Relevant Anatomy
Ventilation (Trachea, Larynx, Bronchi, Syrinx, Air Sacs, Muscles)
Trachea
Complete tracheal rings
Tracheal anatomy is variable in avian species
Emu and male ruddy duck have normal diverticulum
Larynx
Bronchi
Syrinx
Located at trachea and bronchi junction
Sound production
Syringeal bulla (normal finding on radiographs of male waterfowl)
Air Sacs
No gas exchange
Diverticula to some vertebra and ribs, humerus, and femur
Serve as bellows to normally rigid avian lung
Can use to ventilate with upper airway obstruction, surgery, etc.
Muscles
Inspiration and expiration are active processes
Contract inspiratory muscles to achieve volume change in air sacs
Manual restraint can compromise respiration
Gas-Exchange
Parabronchi
Air capillaries anastomose with blood capillaries
Location of gas exchange
Parabronchi Are Either Paleopulmonic Or Neopulmonic
Paleopulmonic parabronchi are present in all bird species (only type in emu and penguin) and have uni-directional flow only
Neopulmonic are found in many species and have bi-directional flow on inspiration and expiration (complex anastomoses)
Cross-current gas exchange (efficient)
Parabronchial gas exchange occurs along entire length (not like mammalian alveoli)
Ventilatory Control
Birds have intrapulmonary chemoreceptors (that differ from mammals)
Inhibited by high lung PCO2
Control rate and depth of breathing
Heart
Lower heart rates and higher cardiac output than mammals of equivalent size
Excitement and handling will increase release of norepinephrine and epinephrine from sympathetic nerves innervating the atria and ventricles
Cardiac arrhythmia and suppressed cardiac function can ensue
Cardiac function is easily depressed by hypoxia, hypercapnia, and inhalant anesthetics
Renal-Portal System
Has little effect on anesthetic induction and distribution even when anesthetic agents are injected into legs
Anesthesia
Apnea and Bradycardia
Has been termed "dive" response (waterfowl)
Actually stress response
Anesthetic facemask stimulates trigeminal nerve receptors in the beak
Can compromise induction
May blunt response with premedication (butorphanol or midazolam)
May explain why waterfowl are considered higher risk for anesthesia
Birds lack significant functional residual volume (birds do not tolerate periods of apnea)
Prior to Anesthetic Induction
Physical examination
Minimize stress/handling
May fast for several hours (depends on species and procedure)
Premedication will allow for a smoother anesthetic induction (butorphanol, midazolam or combination of the two)
Induction
Pre-oxygenate when possible
Intramuscular or intravenous in large species (ratites, e.g., ostrich)
Ratites: ketamine/alpha-2 combination IM, propofol IV if restrained
Inhalant by mask (isoflurane or sevoflurane in 100% oxygen)
Breathing circuits used are typically non-rebreathing (Bain or Norman elbow)
Fresh gas flows should be two to three times minute volume ~200ml/kg/minute
Intubation
Usually birds over 100 grams body weight
Small tubes can increase resistance
Cautious of mucous plugs and obstructions
Often due to dry and cold anesthetic gases
Anticholinergic may help decrease mucous production
Humidi-vent® may help prevent airway drying (but increases dead-space)
Murphy eye in tube will help against complete airway obstruction
Endotracheal tube should not fit tightly
Complete tracheal rings
May not see tracheal damage for up to 7 days after extubation
MAC
Do not have alveoli so refers to Minimum Anesthetic Concentration
Anesthetic concentration to prevent purposeful movement
Values similar to MAC in mammals
Maintain anesthesia with isoflurane or sevoflurane and oxygen
Anesthetic Index
Measures tendency for inhalant to cause respiratory depression
Anesthetic index can be calculated as apneic concentration divided by MAC
Significantly lower in birds (versus mammals)
Inhalants cause more ventilatory depression in birds versus mammals
Cardiac Arrhythmias
Occurs with moderate hypercapnia
Need to assist ventilation or use controlled ventilation
IPPV (Intermittent positive pressure ventilation)
Inspiratory pressure of 10–15 cm H2O and 6–10 breaths/minute
Anesthetic Monitoring
Heart rate (external or esophageal stethoscope and Doppler probe)
ECG (note difference in QRS versus small mammals)
Monitoring respiratory frequency alone is not sufficient to assess ventilation or anesthetic depth
Not necessarily light with increased respiratory rate
Increased respiratory rate will decrease tidal volume of breath and cause greater dead-space ventilation (not effective ventilation)
During general anesthesia minute ventilation is decreasing over time in spontaneously breathing birds
Life-threatening hypoxemia may result without assisted ventilation
Capnometry (sampling rate may exceed minute ventilation in small birds)
Pulse oximetry
Standardized and calibrated for humans
Absorptive characteristics of hemoglobin are different in birds
May underestimate hemoglobin saturation
Readings do not correlate well with arterial blood gas analysis
Blood pressure
Doppler probe and sphygmomanometer with attached cuff (ulnar or median metatarsal artery)
Direct in larger species via ulnar or median metatarsal artery
Difficult, Doppler probe will help record heart rate
Temperature
Esophageal probe
Hypothermia
Causes decreased anesthetic requirement
Cardiac instability
Prolongs recovery
Decreases oxygen delivery to tissues
Contributes to the development of metabolic acidosis
Shifts hemoglobin dissociation curve
Assess depth
Corneal response, toe, and cloacal pinch
Respiratory and heart rates
Fluid Therapy
Intravenous (basilic or ulnar, metatarsal, and jugular veins)
Intraosseous (easier to maintain a catheter)
Distal ulna or proximal tibiotarsus
Recovery
Don't allow to recover without assistance to prevent self trauma
Hold upright, extubate when jaw tone returns
Continue to provide oxygen by facemask until completely recovered
May need to continue to provide thermal support at recovery
Provide darkened, undisturbed environment for continued recovery
Analgesia
Opioids (butorphanol)
NSAIDS (meloxicam, carprofen, etc.)
Take home message
During avian anesthesia always assist or control ventilation, use supplemental heating devices if warranted, provide analgesia, and provide comprehensive anesthetic monitoring. Vigilant monitoring of the anesthetized bird will help detect endotracheal tube occlusion, changes in ventilation, and cardiac arrhythmias. Assist recovery with manual restraint and provide supplemental oxygen (unless it is too dangerous, e.g., ratite). Maintain minimal disturbance at recovery to minimize stress. You often need to continue to provide thermal and fluid support during the postoperative period. Continue analgesic administration as necessary and at appropriate intervals. Carefully observe respiration after extubation to monitor for airway obstruction or apnea.