The most striking features of birds is their ability to perform very rigorous functions in harsh environments such as diving deeply in cold water, flying at high altitudes, and running in hot deserts. This requires that their cardiovascular system (CVS) must be able to meet the demands of providing adequate delivery of oxygen to vascular beds that are taxed by extreme metabolic demands. The CVS must also efficiently remove metabolic byproducts to maintain function and hence performance.
Avian Heart Function & Adaptations
The avian heart is four-chambered. The right heart receives blood from the systemic circulation and pressurizes the pulmonary circulation. Blood returns to the left heart, where the left ventricle then pressurizes the systemic circulation. Both the right and left ventricles receive blood at the central venous pressure before they enter their respective outflow tract. This resistance to blood flow (peripheral resistance) is less on the pulmonary side when compared to the systemic side. For this reason, the left ventricle has more muscle mass to overcome the increased resistance on the systemic side to produce the same flow rate as that on the pulmonary side.
The relative size of the heart in birds is inversely related to a species’ body mass — the larger species of birds have reduced heart mass in comparison to the smaller ones. This arrangement differs from mammals as their body mass is directly proportional to the size of their hearts. This difference between birds and mammals may result from the fact that the heart from larger birds, such as the barnacle goose, can grow much larger (hypertrophy) prior to migration such that their hearts are proportional to their weight. Hummingbirds have proportionally larger hearts for their size, most likely because of the high aerobic demands of hovering flight.
The 4 chambers of the heart are completely divided into 2 atria and 2 ventricles. The right atrium tends to be larger than the left in most birds. The atria contract to empty blood into the ventricles during ventricular diastole, the time when they are not contracting. The left ventricle is cone-shaped and extends to the apex of the heart, while the right ventricle is a crescent-shaped cavity that does not go to the apex. Its right wall forms the interventricular septum that separates the 2 chambers. This allows the left ventricle to generate systolic (contraction) pressures that are 4 to 5 times greater than those produced by the right ventricle. Additionally, this anatomy is important for interpreting ultrasound diagnostics of the heart. Birds also have coronary arteries and veins that provide oxygen to this rapidly beating heart. There are different dynamics of these vessels so that birds that fly at high altitudes can dilate these vessels to continue to provide the oxygen they need to keep flying.
The pulmonary and systemic circulations have 3 main components: arteries or distributing vessels, the capillaries or exchanges vessels with the tissues, and the veins which store blood prior to return to the heart. The arterioles and venules are muscular vessels on either side of the capillary beds and act to control blood flow, thereby indirectly controlling the exchange of materials across the capillary bed with the tissues. Pressure that is generated by cardiac contraction drives the blood flow through the systemic and pulmonary circuits. Vascular resistance results mainly from the diameter of the blood vessels. Vessel size is controlled in part by muscular tone derived from the autonomic nervous system. All of the vessels are controlled by innervation from the parasympathetic and sympathetic nervous systems except for capillary beds.
Companion Bird Cardiovascular Health
Cardiac disease is not uncommon in pet birds, particularly the longer-lived psittacines that have largely been on high fat diets, with little exercise. Signs of cardiac disease may be subtle as most of our companion birds do little flying. When they do fly, they may breathe hard after exertion and their recovery to normal respirations will be longer than 3-5 minutes. Instead, the owner may notice decreased activity, pickiness regarding food items, reduced appetite, weakness and general lack of interest in activities in the home, such as playing with toys. Owners will relate that their parrot will be sitting on the perch and suddenly has increased respiratory effort and may have short periods of open mouthed breathing or they may not vocalize like they had previously. While this may also be from respiratory disease, your veterinarian needs to rule out if these signs could be cardiovascular in nature. Your bird may have a period of disorientation or may fall off the perch but then often recover relatively quickly. These are just some signs to watch for particularly as your parrot ages. Sometimes there may be no signs and there is just sudden death.
Until recently, equipment was not available that could evaluate avian heart function as well as aid in the diagnosis of atherosclerosis. The physical examination by your avian veterinarian should include watching your bird for a while to look at their posture, their overall character and attitude, and mobility, along with the respiratory rate and quality of breathing. The next step is to do a hands-on exam where the heart is auscultated with a pediatric stethoscope on the ventral chest in multiple locations to listen for soft murmurs or detect any changes from normal. Unfortunately, a normal auscultation does not rule out heart disease so additional tests may be performed.
In addition to auscultation, radiographs provide basic information regarding overall health along with heart size and the presence or absence of calcification of the great vessels to the heart. At our practice, we have observed that birds that do limited flying are more susceptible to heart disease along with those that eat seeds high in fats (like sunflower seeds) and other fatty foods. Radiographs help to determine if there is metabolic bone disease from the lack of flight and changes in the large vessels of the body along with the kidney — an organ that has a large number of blood vessels. Those with atherosclerosis will have calcification of the large vessels along with an increased brightness of the kidney on the radiographs.
Additional tests include monitoring blood pressures, measuring the electrical conductivity of the heart with an ECG, and performing an ultrasound of the heart. Future tests including computed tomography are just around the corner for our bird patients.