrial dilation; fungal infections; and allergic or infectious chronic bronchitis.

Lower airway coughing can be tied to heartworm disease, cardiogenic pulmonary edema, pneumonia and pulmonary neoplasia.

For older, small breeds, like Yorkshire terriers, chihuahuas, cavalier King Charles spaniels and Pomeranians, Visser cited collapsing trachea and chronic bronchitis as top reasons for coughing. But if a murmur is present, he said, add cardiogenic bronchial compression and pulmonary edema to this shortlist.

Weeding out coughs that are not heart-based is key, said Visser. “The murmur status, or lack thereof, is going to be really helpful here.”

In older dogs, lack of a murmur basically eliminates heart disease as the cough trigger. But presence of one doesn’t lend the same diagnostic certainty, as a murmur can be incidental to a respiratory problem. “Myxomatous mitral valve disease is so common, particularly in our middle-aged to older dogs, that sometimes (a murmur) is just a red herring,” he explained.

The relevance of cardiomegaly

Absence of significant cardiomegaly in a coughing patient with a heart murmur essentially rules out cardiac cough and points to airway disease as the culprit. Therefore, ground 0 in the coughing, murmurous dog is heart measurement. The gold standard for this is echocardiography, he said, but it can be costly. “Not all dogs with a cough and a murmur have the luxury of getting an echo.”

Not to worry, he added, Thoracic radiographs are a reliable work-around for flagging cardiomegaly, and a good diagnostic screening tool for collapsing trachea, heartworm disease and pulmonary parenchymal pathology.

The American College of Veterinary Internal Medicine supports 2F heart measurement modalities—vertebral heart score (VHS) and the newer vertebral left atrial size (VLAS), assessed using lateral views.⁷

“Pick one,” Visser said, “and at least do one of these quantitative measurements.”

The VHS is calculated by measuring the long axis of the cardiac silhouette,⁸ running from the carina to the apex; the short axis is measured at the widest part of the cardiac silhouette in the dorsoventral plane, perpendicular to the long axis. These axes are summed and indexed to thoracic vertebral bodies starting at T4. A VHS>10.7 is considered enlarged; over 11.5 is clinically significant cardiomegaly.

The VLAS is obtained by drawing a line from the carina to the left atrium where it intersects with the dorsal border of the caudal vena cava.⁹ This line is then indexed to thoracic vertebrae starting at T4. A VLAS>2.2 and 2.5 are rated, respectively, as enlarged and clinically significant.

Quantitative measurements should be corroborated with subjective assessments of cardiomegaly, like loss of the dorsocaudal cardiac waist, and dorsal deviation of the trachea, carina and mainstem bronchi, as seen on lateral projection. Dorsoventral or ventrodorsal views might reveal a widened cardiac silhouette, a bulge at 2-3 o’clock representing an enlarged left auricle and increased soft tissue opacity between the mainstem bronchi.

Measurements aside, Visser cautioned: “Not all dogs with heart enlargement and a loud murmur are in left-sided congestive heart failure and cough for that reason.”

He pointed to a study minimizing the link between congestive heart failure (CHF) and cough in dogs.¹⁰ Cough receptors, he explained, reside in the larynx, trachea, and bronchi, but not in the alveoli. The study found that coughing matches up well with abnormal radiographic airway patterns and left atrial enlargement, but often dissipates once pulmonary edema unfolds.

Visser recommended treatment with the drug pimobendan for the coughing dog with a minimum grade 3 murmur and cardiomegaly that meets the threshold cardiac scores, VHS>11.5 or VLAS>2.5. But for dogs that fall short of these numbers, he said to hold off.

“Mitral valve disease in most dogs is slow. So, if they don’t meet the cutoffs, don’t treat yet. Just re-X-ray in four months.”

Pimobendan lowers left atrial pressure and reduces left heart size, thereby alleviating left mainstem bronchial compression, airway irritation and cough.¹¹ it has been shown to delay heart failure by some 15 months.Visser sometimes adds an ACE inhibitor like enalapril or benazepril, though studies show mixed results in staving off pulmonary edema.

Pulmonary edema: Life rafts that prevent drowning

Furosemide should not be used casually, said Visser. The presence of CHF should be verified before instituting the drug. “That’s a whole lot of pee spots on the carpet,” he said.

Cardiogenic pulmonary edema announces itself radiographically with increased interstitial infiltrate in the perihilar and caudodorsal lung fields. Severe cases could also feature an alveolar lung pattern with spread to other lung fields, pleural lines, and caudal vena caval and pulmonary venous distention. Abdominal serosal detail may be absent due to ascites.

Visser said dogs with radiographic evidence of cardiogenic pulmonary edema and tachypnea/dyspnea should be started on furosemide and pimobendan, and reevaluated five to seven days later. If still dyspneic, next steps might include addition of an ACE inhibitor and spironolactone.

In the non-dyspneic, stable patient, furosemide should not be initiated unless the resting respiratory rate exceeds 30 breaths per minute. If coughing persists despite treatment, thoracic radiographs should be repeated.

Addressing the cough when the heart is not to blame

Once heart disease has been quashed as the reason for a cough, a heartworm antigen test should be performed to confirm negative status. A bronchoscopy can assess for upper airway pathology. If he suspects chronic bronchitis or collapsing trachea, Visser said he often prescribes a 2 to 4 week course of doxycycline, sometime augmented with a tapering course of prednisone.

Other measures that can alleviate chronic coughing include long-term inhaled corticosteroids or bronchodilators, cough suppressants, antacids (for cough related to gastroesophageal reflux) and weight management.

References

1. Visser L. Recommended Approach to the Coughing Dog with a Murmur. Presented at: AVMA Convention; Denver, Colorado. July 14-18, 2023.
2. Ferasin L, Linney C. Coughing in dogs: what is the evidence for and against a cardiac cough?. J Small Anim Pract. 2019;60(3):139-145. doi:10.1111/jsap.12976
3. Singh MK, Johnson LR, Kittleson MD, Pollard RE. Bronchomalacia in dogs with myxomatous mitral valve degeneration. J Vet Intern Med. 2012;26(2):312-319. doi:10.1111/j.1939-1676.2012.00887.x
4. Lebastard M, Le Boedec K, Howes M, Joslyn S, Matheson JS, O’Brien RT. Evaluation of bronchial narrowing in coughing dogs with heart murmurs using computed tomography. J Vet Intern Med. 2021;35(3):1509-1518. doi:10.1111/jvim.16115
5. Szatmári V, Kittleson MD, Johnson LR, Pollard RE, Singh MK. Letter regarding "Evaluation of bronchial narrowing in coughing dogs with heart murmurs using computed tomography". J Vet Intern Med. 2021;35(6):2555-2556. doi:10.1111/jvim.16266
6. Le Boedec K, Lebastard M, Howes M, Joslyn S, Matheson JS, O'Brien RT. Response to letter regarding "Evaluation of bronchial narrowing in coughing dogs with heart murmurs using computed tomography". J Vet Intern Med. 2021;35(6):2557-2558. doi:10.1111/jvim.16265
7. Keene BW, Atkins CE, Bonagura JD, et al. ACVIM consensus guidelines for the diagnosis and treatment of myxomatous mitral valve disease in dogs. J Vet Intern Med. 2019;33(3):1127-1140. doi:10.1111/jvim.15488
8. Buchanan JW, Bücheler J. Vertebral scale system to measure canine heart size in radiographs. J Am Vet Med Assoc. 1995;206(2):194-199.
9. Malcolm EL, Visser LC, Phillips KL, Johnson LR. Diagnostic value of vertebral left atrial size as determined from thoracic radiographs for assessment of left atrial size in dogs with myxomatous mitral valve disease. J Am Vet Med Assoc. 2018;253(8):1038-1045. doi:10.2460/javma.253.8.1038
10. Ferasin L, Crews L, Biller DS, Lamb KE, Borgarelli M. Risk factors for coughing in dogs with naturally acquired myxomatous mitral valve disease. J Vet Intern Med. 2013;27(2):286-292. doi:10.1111/jvim.12039
11. Boswood A, Häggström J, Gordon SG, et al. Effect of Pimobendan in Dogs with Preclinical Myxomatous Mitral Valve Disease and Cardiomegaly: The EPIC Study-A Randomized Clinical Trial. J Vet Intern Med. 2016;30(6):1765-1779. doi:10.1111/jvim.14586