The Heart-Lung Interaction: Is It Cardiac or Is It Respiratory Disease
World Small Animal Veterinary Association World Congress Proceedings, 2004
Brendan M. Corcoran, MVB, Dip Pharm, PhD, MRCVS
Hospital for Small Animals, The University of Edinburgh, Easterbush Veterinary Centre Roslin
Midlothian, Scotland

A major difficulty for many clinicians can be deciding if a case has a primary cardiac problem or primary respiratory problem. By their very nature diseases of the cardiovascular and respiratory system in dogs present with similar clinical signs. These include coughing, tachypnoea, dyspnoea and exercise intolerance, but also can include cyanosis, collapse and syncope. Sneezing is typically a respiratory sign, and in cats coughing is invariably due to respiratory disease. Furthermore, some of the clinical signs of cardiopulmonary disease can be exhibited by diseases of the musculoskeletal system, and endocrine and neurological diseases. To complicate the picture further there can be concurrent co-existing diseases of the cardiac and respiratory systems in the same patient, with the associated complexity of interaction.

When presented with a patient with signs of cardiopulmonary disease, the first step is to determine which system is likely to be affected. This is primarily because different diagnostic tests are used to evaluate the different systems. In the case of cardiac disease, radiography, electrocardiography, blood pressure measurement and echocardiography are the required ancillary diagnostic techniques, and will give a diagnosis rapidly. In the case of respiratory disease, radiography, bronchoscopy and tracheobronchial and broncho-alveolar lavage sampling for cytology and culture sampling are the usual techniques, and it may be several days before a definitive diagnosis can be made. For both systems haematology and biochemistry profiles are undertaken, and 24 hour ECG monitoring or event-recording may be used in cardiology, while blood gas analysis tends to be used more in respiratory patients. Computed tomography and ultrasonography also have a role to play in the investigation of respiratory disease. Crucially, the cardiac patient is rarely if ever anaesthetised for investigation, while this is common in respiratory patients, particularly for those with chronic respiratory disease. It is, therefore useful to know in advance of undertaking a complicated investigation as to which system is causing the problem. Also the financial cost to the client must be understood, and therefore, the most appropriate tests chosen.

The clinical history can be of particular use in determining the likely system that is affected. Breed predisposition for a variety of cardiovascular and respiratory diseases can be very useful in diagnosis, coupled with significant findings on physical examination. Examples of cardiac diseases with a high breed pre-disposition include mitral valve endocardiosis in the cavalier King Charles spaniel, dilated cardiomyopathy in the Doberman pinscher, Newfoundland and other giant breeds and cocker spaniel, aortic stenosis in the boxer and mitral dysplasia in the English bull terrier. Most textbooks have exhaustive lists of the lesser know breed predispositions, and they are worth consulting if a particular disease is suspected. Breed predisposition for respiratory diseases is not as obvious as for cardiac diseases, but good examples include tracheal collapse in the Yorkshire terrier and other toy breeds, idiopathic pulmonary fibrosis in the West Highland white terrier and laryngeal paralysis in the Labrador retriever. All brachycephalic breeds have brachycephalic airway syndrome to varying degrees and should be suspected as the likely cause of the dogs problem unless evidence suggests otherwise. Interestingly, upper airway obstruction can have effects on cardiac rhythm, which might be the cause of problems and also have secondary effects on the lung resulting in non-cardiogenic pulmonary oedema, which can be similar to the cardiogenic form. This illustrates the potential complexity of the inter-action between the two systems. Another example is the equal susceptibility of toy breeds to mitral valve endocardiosis and tracheal collapse, both of which can cause chronic coughing.

The clinical signs of cardiac and respiratory disease can be so similar as to make deciding which system is affected difficult. For example, a middle aged Labrador presenting with a gradual onset of coughing and dyspnoea might have dilated cardiomyopathy or pulmonary neoplasia. On the other hand a dog presenting with syncope episodes during exercise, unless it has neurological or other systemic illness, is likely to have a cardiac rather than a respiratory problem. A further example is a dog with intermittent cyanosis which is more likely to have a upper airway obstruction than an intra-cardiac shunt, but a dog or cat with cyanosis at rest might have fulminant cardiogenic pulmonary oedema or severe airway, lung or pleural disease. Physical examination hopefully will narrow down the possible options.

The simplest approach to deciding which category a patient fits in to is to evaluate the cardiac system first. This is because on routine physical examination the cardiovascular system can be readily evaluated in the majority of cases. Assessment of mucous membrane colour and capillary refill time can allow assessment of cardiac output and peripheral vascular resistance. Palpating the femoral pulses can give an estimation of cardiac output and measurement of heart (pulse) rate and identification of dysrhythmias. In some instances, and with additional information, pulse quality, strength and rhythm can be used to diagnose certain cardiac diseases. Inspecting the jugular veins for pulsation and distension, augmenting with the hepato-jugular reflux technique, can give information on the existence of right sided heart failure. Auscultating the thorax allows measurement of heart rate, identification of dysrhythmias (and pulse deficits if compared to pulse rate) and abnormal heart sounds such as murmurs and gallop sounds. Further examination will allow localisation a murmurs' point of maximal intensity (heart base or apex, and likely valve source), the degree of radiation (dependent on the source of the murmur and loudness), the loudness of the murmur (grade 1-6), what part of the cardiac cycle it is present in (systolic, diastolic, continuous), if systolic to what extend (pan/holo systolic, crescendo-decrescendo early to mid-systolic) and the character of the murmur (harsh/ejection or soft / regurgitant). As an example, a Great Dane dog presenting with a history of exercise intolerance, weakness, tachypnoea and dyspnoea that has a chaotic heart rhythm, a heart rate of approximately 200 beats per minute, a grade 3 soft left heart apical murmur and weak femoral pulses, is likely to have idiopathic dilated cardiomyopathy with congestive heart failure and is unlikely to have respiratory disease. This case would need echocardiography to confirm the disease and vigorous treatment for congestive heart failure.

To complete the examination of the cardiopulmonary systems, the respiratory rate is noted and the presence of respiratory effort assessed. This needs to be undertaken when the dog or cat is calm, as the stress of the consultation room can have marked effects on respiratory pattern. The particular problem of interpreting respiration in the presence of panting in dogs needs to be recognised. The presence of audible respiratory noise should be noted and the upper airway auscultated to determine if that is the source (stridor or stertor). The ability to elicit coughing by tracheal pinching should be assessed, but its' exact interpretation is often difficult. Chest percussion should be undertaken to identify regional changes in resonance (free fluid, air or consolidate/mass lesions). This can also sometimes induce coughing and assist in the approximate localisation of where a lesion is. In cats, chests compliance can be useful for identifying the presence of mediastinal masses. Chest auscultation to listen for abnormal respiratory sounds, should be undertaken. In calm, co-operative normal cats minimal sounds are heard. In normal dogs, background respiratory sounds (so called broncho-vesicular) can be heard, but even with minimal excitement, and definitely with panting, low pitched rhonchus sounds can be heard. If rhonchi are very loud and inappropriate for the level of excitement, then it is abnormal. Inspiratory crackles are always abnormal and are a typical finding of extensive and severe cardiogenic pulmonary oedema, but can also be heard with respiratory diseases such as chronic bronchitis and pulmonary fibrosis. Wheezes are also always abnormal and are more typically heard in cats with small airway disease, such as asthma. A mixture of all these sounds can also be heard with both cardiac and respiratory disease. Equally, in diseased cases respiratory sounds may appear normal, while if inaudible, the likely explanation is pleural effusion or obesity.

Once the likely system affected is identified then it is necessary to carry out the appropriate diagnostic tests. This will depend on the client's finances, the equipment and expertise available. Routine haematology and biochemistry tests tend to reveal very little. In cardiac cases the existence of pre-renal azotaemia can be identified and leucocytosis with neutrophilia may be noted in respiratory infections. Thoracic radiography will allow assessment of the heart for the presence of cardiomegaly, specific chamber enlargement, vascular engorgement (congestive heart failure) and pulmonary oedema. Radiography is notoriously insensitive for the diagnosis of congenital heart disease and rarely of use in cases with dysrhythmia where there is no evident changes in the cardiac silhouette. If respiratory disease is suspected then the films should be assessed for the presence of bronchial, alveolar and interstitial patterns and the trachea inspected. The presence of pleural effusion should also be noted (respiratory and cardiac causes) and the mediastinum assessed. Hopefully, radiography will significantly narrow down the possible disease differential list.

If cardiac disease is suspect, an ECG should be obtained and the rhythm assessed. The presence of chamber enlargement can be inferred from the change in complex morphology, but should be interpreted with caution. If a dysrhythmia as a cause of the clinical presentation is suspected then a 24 hour Holter monitoring of the ECG will be needed, and if this is unsuccessful, then an event recorder may need to be used. Both of these techniques are usually only available at referral centres. Echocardiography is by far the most sensitive technique for the diagnosis of cardiac disease in dogs, but it requires expensive equipment and a high level of operator expertise. 2-dimensional echocardiography allows identification of gross changes, while M-mode allows assessment of ventricular function, and Doppler and colour Doppler echocardiography allows identification of flow abnormalities (typically causing murmurs). Blood pressure measurement is particularly useful in the investigation of cats with heart disease or diseases that cause hypertension and then affect cardiac function.

If respiratory disease is suspected then bronchoscopy should be considered as it is more likely to allow definitive diagnosis than relying on radiography alone. Again it requires expensive equipment and a significant level of expertise, but allows direct visualisation of the tracheo-bronchial tree and the accurate collection of samples for cytological analysis. Even if the airways look normal this is diagnostically useful as it rules out important conditions such as chronic bronchitis, and if there is radiographic evidence of disease it helps more accurately localise the pathology. The cytological profile form bronchial washes and broncho-alveolar lavage samples is very useful as it gives more direct information on the type of pathology present. If a pleural effusion is present or a well defined mass visible, then fine needle aspiration (thoracocentesis / mass biopsy) again is very useful as it gives a pathological profile as to what is happening within the chest, and in some instances, such as neoplasia, will give a definitive diagnosis. With particularly difficult cases that are not responding to treatment and the exact diagnosis has not been achieved them the obvious next step is to do open chest lung biopsy. In a small number of referral centres video-assisted thoracoscopic lung biopsy equipment is available. But if there is no access to high technology diagnostic tools, simple blind tracheo-bronchial washings can be a very powerful technique in the diagnosis of respiratory disease.

With a logical approach, the possible contribution of cardiac or respiratory disease to the clinical presentation of a patient can usually be ascertained with minimal intervention. Identifying the exact cause of a problem may require specialist intervention, and this is particularly so if the case is complicated.

References

For further information consult standard medicine, cardiology and respiratory textbooks, particularly those chapters that deal with clinical manifestations of disease or discussion of clinical syndromes and their diagnosis.

Speaker Information
(click the speaker's name to view other papers and abstracts submitted by this speaker)

Brendan M. Corcoran, MVB, Dip Pharm, PhD, MRCVS
Hospital for Small Animals, The University of Edinburgh
Easterbush Veterinary Centre Roslin
Midlothian, Scotland


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