The avian heart displays very similar anatomy and physiology to our companion animal counterparts; however, cardiac disease in our avian patients is often overlooked and underdiagnosed. Symptoms are often more subtle than we see in companion animals, as captive birds have the ability to self-restrict exercise and hide signs of illness until they reach congestive heart failure (Welle, 2016). The avian heart is relatively larger than mammalian species, and their heart rates are much faster (Sandmeier, 2018). Avian hearts have four chambers and four valves; however, the right atrioventricular valve lacks chordae tendineae, functioning as a muscular flap (Beaufrère and Guzman, 2016).
Common clinical signs of cardiac disease include lethargy, exercise intolerance (which is more often noticed in athletic birds such as birds of prey), hyporexia and, in severe cases, dyspnoea as a result of ascites or hepatic congestion and hepatomegaly. Coughing is not usually observed as birds lack a diaphragm and have fixed non-expansile lungs (Sandmeier, 2018).
The ideal examination of the avian cardiovascular system would occur on a non-restrained patient to reduce stress and vocalisation; however, as many birds are unfamiliar with a veterinary setting light restraint in a towel can facilitate a quicker examination. The heart can be auscultated over the left and right pectoral muscles (Figure 1), at the thoracic inlet or between the scapulae. Due to the higher heart rate of avian species, murmurs and arrhythmias can be difficult to identify and often are only apparent when the patient is anaesthetised and the heart rate slows. If possible, peripheral pulses should be palpated over the basilic artery, ventrally at the point of the elbow or at the tibiotarsal artery on the medial aspect of the tarsometatarsal joint (Hunt, 2018). Mucous membranes can be visualised to assess pallor, but this is difficult in birds with naturally pigmented mucous membranes such as most domesticated parrots, and so this is usually of limited use.
If a cardiac abnormality is identified or suspected, then further diagnostics should be pursued. Echocardiography is a very useful tool to assess structural heart changes, identify turbulent flow within the heart and detect pericardial effusion (Fitzgerald and Beaufrère, 2016). Coelomic ultrasound to assess for ascites can also be performed at the same time. In very well-behaved patients this can be performed conscious; however, most patients will need to be sedated or receive gaseous anaesthesia. Radiographs taken under sedation or general anaesthesia can be useful to assess the cardiac silhouette and great vessels, as well as identify any hepatic congestion or ascites. Cardiac silhouette measurements have been described for many avian species, including a number of falcons (Barbon et al., 2010), psittacines (Straub et al., 2002) and numerous other species, which can be used to assess for cardiomegaly.
Electrocardiography (ECG) can be useful to characterise arrhythmias and can be used in conjunction with other diagnostic methods to identify structural abnormalities, such as chamber enlargement (Fitzgerald and Beaufrère, 2016). Ideally this is performed on an anaesthetised or sedated patient (Figure 2) to avoid motion artefact; however, the clinician’s discretion must be used to ensure the patient is stable enough to tolerate this. Blood pressure measurements can be taken conscious in well-behaved patients. This is achieved by placing a blood pressure cuff of 30 to 40 percent of the limb circumference over the superficial ulnar artery or the cranial tibial artery (Figure 3) and using a Doppler ultrasonic flow detector and sphygmomanometer to gain a systolic blood pressure (Fitzgerald and Beaufrère, 2016). Systolic blood pressure in avian patients is much higher than that of mammals, but indirect measurements do not always correlate with direct measurements (Acierno et al., 2008), and therefore its use as a diagnostic tool can be limited.
Computed tomography (CT) can also be used as a diagnostic tool and is an excellent method for detecting the presence of atherosclerosis in vessels; however, it is of limited diagnostic use for the heart itself, due to the high resting heart rate of avian species (Fitzgerald and Beaufrère, 2016). As patients are required to be completely still during CT scans, this method does require the use of a general anaesthesia (Figure 4) and should therefore only be considered in stable patients.
Cardiac disease in avian patients can be of congenital, degenerative, nutritional, infectious, toxic, neoplastic or idiopathic causes (Beaufrère and Guzman, 2016). There are multiple aetiologies to be considered, but the commonly encountered causes are discussed below.
Atherosclerosis, a chronic, inflammatory disease of the arterial walls, narrows the diameters of arteries with accumulations of cholesterol, fat, calcium and inflammatory cells (Beaufrère and Guzman, 2016). It has been reported to result from chronic excess calorific intake or low exercise, but genetic factors have also been identified (Chitty, 2008).
Atherosclerosis can result in episodes of ischaemia, thrombosis, haemorrhage and stenosis, often with sudden death being the only clinical sign (Jones, 2008). It is reported commonly in older psittacines at post-mortem; however, evidence of calcified vessels can be seen on radiographs (Lawrie, 2005). In psittacines, it is most commonly encountered in African grey parrots (Psittacus erithacus) and Amazons (Amazona spp.) (Beaufrère, 2013). Atherosclerosis is reported in birds of prey that become overweight and have limited physical activity in captivity (Jones, 2008). Risk factors include female birds, high-calorie diets, hypercholesterolaemia and limited physical activity (Fitzgerald and Beaufrère, 2016). Treatment involves dietary management and the use of vasodilators; however, ante-mortem diagnosis is difficult. Prevention, with appropriate weight management, reduced-calorie diets and encouraging physical activity, is key.
Congestive heart failure is the end stage of heart failure, occurring when cardiac output can no longer maintain arterial blood pressure. Right-sided congestive heart failure is more common than left-sided in avian patients, which results in the majority of cases displaying ascites, hepatic congestion, pericardial effusion, jugular distension and dyspnoea due to air sac compression (Beaufrère and Guzman, 2016). Effusions can be drained with ultrasound guidance, with transudate being identified via refractometry and cytology. Diagnostics are as described previously. Treatment involves stabilising the patient with oxygen therapy and draining any effusions that are large enough to cause dyspnoea or cardiac tamponade. Diuretics can be used to decrease smaller coelomic effusions and reduce fluid overload, and angiotensin converting enzyme (ACE) inhibitors assist ventricular filling and maintain cardiac output (Beaufrère and Guzman, 2016). The use of cardiac drugs in avian patients should be up to the veterinary practitioner’s discretion and follow the prescribing cascade.
Cardiotoxicity can be seen with a number of agents, but in pet psittacines there are a number to avoid in the home. Avocado flesh and seeds can cause pericardial effusion, myocarditis, myocardial degeneration and cardiac necrosis (Fitzgerald and Beaufrère, 2016). Acute death often follows, but supportive care for those that survive includes fluids and oxygen therapy (Lawrie, 2005). Chocolate contains caffeine and theobromine, resulting in tachycardia, hypertension and arrhythmias (Fitzgerald and Beaufrère, 2016). Treatment involves the use of gastroprotectants, activated charcoal and symptomatic supportive care, as for canine and feline patients (Lawrie, 2005).
Patients with cardiac disease should be closely monitored for demeanour, appetite, respiratory effort and signs of effusion. Owners should be encouraged to keep diaries of their animal’s progress, weight and clinical signs, which can be used in conjunction with follow-up examinations and repeat echocardiography.