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August 1998
Upper airway reflexes during a combination of propofol and fentanyl anesthesia.
Tagaito Y, Isono S, Nishino T. Anesthesiology. 1998;88:1459-1466.
[ see abstract below ]
Laryngeal reflexes: Exploring terra incognita.
Warner DO. Anesthesiology. 1998;88:1433-1434.
No abstract available
Most anesthesiologists venture only a short distance into their training before they develop an intense interest in the treatment and prevention of laryngospasm. And this interest remains throughout a lifetime of clinical practice, particularly when that practice involves infants and children. Because in vivo evaluation of laryngeal function has been difficult, systematic clinical investigation of laryngeal reflexes has largely been absent since the work of Fink (Anesthesiology. 1956;17:569-77 and The Human Larynx: A Functional Study, Raven Press, New York, 1975). As reported in the above extension of the previous ground-breaking work from the same institution (Anesthesiology. 1996; 84:70-74), Tagaito and colleagues at Chiba University School of Medicine in Japan have addressed this difficulty. The accompanying editorial by David Warner nicely puts their accomplishments into perspective and provides a list of questions that could keep several teams of investigators busy assessing the components of various airway reflexes for years to come. Briefly, the authors used simultaneous measurements of laryngeal behavior (as seen through a fiberoptic endoscope inserted through a laryngeal mask airway) and function (gas flow). Measurements were made in propofol-anesthetized patients during quiet breathing, after stimulation of upper airway receptors (0.2 mL distilled water injected onto the laryngeal mucosa), and after stimulation in the presence of increasing doses of fentanyl. They also independently measured the effects of the progressive hypercapnia associated with fentanyl administration by dividing the fentanyl patients into subgroups with and without ventilation controlled to normocapnia. They found that:
- Laryngeal stimulation in propofol-anesthetized patients activates vigorous airway reflexes.
- Incremental doses of fentanyl (in the presence and absence of controlled ventilation) depress the incidences of the expiration reflex, spasmodic panting and (particularly) cough produced by stimulation.
- In spontaneously breathing (hypercapnic) patients, incremental doses of fentanyl also suppress apnea with laryngospasm (incidence <20% after 200 mcg fentanyl).
- Normocapnic patients had a greater incidence and duration of apnea with laryngospasm after low (50 mcg) doses of fentanyl than those anesthetized with propofol alone. After increasing doses, apnea decreased progressively. More important than the results, however, are the methods themselves.
As Warner points out, the answers to many additional clinically relevant questions can be addressed using this measurement system. We will await these answers with interest. An even greater challenge will be for these and/or other investigators to devise a way to extend the methods to allow evaluation of patient safety during lighter levels of sedation. Objective evaluation of the extent to which protective laryngeal reflexes are attenuated by differing amounts of individual drugs and commonly used drug combinations would provide guidance for rational selection for sedation of patients with "full stomachs".
An MPEG video file related to the Tagaito paper can be downloaded from the Anesthesiology web site. This is the "additional material" promised to readers of the journal Anesthesiology, and it is worth its download time.
Return to the Current Literature Review Front Page, or read the abstract:
ABSTRACT
Background: The effects of intravenous anesthetics on airway protective reflexes have not been fully explored. The purpose of the present study was to characterize respiratory and laryngeal responses to laryngeal irritation during increasing doses of fentanyl under propofol anesthesia.
Methods: Twenty-two female patients anesthetized with propofol and breathing through the laryngeal mask airway were randomly allocated to three groups: (1) eight patients who received cumulative total doses of 200 µg fentanyl given in the form of two doses of 50 µg and one dose of 100 µg spaced 6 min under mechanical controlled ventilation while
end-tidal carbon dioxide tension (PCO2) was maintained at 38 mmHg (fentanyl-controlled ventilation group), (2) eight patients who received cumulative total doses of 200 µg fentanyl while breathing spontaneously while end-tidal PCO2 was allowed to increase spontaneously (fentanyl-spontaneous ventilation group), and (3) six spontaneously breathing patients who were anesthetized with propofol alone (propofol group). The laryngeal mucosa of each patient was stimulated by spraying the cord with distilled water, and the evoked responses were assessed by analyzing the respiratory variables and endoscopic images.
Results: Before administration of fentanyl, laryngeal stimulation caused vigorous reflex responses, such as expiration reflex spasmodic panting, cough reflex, and apnea with laryngospasm. Increasing doses of fentanyl reduced the incidences of all these responses, except for apnea with laryngospasm, in a dose-related manner in both the fentanyl-controlled ventilation and the fentanyl-spontaneous ventilation groups. Detailed analysis of endoscopic images revealed several characteristics of laryngeal behavior during the airway reflex responses.
Conclusion: Incremental doses of fentanyl depress airway reflex responses in a dose-related manner, except for apnea with laryngospasm. (Key words: Airway; fentanyl; intravenous anesthetics; opioids; propofol; reflex.)
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