Cough, like many other signs/symptoms, occurs commonly in dogs due to either heart disease (most often due to mitral regurgitation)
or lung disease (often due to pulmonary fibrosis, tracheal collapse, tracheo-bronchitis), and when disease of both heart and
lungs are present (as occurs commonly in ageing dogs), it is extremely difficult to determine which organ system is responsible.
But etiology is important, since drugs are selected based upon etiology, and treating cough sue to heart disease, when it
is caused by lung disease, may be harmful. We can often determine whether heart disease or lung disease is the culprit by
physical examination and thoracic radiology.
The following chart identifies signs/symptoms that, when combined, help to determine if the dog is ill from heart disease
or from lung disease. Any sign/symptom taken by itself may be in error; all or many taken together are usually correct.
Functions of the heart and lung are interdependent because they occupy space in the same cavity, because the heart pumps blood
through the lungs, the lung hinders flow of blood from right ventricle to left ventricle; so when disease strikes either,
it also affects the other.
The lungs have an apex starting at the thoracic inlet and a base covered by the diaphragm. It has a huge single airway—the
trachea—through which all of the air flows, and hundreds of thousands of small airways where gas exchange occurs. The trachea
is huge, but there's only 1; the small airways are tiny, but there are many, many of the them. Thus air rushes at a high velocity
into and out of the trachea—producing a loud vesicular breath sound heard well with a stethoscope—but flows very slowly into
and out of the small airways, producing no sound. Narrowing of the trachea will increase flow velocity making the tracheal
vesicular breath sound louder then normal. So you better learn what is normal! How will you do it?
A large left atrium compresses and displaces the left mainstem bronchus. Injury to a large airway produces a hacking cough.
As you'll learn, mitral regurgitation (MR) produces a large left atrium, therefore dogs with MR often have hacking coughs,
not because of heart failure, but because of MR.
With pleural effusion caused by cancer (usually lymphoma), an obstructed thoracic duct, or hypertrophic cardiomyopathy in
cats, there may be a dense substance (pleural effusion) between the outside of the lung and the chest wall. Remember that
the tracheal vesicular breath sound is sucked into the lung with the inspiratory air flow, thus you hear it on the chest wall,
not because it was made underneath where you're listening, but because it was transmitted there by the flow of air. With the
pleural effusion, the normal vesicular breath sound heard over the base of lung is absent or diminished, because the sound
carried by the air is reflected and not transmitted. Therefore you can identify pleural effusion by the absence of a vesicular
breath sound. Also, because of the dense effusion, when you thump on the chest wall over it (that is known as percussion),
there will be a dull note of percussion. How will you know if the note is duller than normal? You better!
When there is pneumothorax with air between the lung and chest wall, the vesicular breath is equally soft or absent to when
there's pleural effusion...not because sound is reflected, but because the sound "dies" in poorly-conducting air. When you
percuss the chest with pneumothorax, there is a loud, hyper resonant note.
With distension of pulmonary veins—due most often to left-sided heart failure (from MR or dilated cardiomyopathy), a bronchoconstrictory
reflex is elicited which results in asthmatic wheezes.
Pulmonary edema may be identified readily by increased intensity of vesicular breath sounds during both inspiration and expiration.
This is caused by the wet lung transmitting sounds better to the chest wall. These sounds are termed bronchial or bronchovesicular....but
they are also heard with bronchial pneumonia (a rarely-occurring disease, however). Because the lung I wet and dense, there
will be a dull note of percussion (You better know what dull means, but you can learn it easily!)