Effects of inert gas narcosis on breathing and effort sensations
during exercise and inspiratory resistive loading.
Fothergill, D. M., and N. A. Carlson.
Naval Medical Research Institute, Bethesda, MD, 20889-5607
APStracts 3:0291A, 1996.
The influence of inert gas narcosis on exercise tolerance and on the
psychophysical perceptions of exertion and respiratory responses to
steady state exercise with inspiratory resistive loading was studied
in thirteen male U.S. Navy divers. Each diver performed an
incremental bicycle exercise test at 1 atm abs to volitional
exhaustion while breathing a narcotic gas mixture (23% N2O, 44% O2,
15% He, balance N2) and a non-narcotic gas mixture (44% O2, balance
N2) of the same PO2, density, and viscosity. The same gas mixtures
were used during four subsequent trials in which the subjects
breathed the mixtures both with and without an inspiratory resistance
(5.5 vs 1.1 cmH2O/l/s at 1 l/s) during 10 min of bicycle exercise at
40% of maximum aerobic workload (Wmax) followed by 20 min at 75%
Wmax. Throughout each test subjective ratings of respiratory effort
(RE), leg exertion (LE) and narcosis were obtained at 3-min intervals
using a category-ratio scale. The mean level of narcosis was rated
between slight and moderate for the N2O mixture but showed great
individual variation. The time to exhaustion was not significantly
different with the two breathing mixtures (P > 0.05). Heart rate
was unaffected by gas mixture and the inspiratory resistance at rest
and during steady state exercise, but was significantly lower with
the narcotic mixture during incremental exercise (P < 0.05).
Mean LE ratings were not significantly different between the narcotic
and non-narcotic trials (P > 0.05). However, mean RE responses
during both steady state and incremental exercise were 25% lower with
the narcotic gas than with the non-narcotic mixture (P < 0.05).
The respiratory responses to N2O narcosis included significant
increases in mouth pressure 0.1 s after the onset of an occluded
inspiration (+13%; P < 0.05), esophageal pressure (+12%; P
< 0.001), and the work rate of breathing (+15%; P < 0.001)
compared with the non-narcotic trials. The narcotic gas also produced
a small but significant increase in E (+4%; P < 0.01), which
resulted in slightly lower PETCO2 values (-2%; P < 0.05).
Inspiratory duty cycle was similar while breathing the narcotic and
non-narcotic mixtures (P > 0.05). We conclude that the narcotic
mediated changes in ventilation, exercise heart rate and RE induced
by 23% N2O are not of sufficient magnitude to influence exercise
tolerance at surface pressure. Furthermore, the load-compensating
respiratory reflexes responsible for maintaining ventilation during
inspiratory resistive loading are not depressed by inert gas
narcosis.
Received 28 December 1995; accepted in final form 3 June 1996.
APS Manuscript Number A1356-5.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 28 June 96