The Effects of General Anesthetics on Upper Airway Collapsibility in Healthy Subjects

Overview

The investigators hypothesize that propofol, when compared to sevoflurane, causes the upper airway to collapse more easily and causes less activity in the tongue muscle. Additionally, the investigators hypothesize that, under increased carbon dioxide concentrations of the air inhaled, the upper airway will be less likely to collapse under anesthesia and there will be increased activity in the tongue muscle under both propofol and sevoflurane, when compared to breathing normal concentrations of carbon dioxide, as in room air. Furthermore the investigators hypothesize that anesthesia disrupt the breathing swallow coordination, an effect additionally altered by increased carbon dioxide through increased respiratory drive.

Upper airway patency depends on an appropriate balance between the dilating force of pharyngeal muscles and the collapsing force of negative intraluminal pressure, which is generated by respiratory "pump" muscles. The genioglossus (GG) protects pharyngeal patency in humans. This muscle receives various types of neural drive, distributed differentially across the hypoglossal motoneuron pool, including phasic (inspiratory) and tonic (non-respiratory) drives. In addition, reflex GG activation in response to negative pharyngeal pressure stabilizes upper airway patency both in humans and in rats. General anesthetic agents, including propofol and sevoflurane, predispose the upper airway to collapse, at least in part by decreasing upper airway muscle activity. Theoretically anesthetics could affect upper airway dilator activity by several mechanisms, including an anesthetic-induced, dose-dependent decrease in hypercapnic and hypoxic ventilatory drive, hypoglossal motoneuron depression, decreased skeletal muscle contractility, an increase in phasic GG activity as a result of decreased arterial blood pressure, and an increase in phasic hypoglossal nerve discharge. Previous studies have shown that certain anesthetics, including pentobarbital and isoflurane, can increase genioglossus phasic activity in rats and in humans. The effects of propofol on airway collapsibility have been studied in humans however, to our knowledge, they have not been measured under conditions of hypercapnia. Studies of airway collapsibility under sevoflurane anesthesia have been performed in children, but no data exists for airway collapsibility in sevoflurane-anesthetized adults. Similarly no data exists on the effects of sevoflurane on GG activity In a previous trial of pentobarbital-anesthetized volunteers, the investigators observed that mild hypercapnia (5 - 10 mmHg above baseline) produced a significant increase in flow rate and GG phasic activity, as well as a smaller increase in GG tonic activity. If our proposed study shows a beneficial effect, then the investigators plan a follow-up study addressing the possibility that hypercapnia may be used therapeutically for airway protection. A similar concept has already been considered for critically ill ICU patients. However, previous studies have shown that a hypercapnia-induced increase in ventilatory drive can inhibit airway protective reflexes by disrupting the breathing swallowing coordination. In order to assess the safety of induced mild hypercapnia as an intervention for airway protection, we evaluated whether variable levels of hypercapnia occurring during anesthesia with sevoflurane and propofol impair the coordination of breathing and swallowing compared with the effects of anesthesia alone. With this pharmaco-physiological interaction study on healthy adults we aim to: 1. Compare the effects of sevoflurane and propofol on upper airway closing pressure, upper airway muscle control and breathing. 2. Assess the effects of evoked hypercapnia (carbon dioxide reversal) on propofol-induced upper airway collapsibility 3. Evaluate the effects of sevoflurane, propofol, and induced hypercapnia on coordination of breathing and swallowing. Comparative drug studies on airway effects of anesthetics in humans are important for defining an optimal anesthetic regimen for patients at risk of airway collapse, such as patients with obstructive sleep apnea. Our studies are also particularly relevant for patients undergoing procedural sedation, which is typically being conducted under spontaneous ventilation with the upper airway being unprotected. In addition, our results may increase our understanding of postoperative airway obstruction, a common complication in the post-anesthesia recovery room.