Specific Effects of Anesthetics
Postoperative hypoxemia and atelectasis are common causes of perioperative
morbidity. Many factors may affect mucociliary function in the mechanically ventilated
patient. Poorly humidified inspired gases are known to reduce ciliary movement and
desiccate mucus. The flow rates of mucus were maintained in the normal range during
a 40-minute exposure to an inspired air temperature greater than 32°C when inspired
water vapor content was at least 33 mg/L in dogs.[97]
However, 3 hours of inhalation of dry air resulted in a complete cessation of the
flow of tracheal mucus, which was restored by subsequent use of inspired gases with
100% relative humidity at 38°C.[98]
Other factors
that reduce the rate of mucus movement are high inspired oxygen concentration, inflation
of an endotracheal tube cuff, and positive-pressure ventilation.[99]
The effects of inhaled anesthetics on mucociliary flow have been
determined with scintillation counters measuring radioactive droplet progression
in the trachea of dogs ( Fig. 6-6
).
[99]
[100]
[101]
Halothane alone, enflurane alone, nitrous oxide with halothane, and nitrous oxide
with morphine produced similar dose-dependent decreases in mucociliary movement.
[100]
[101]
Clearance
of mucus from central and peripheral airways after a 6-hour exposure to 1.2 MAC of
halothane was delayed by at least 3 hours.[102]
Volatile anesthetics and nitrous oxide may diminish rates of mucus clearance by
decreasing ciliary beat frequency, disrupting metachronism, or altering the physical
characteristics or quantity of mucus. Exposure to inhaled anesthetic caused dose-related
decreases in ciliary activity and cellular mobility of the ciliated protozoan Tetrahymena
pyriformis.[103]
The dose that effectively
terminated organism and ciliary movement in 50% of the protozoans closely corresponded
to MAC values of the various anesthetics. The mechanism by which cilia were affected
was not delineated in this study, but the speed and reversibility of ciliary depression
suggested that ATP metabolism was not responsible. Extrapolation from the protozoan
to mammalian airway epithelia must be performed with caution. Nevertheless, the
results of this interesting study suggest that anesthetics produce a direct inhibitory
effect on mucus clearance. High concentrations of halothane (at doses greater than
3%) also depressed ciliary movement in cultures of canine ciliated epithelium.[104]
The sensitivity of cilia to halothane appeared to be less pronounced than that observed
in T. pyriformis.[103]
The effects of inhaled anesthetics on ciliary beat frequency in humans have yet
to be elucidated.
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