Brendan M. Corcoran, MVB, PhD, MRCVS
Hospital for Small Animals, Royal (Dick) School of Veterinary Studies, The University of Edinburgh
Dyspnoea is a common clinical presentation of cardiorespiratory diseases and indicates a significant degree of respiratory impairment. Dyspnoeic cases always appear particularly challenging since the consequence of unsuccessful treatment is usually death. In order to properly manage such cases it is important the clinician takes a logical approach. In many instances it is important first to stabilise the patient and then to attempt to achieve a definitive diagnosis. For the purposes of stabilisation, some facility for giving supplemental oxygen is of immense value and gives some time to properly review the situation and develop a logical assessment and treatment plan. It is also worth remembering that there are common causes of dyspnoea and where the condition is immediately life-threatening the clinician should be willing to make an educated clinical guess as to what is the likely cause, and then select the best course of action.
Definitions and Terminology
Tachypnoea refers to an increased respiratory rate and should not be confused with panting. Dyspnoea is best described as laboured or difficult breathing or alternatively as inappropriate breathing effort. It tends to be best recognised when breathing is slow and purposeful. Where there is inspiratory dyspnoea this is often associated with upper airway obstruction (stridor or stertor). Expiratory dyspnoea can be recognised as a discrete end-expiratory effort or grunt, but might only be heard on auscultation, although abdominal effort may be apparent. Hyperpnoea describes deep and rapid breathing and orthopnoea (severe and life-threatening) describes adopting a position to ease breathing. Often this is seen as adopting a standing or sternal position. Usually cats stay in sternal recumbency, with abducted elbows, extended neck and open-mouth breathing. Dogs often remain standing for prolonged periods, being reluctant to lie down as this only exacerbates their dyspnoea.
Initial Approach to the Dyspnoeic Case
As soon as it is recognised the case has significant respiratory distress then the patient should be removed to a quiet environment and administered supplemental oxygen. The case should be minimally handled, and restraint should be avoided or carried out cautiously, but this depends on the degree of dyspnoea. Cases with cyanosis at rest are at particular risk. Increasing inspired O2 to 40% can result in a marked improvement in arterial oxygen tension. Using a purpose built oxygen cage with temperature and humidity control is ideal, but such facilities are expensive and tend to be found only in specialist referral centres. Delivering humidified supplemental oxygen via nasal prong catheters is a highly effective method, requires minimal equipment and does not need a CO2 scavenging system or temperature control, which is necessary for sealed cage environments. Facemask delivery of O2 can also be used, but only as a short term approach until nasal catheters are placed. Most patients will actively resent a face mask being placed too close to the head and the restraint necessary may be counterproductive.
Once stabilised as much as possible, a more detailed clinical examination can be undertaken to attempt to make a diagnosis. This can include diagnostic tests such as thoracic radiography. However, on the basis of identifying a probable cause, usually suspected from the physical examination, further treatment can be instituted without undergoing diagnostic tests. Typical examples include thoracocentesis of cats with suspect pleural effusion, and standard emergency therapy of suspect congestive heart failure (intravenous furosemide, percutaneous nitroglycerine). Such treatments can be sufficient to save the patient and then a proper diagnostic work-up can be undertaken.
Diagnostic Tests for Dyspnoeic Patients
This is the most important diagnostic test for investigation of dyspnoeic patients as it will allow identification of pleural effusion, severe lung parenchymal disease and congestive heart failure. It can also identify severe airway obstruction caused by foreign bodies (rare) and tracheal collapse (common). It will also allow all these problems to be excluded from the differential list. Radiography has limited use in identifying upper airway obstruction, where direct visualisation is of greater use. The restraint necessary for radiography, particularly for lateral radiographs, is of concern in severely dyspnoeic patients. Low level sedation can be used with facemask administration of O2 to reduce potential risk. In dyspnoeic cats a dorso-ventral view can be achieved with minimal restraint and will identify severe pleural effusion, but identification of other thoracic changes with confidence requires lateral views.
Laryngoscopy and Bronchoscopy
These can be used to identify upper airway obstruction, tracheal collapse and hypoplastic trachea, and help confirm lower airway and lung parenchyma diseases causing dyspnoea. The necessity for anaesthesia is a limiting risk factor, and these procedures should only be carried out if safe to do so and the quality of information that is likely to be obtained warrants the risk. In the case of upper airway obstruction, confirmation of the obstruction is best followed immediately by surgical correction rather than repeating anaesthesia. The good example of this approach is with laryngeal paralysis.
Blood Gas Analysis (BG)
The degree of respiratory impairment is best evaluated using blood gas analysis. However, the restraint used to obtain a sample can make the risk too great. The degree of hypoxia can be determined and any changes monitored over time. The contribution of ventilation-perfusion (V/Q) mismatch or hypoventilation to the BG abnormalities can be determined by calculating the alveolar (A) to arterial (a) pressure gradient ([A-a]PO2), and this can also support diagnosis. The gradient is calculated as follows:
PAO2 = 150 - (PaCO2 x 1.1)
P[A-a]O2 = PaO2 - PaO2
A gradient greater than 20 mmHg suggests V/Q mis-match, while less than 15 mmHg suggests hypoventilation. Normal values are typically less than 10 mmHg.
Haematology and Biochemistry
Routine haematology and biochemistry profiles will provide additional information as to the patient's health status but are unlikely to significantly assist diagnosis.
This will be necessary to make a definitive diagnosis of heart disease, but identification of congestive heart failure is made on thoracic radiography (cardiomegaly, left-atrial enlargement, pulmonary vascular congestion, pulmonary oedema), and diagnosis can often be confidently made by considering the breed and physical findings.
Differential Diagnosis of Causes of Dyspnoea
Young animals are more likely to be affected by infectious diseases, and older animals are more predisposed to chronic illness such as idiopathic pulmonary fibrosis (IPF) and neoplasia. Certain breeds are predisposed to respiratory diseases that can cause dyspnoea. Brachycephalic airway syndrome is a common cause of dyspnoea, tracheal collapse is seen mainly in toy breeds and IPF is most common in West Highland white terriers. Being aware of breed-associated cardiac diseases can also help diagnosis. Possible exposure to an aetiological agent (infections, toxins etc) needs to be considered. Clinical signs attributable to other body systems (particularly gastro-intestinal) should be noted. Many case of severe bacterial bronchopneumonia are a result of aspiration of food or vomitus. Also a history of prior trauma or surgery should be noted. Documented response to therapy, and presumably a documented diagnosis, such as with congestive heart failure, can be very useful. The presence of other cardiorespiratory signs, their duration and severity should be noted. These include cough, exercise intolerance collapse or syncope, cyanosis and ascites.
Care has to be taken in handling the dyspnoeic patient and the physical examination might have to be curtailed. Also observation of the breathing pattern may have to take place once the patient is hospitalised and in a quiet, undisturbed environment. A standard physical examination is carried out and particular attention paid to the respiratory rate, heart rate, heart rhythm (see below), pulse rate and strength, mucosal colour and capillary refill time, jugular distension and abdominal distension. The head conformity is examined, nasal patency assessed and the trachea palpated. Chest percussion, mainly to identify pleural effusion, is worthwhile but needs to be interpreted with caution. Auscultation is carried out along the entire length of the respiratory tract and the presence of abnormal respiratory sounds noted. These should be classified as normal, inaudible, rhonchi, crackles and wheezes or mixed and the predominance of respiratory noise to a particular part of the respiratory cycle noted. Heart rhythm should be assessed, but exact rhythm identification requires electrocardiography, and the presence of murmurs and gallop sounds identified. The presence of pyrexia should be noted as should the presence of obesity.
Localisation of Source and Cause of Dyspnoea
Upper airway obstruction identified on auscultation tends to result in inspiratory stridor or stertor and typically is associated with nasal, pharyngeal, laryngeal and tracheal disease. Inspiratory crackles are found with restrictive lung diseases, such as pulmonary oedema and pulmonary fibrosis, but can also be present with chronic bronchitis. End-expiratory dyspnoea is associated with non-fixed airway obstruction, such as intra-thoracic tracheal collapse, neoplasia and feline asthma (lung hyperinflation). If there is a combination of inspiratory dyspnoea, expiratory dyspnoea and wheezing then a fixed airway obstruction is possible, such as with bronchoconstriction with feline asthma. Tachypnoea alone can be seen with diseases that restrict lung expansion, such as pneumonia, pleural effusion, chest wall abnormalities and systemic illnesses (fever, shock, anaemia and acidosis), and in pulmonary thrombo-embolism.
Therapy of Specific Diseases
Once a diagnosis has been made then the treatment programme has to target that disease. This may involve surgical alteration of anatomical abnormalities, such as brachycephalic airway syndrome and tracheal collapse, and medical treatment of primary respiratory diseases such as pneumonia, and cardiac disease causing congestive heart failure, such as myxomatous mitral valve disease and dilated cardiomyopathy.
Table 1. Common causes of dyspnoea.
--Brachycephalic airway syndrome
--Extended soft palate
--Airway foreign bodies
--Feline asthma syndrome