- Air sac cannulas are routinely used to ventilate birds by a route other than endotracheal intubation. Air sac tubes are used for oxygenation and anesthesia, especially during surgery of the head or trachea where tracheal intubation would be cumbersome. In addition, air sac cannulas provide a means to medicate air sacs directly, and they are also used to aid dyspneic birds with tracheosyringeal obstruction from foreign bodies, granulomas, or tumors.
- Air sac cannulas are commercially available. Makeshift cannulas may be created from sterile endotracheal tubes or plastic tubing cut to the appropriate length.
- In parrots, pigeons, doves, and songbirds, insert the air sac cannula into the caudal thoracic or abdominal air sac.
- Air sac cannulae may be left in place for up to 7 days. However, they ideally should be removed within 4 days because of increased risk of bacterial or fungal infection with prolonged tube placement.
The lungs of birds are small, compact, spongy structures that are fitted closely against the contours of the ribs on either side of the spine. Avian lungs weigh as much as the lungs of mammals of similar body weight, but because avian lungs have much greater tissue density, they occupy about one-half the volume.
The air sac system is a vital part of the avian respiratory system. Air sacs are thin-walled structures, one to two cell layers thick, that extend throughout the body cavity and into the wing and leg bones. Most birds have nine air sacs: two cervical, two cranial thoracic, two caudal thoracic, two abdominal, and a single interclavicular air sac. The abdominal air sacs carry air to leg and pelvic bones; the interclavicular sac branches into the wing bones, sternum, and syrinx.
The air sacs connect directly to the bronchi and make possible an effective continuous flow of air through the lungs. Air flows into the avian lungs through a set of bronchi into the air-capillary system, which is the primary site of oxygen and carbon dioxide exchange. There is a constant flow of air through the avian lung, allowing a more efficient extraction of oxygen than occurs in the mammalian lung. Half of the inhaled air during the first inhalation in this cycle passes through the primary bronchi to the posterior air sacs. During the exhalation of the first breath, the inhaled air moves from the posterior air sacs into the lungs, where it flows through the gas-exchange areas. The next time the bird inhales, this oxygen-depleted air moves into the anterior air sacs. During the second exhalation, the carbon dioxide-rich air is then expelled from the anterior air sacs, bronchi, and trachea back into the atmosphere. The constant airflow through the avian lung allows more efficient extraction of oxygen than occurs in the mammalian lung.
Air sac cannulas or tubes are used routinely to ventilate birds by a method other than endotracheal intubation. Air sac cannulas are used for oxygenation and anesthesia, especially during head or trachea surgery where tracheal intubation would be cumbersome. Air sac cannulation is also used therapeutically to aid dyspneic birds. These are often birds with upper respiratory tract foreign bodies, or tracheosyringeal obstruction from granulomas or tumors. Less commonly, air sac cannulas are placed because of tracheal trauma, collapse, or obstruction. In addition, air sac cannulas provide a means to medicate air sacs directly.
Air sac cannulas have also been used in research. Small songbirds are used for studies on neural mechanisms that underlie vocal learning. Researchers have performed survival surgeries for localization of specific brain regions and have used air sac cannulation to deliver anesthetic gas to zebra finches (Taeniopygia guttata) during neurosurgery. Included among the advantages of this method are that it leaves the birdís head free for stereotaxic targeting and does not interfere with the beak clamps that are often used to position and stabilize the animalís head. In addition, this method allows for the use of isoflurane, allowing fast, minimally stressful induction, and preventing lengthy postoperative recovery times.
Air sac cannulas can be left in place for up to 7 days, however cannulas should ideally be removed within 4 days because of increased risk of bacterial or fungal infection and subsequent air sacculitis associated with prolonged tube placement. Other possible complications of air sac tubes include severe air sac damage, occlusion of the cannula with exudate or fluid, and abdominal organ damage. Life-threatening blood loss can occur during placement of the cannula if an internal organ is penetrated. Subcutaneous emphysema commonly occurs when the cannula is removed, but is self-limiting and will diminish with time.
Minimal equipment is necessary for air sac cannula placement. A surgical blade (size no.11 or no. 15), a fine mosquito hemostat, a needle holder, a pair of scissors, and suture material are all that is required to perform the procedure (Fig 1). A sterile endotracheal tube cut to the appropriate length, trimmed rubber tubing, a commercially available air sac cannula, or plastic tubing from an IV extension set can be used as an air sac cannula.
The surgical site should be aseptically prepared. Sterile, water-soluble lubricant can be used to keep surrounding feathers from getting into the surgical site. A small, sterile, self-adhesive drape is used to isolate the surgical site. The size of the cannula and suture material will vary depending on the size of the bird.
In parrots, pigeons and doves, and passerine birds, the aim is to insert the air sac cannula into the caudal thoracic or abdominal air sac (Fig 2). The bird is placed in lateral recumbency and the upper leg is pulled cranially. The entry site is at the junction of the caudal edge of the last rib and the flexor cruris medialis muscle. The pubic bone is an additional landmark that outlines a triangular shaped region for cannula placement (Fig 3).
The site is prepared by gently plucking feathers and performing a sterile surgical prep of the skin. A sterile drape is placed, a small incision is made through the skin, and the mosquito forceps are used to bluntly dissect through the muscle wall and enter the air sac. The jaws of the mosquito forceps are opened and the cannula is inserted between them into the air sac (Fig 5 and Fig 6).
If the cannula is placed appropriately, condensation will appear within the cannula tube or on a glass slide placed at the end of the cannula. A down feather can also be held at the end of the cannula to check for gentle air movement as the bird respires (Fig 7). Use caution not to allow the feather to be inhaled into the air sac. To secure the tube in place, a piece of tape can be affixed around the proximal aspect of the cannula, which is then secured to the skin. Alternatively, a finger trap suture technique can be used to secure the tube in place (Fig 8). It is important to re-assess patency after the cannula is sutured in place, as the cannula’s position may shift, resulting in an obstructed airway if the cannula is compressed against an internal structure. Radiographs are often used to assess proper tube placement by determining the length of the tube, its location in the air sac, and whether the tube is affecting other internal organs. Until experience is developed with this procedure, it is advisable to obtain both ventrodorsal and lateral radiographs of the bird once the tube is in place.
- Culture the air sac tube after removal. Cytologic analysis of the tube tip may also be warranted.
- If an endotracheal tube is used as an air sac cannula, keep the adaptor nearby for emergency oxygen supplementation.
- Radiographs may be necessary before air sac tube placement to ensure the tube is not placed into a severely diseased air sac. With diffuse airsacculitis, air sac tube placement is not generally recommended.
- Air sac cannulas can be placed endoscopically to ensure they are placed correctly.