Comparison of Anesthetic Effects on the Airway
General anesthesia creates a host of complications for normal respiration, from simple changes in posture to the loss of consciousness and natural reflexes associated with anesthetic and neuromuscular blocking agents. Understanding the complex ways in which an anesthetic impacts a patient’s ability to maintain their airway and breathe, both independently and in tandem with other medications, is essential to ensuring safe care.1
Ventilation is most immediately inhibited under general anesthesia because of the loss of a patent airway: when the pharyngeal muscles are relaxed, the tongue is displaced posteriorly and a natural pathway for ventilation is lost.2 An endotracheal tube or laryngeal masked airway is used to promote breathing under anesthesia.3 However, commonly used drugs like propofol, which induces anesthesia, can inhibit normal coughing and secretion management, leading to hypoxia (low oxygen) and hypercapnia (high carbon dioxide levels).1,2 Other anesthetic agents, like isoflurane and ketamine, can exacerbate mucous and saliva production—isoflurane is particularly pungent and can increase coughing and airway hyper-reactivity.1,4 An advantage of ketamine, however, is that the drug preserves laryngeal reflexes, allowing the patient to sustain a patent airway.2 Ketamine is known to have limited effects on both the patient’s respiration rate as well as their gas exchange.5
Gas exchange is also impacted by most drugs used in anesthesia. When the anesthetic is administered, the closure of the airway (described above) and the rapid collapse of the lungs’ alveoli result in reduced oxygenation.6 Alveolar collapse prevents ventilation, or the distribution of gas through the alveoli, but does not affect perfusion, or the distribution of blood flowing through the pulmonary capillaries. As a result, blood passing through the alveoli may not become oxygenated, a condition called “shunt,” and hypoxemia can occur.2 Hypoxic pulmonary vasoconstriction (HPV) is a protective mechanism for constricting pulmonary capillaries when alveolar oxygen is low, but volatile anesthetics inhibit HPV in a dose-dependent manner. These volatile drugs include sevoflurane7 and halothane, an older drug that has largely been replaced by sevoflurane.2 Halothane and a more commonly used drug, isoflurane, may inhibit HPV by as much as 50 percent. Drugs that induce anesthesia through an IV do not appear to create this effect, and propofol may help preserve HPV. 1
Anesthesia providers must employ a number of tactics to preserve oxygenation and prevent shunt. These include pre-oxygenation, which can maximize the oxygen content in the amount of gas in the lungs at the end of normal expiration (also called maximizing the functional residual capacity, or FRC). Mechanical ventilation is also used, but proper precautions must be taken to ensure the lungs are not damaged with its use. High positive end-expiratory pressure (PEEP) can also be used to support the alveoli and even open collapsed portions of the lung.2 By assiduously monitoring a patient’s respiration throughout surgery, anesthesia providers can work to counter the pulmonary impairment inherent to their practice.
References
1. Saraswat V. Effects of anaesthesia techniques and drugs on pulmonary function. Indian J Anaesth. 2015;59(9):557. doi:10.4103/0019-5049.165850
2. Davison R, Cottle D. The Effects of Anesthesia on Respiratory Function. Anaesthesia Tutorial of the Week. World Federation of Societies of Anesthesiologists. https://www.frca.co.uk/Documents/205%20The%20effects%20of%20anaesthesia%20on%20respiratory%20function.pdf. Published 2010.
3. General Anesthesia | Michigan Medicine. University of Michigan Health. https://www.uofmhealth.org/health-library/rt1584. Published 2019.
4. Sahu D, Parampill R, Kaul V. Comparison of isoflurane and sevoflurane in anaesthesia for day care surgeries using classical laryngeal mask airway. Indian J Anaesth. 2011;55(4):364. doi:10.4103/0019-5049.84857
5. Mankikian B, Cantineau J, Sartene R, Clergue F, Viars P. Ventilatory Pattern and Chest Wall Mechanics during Ketamine Anesthesia in Humans. Anesthesiology. 1986;65(5):492-499. doi:10.1097/00000542-198611000-00007
6. Hedenstierna G. Alveolar collapse and closure of airways: regular effects of anaesthesia. Clin Physiol Funct Imaging. 2003;23(3):123-129. doi:10.1046/j.1475-097x.2003.00483.x
7. Oksar M, Koyuncu O, Turhanoglu S. Severe hypoxemia follows hypoxic pulmonary vasoconstriction and/or hypoxic pulmonary vasoconstriction inhibition by inhaled anesthetics: prognostic potential of 100% shunt fractions. Brazilian Journal of Anesthesiology (English Edition). 2017;67(6):664-665. doi:10.1016/j.bjane.2016.02.003