Avian respiratory and thoracic surgery (Proceedings)
Anatomy and Physiology
The respiratory system of birds is significantly different, both physiologically and anatomically, from that of mammals. These differences are a result of adaptations to the demands associated with flight resulting in an extremely efficient gas exchange system allowing them to breathe at high altitudes (low O 2 levels) and maintain their high metabolic rate. The nares are the beginning of the avian respiratory tract and may be located anywhere along the beak from the tip (kiwis) to the base. Some species have an operculum to filter large particles from inspired air. Birds have three conchae (rostral, middle, and caudal). The infraorbital sinus is the only paranasal sinus of birds and has a rostral diverticulum within the beak, the preorbital diverticulum rostral to the eye, the infraorbital diverticulum ventral to the eye, the postorbital diverticulum caudal to the eye and the mandibular diverticulum that extends into the caudal mandible. The nasal passages and sinus communicate with the choana, a normal anatomic cleft in the roof of the mouth. The choana has fine papillae along its margins. During respiration, the glottis abuts the choana allowing birds to breathe through the nares. The glottis of birds is simple with no epiglottis making them more susceptible to foreign body aspiration. The glottis is located at the base of the tongue and is usually accessible even in awake birds but is not involved in sound production.
The trachea is long compared with mammals with complete cartilagenous rings it bifurcates just cranial to the heart as the syrinx. The syrinx produces sound and is composed of modified tracheal cartilages forming flexible membranes to which muscles attach. The muscles are used to cause vibration producing sound. The trachea becomes narrower progressing caudally with the syrinx being the narrowest portion of the trachea which predisposes foreign objects to lodge and granulomas to form there. Each primary bronchus enters one lung and divides into secondary bronchi which further subdivide into parabronchi. Inspired air moves from the parabronchi into atria which open along the walls of the parabronchi. Air then flows into air capillaries, the avian analog of alveoli being the gas exchange site. However, unlike mammalian alveoli, air capillaries communicate with each other and with other atria so air cannot become entrapped and emphysema does not occur in birds. The lungs of birds are fixed adhered dorsally to the ribs and vertebrae. They do not expand and collapse during respiration. The lungs are directly connected to the air sacs of which there are 4-5 sets (cervical, interclavicular, cranial thoracic, caudal thoracic, and abdominal). The cervicocephalic air sac communicates with the infraorbital sinus and not directly with the lungs.The respiratory cycle in birds is complicated but the majority of air requires two complete breathes to make it through. Most of the inspired air bypasses the lungs and goes into the caudal thoracic and abdominal air sacs during the first inspiration. During the first exhalation, air moves from these caudal air sacs into the lungs where gas exchange occurs. With the second inspiration, air moves from the lungs into the cranial air sacs and during the second exhalation it leaves through the trachea. This creates a unidirectional flow of air through the lungs. Blood moves in a direction opposite to that of air flow creating a counter-current exchange which is very efficient for gas exchange. Air is moved through the respiratory system by movement of the sternum so if a bird cannot move its sternum it will not be able to move air.
Air Sac Cannulae
A tube inserted into an air sac can obviate, at least temporarily, the need for respiration through the trachea. This can be a life-saving procedure in birds with upper airway obstruction. An air sac cannula can also provide a means by which oxygen and anesthetic gases can be supplied to birds while working on the mouth, sinus, trachea, glottis, choana, and the cranial coelomic cavity. The tube can be placed either in the caudal thoracic or abdominal air sac using one of two insertion sites. To place an air sac tube into the caudal thoracic, a nick incision is made just cranial to the thigh muscles over the last intercostal space. The intercostal muscles are penetrated using a mosquito hemostat to create a hole large enough to insert the tube. If the bird has a respiratory obstruction, as soon as the hole is created, air can be heard passing into and out of the hole. The tube is only advanced a few millimeters so it does not butt against internal structures. Suture the tube in place using a finger trap or butterfly tape technique.
The second site commonly used for insertion of an air sac cannula is caudal to the thigh muscles into the abdominal air sac. I find tubes placed in this location dislodge more easily. The leg is pulled cranially and a nick incision is made where the caudal thigh muscles cross the last rib. A mosquito hemostat is used to bluntly dissect just caudal to the last rib entering into the air sac. If the hemostat is too parallel to the body wall it will not penetrate the body wall but will dissect subcutaneously. Once the tube is inserted the leg is allowed to fall into a normal position and the tube is secured in place. It is probably best not to leave an air sac cannula in place more than 5 days and birds with air sac tube should be placed on a broad spectrum, systemic antibiotic.