Cough...Heart disease or lung disease? (Proceedings)

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!)

Crackles are breath sounds that sound like crackle! They are caused by airways scrunching closed and popping open. This occurs because the lubing is wet and heavy with either edema or pneumonia, it settles down, airways become smaller, therefore are more likely to close as the lung gets smaller during exhalation. Then, during inspiration when the lung gets bigger, the collapsed small airways pop open again.

Cyanosis is a blue tinge to the mucous membranes. It is caused only by increased amounts of unoxygenated hemoglobin in the arterial blood. This occurs because some of the pulmonary blood flows through regions of the lung where the airways have collapsed (see above). This blood picks up now oxygen, and when added to the blood that did pick up oxygen, may make the arterial blood have too little oxygen. Dogs cyanotic because of with pneumonia and edema are not cyanotic long.....the either die or are treated and get better. Dogs with pulmonary disease (e.g., fibrosis, chronic bronchitis) remain cyanotic forever, because there lungs are permanently damaged.

Dogs with stiff lungs due to edema, pneumonia, fibrosis are difficult to ventilate. Therefore the dog will breath rapidly and shallowly, will fatigue muscles of respiration, and may die from asphyxia.

Pulmonary fibrosis occurs VERY commonly in small breed, ageing dogs. The lungs are shrunken and gnarled, ugly-looking, very stiff, and produce cacophonous crackles during both inspiration and expiration. Because the lung is so small, airways are closed, and cyanosis is almost always present. Because they impose such a hindrance to blood flow from right ventricle to left atrium, the right ventricle hypertrophies. Because the chest wall works so hard to ventilate the stiff lungs, muscle of respiration fatigue and pulmonary retractions (flailing-in of the chest wall) occurs. Also because the pleural pressure must fall so low in order to suck open the stiff lungs, air is sucked into the esophagus, then when the dog exhales, it is forced into the stomach and GI tract....therefore gas in the gut is common with lung disease.

With lung disease the muscles must work very hard to ventilate the sick lungs. The only way they can do that is if they get exaggerated nervous impulses from the brain. As these impulses leave the respiratory centers they affect the cardioregulatory (rate) centers and produce exaggeration of the sinus arrhythmia (speeding of heart rate during inspiration, slowing during expiration).