Heart disease is usually progressive while respiratory disease can wax and wane for long periods of time. A loud, hacking cough is usually associated with disease of the large airways including compression of the mainstem bronchi, chronic bronchitis and collapsing trachea. Coughing due to left heart failure is usually soft and associated with dyspnea. A nocturnal cough suggests cardiac disease because recumbency tends to increase intrathoracic blood volume which can excerbate signs of pulmonary congestion. Physical examination Patients that cough due to heart disease tend to be thin.
Obesity suggests pulmonary disease.
Many normal dogs have a respiratory sinus arrhythmia which is a regularly, irregular change in heart rate. Heart rate increases with inspiration and decreases with expiration due to a waxing and waning of vagal tone. Dogs with respiratory signs due to heart failure, do not have respiratory sinus arrhythmia whereas, dogs with primary respiratory disease frequently have respiratory sinus arrhythmia. Indeed, dogs with severe pulmonary disease can have an exaggerated sinus arrhythmia with bradycardia. In older, small breed dogs, the absence of a cardiac murmur largely eliminates chronic valvular disease as a cause of respiratory disease.