Because the airway extends from the oral or nasal cavity to the alveoli, respiratory compromise has numerous manifestations.
Complications can be encountered in both the upper and lower airways. Anesthesia can cause further complications since anesthetic
drugs and equipment can exacerbate or even cause airway difficulties. Patients with compromised airways may present for surgery
of the respiratory system or may present for surgery of other organ systems with airway compromise as a complication. Regardless
of the reason for presentation, the patient should be critically evaluated before induction to anesthesia. The location, extent,
and severity of the problem should be carefully assessed and the degree of respiratory dysfunction should be determined. During
the evaluation, the patient should be handled quietly and carefully to avoid stress or fear-induced tachypnea with subsequent increased work of breathing and possible further respiratory dysfunction.
Tranquilization is often necessary to keep the patient calm and analgesia is required for any painful patients. Oxygen should be administered to all patients during handling. For induction to and maintenance of anesthesia, the choice of anesthetic
drugs is not necessarily dictated by the presence of respiratory compromise, but rather by the overall health of the patient. The
choice of anesthetic technique (eg, method of induction, method of intubation, use of positive pressure ventilation, etc.), on the other hand, is often
critical. For all patients, anesthesia should be thought of as 4 distinct and equally important phases (preanesthesia, induction,
maintenance and recovery) and a plan should be developed for each phase.
Anesthesia for Patients with Upper Airway Dysfunction or Disease
Preanesthesia: Patients presenting for surgery of the upper airway are often at risk for airway obstruction. Stress, pain or fear-induced
tachypnea, with subsequent increased work of breathing, will cause a tremendous increase in negative pressure in the airway
and this negative pressure can cause or exacerbate airway collapse or obstruction. Acid-base derangements and hypoxia will
occur due to ineffective ventilation through the narrowed airway and negative-pressure pulmonary edema can occur. Thus, the
patient should be adequately sedated prior to examination or handling. Acceptable sedatives include the opioids, acepromazine
and benzodiazepines. Alpha-2 agonists may be appropriate in some patients. Low dosages should be used in order to avoid excessive respiratory depression and the patient should not be left unobserved after sedation.
Analgesia should also be provided and the opioids are generally the most logical choice.
Administration of oxygen via face mask or nasal cannula is recommended, and sometimes critical. Delivery of 100% oxygen for
2-5 minutes ('preoxygenation') allows the functional residual capacity (or 'reservoir' of air in the lungs) to fill with oxygen.
With this technique, the patient can be apneic for 3-4 minutes without becoming hypoxemic. Patients that are not preoxygenated
will become hypoxemic following approximately 90 seconds of apnea.
The use of anticholinergics in the preoperative period is controversial. Laryngeal manipulation can cause a profound vagally-mediated
bradycardia, thus, anticholinergics are often used in brachycephalic breeds and other patients in whom intubation might be
difficult. The use of anticholinergics as antisialagogues and to decrease respiratory tract secretions is generally not recommended,
although exceptions exist. Anticholinergics change secretory composition from a watery fluid to a thick mucous. This mucous
may not be adequately cleared by the mucociliary system and may remain in the respiratory tree. A major drawback to the use
of anticholinergics is the subsequent tachycardia which increases cardiac work and cardiac oxygen demand. Cardiac oxygen demand
may not be met by oxygen supply (creating an oxygen debt) if ventilation is impaired in any manner, including airway obstruction
(ie, upper airway disease) and/or decreased diffusion of oxygen across alveolar walls (ie, lower airway disease).