Differential effects of isoflurane and propofol on upper airway dilator muscle activity and breathing.
BACKGROUND
Anesthesia impairs upper airway integrity, but recent data suggest that low doses of some anesthetics increase upper airway dilator muscle activity, an apparent paradox. The authors sought to understand which anesthetics increase or decrease upper airway dilator muscle activity and to study the mechanisms mediating the effect.


METHODS
The authors recorded genioglossus electromyogram, breathing, arterial blood pressure, and expiratory carbon dioxide in 58 spontaneously breathing rats at an estimated ED50 (median effective dose) of isoflurane or propofol. The authors further evaluated the dose-response relations of isoflurane under different study conditions: (1) normalization of mean arterial pressure, or end-expiratory carbon dioxide; (2) bilateral lesion of the Kölliker-Fuse nucleus; and (3) vagotomy. To evaluate whether the markedly lower inspiratory genioglossus activity during propofol could be recovered by increasing flow rate, a measure of respiratory drive, the authors performed an additional set of experiments during hypoxia or hypercapnia.


RESULTS
In vagally intact rats, tonic and phasic genioglossus activity were markedly higher with isoflurane compared with propofol. Both anesthetics abolished the genioglossus negative pressure reflex. Inspiratory flow rate and anesthetic agent predicted independently phasic genioglossus activity. Isoflurane dose-dependently decreased tonic and increased phasic genioglossus activity, and increased flow rate, and its increasing effects were abolished after vagotomy. Impairment of phasic genioglossus activity during propofol anesthesia was reversed during evoked increase in respiratory drive.


CONCLUSIONS
Isoflurane compared with propofol anesthesia yields higher tonic and phasic genioglossus muscle activity. The level of respiratory depression rather than the level of effective anesthesia correlates closely with the airway dilator muscle function during anesthesia.
Eikermann
Matthias
M
Department of Anesthesia, Massachusetts General Hospital, and Harvard Medical School, Boston, Massachusetts 02114-2696, USA. meikermann@partners.org
Malhotra
Atul
A
Fassbender
Philipp
P
Zaremba
Sebastian
S
Jordan
Amy S
AS
Gautam
Shiva
S
White
David P
DP
Chamberlin
Nancy L
NL
D000818:Animals; D001794:Blood Pressure / Q000187:drug effects; D004305:Dose-Response Relationship, Drug; D004576:Electromyography; D007530:Isoflurane / Q000494:pharmacology*; D008297:Male; D023421:Models, Animal; D009043:Motor Activity / Q000187:drug effects; D018482:Muscle, Skeletal / Q000187:drug effects / Q000502:physiology*; D015742:Propofol / Q000494:pharmacology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D012132:Respiratory Muscles / Q000187:drug effects / Q000502:physiology*; D012143:Respiratory Physiological Phenomena / Q000187:drug effects*
D007530:Isoflurane; D015742:Propofol