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.