2007, Piran, Slovenia

Environmental Ergonomics XII
Igor B. Mekjavic, Stelios N. Kounalakis & Nigel A.S. Taylor (Eds.), © BIOMED, Ljubljana 2007
GENDER DIFFERENCES IN SUSCEPTIBILITY TO
SUBANESTHETIC CONCENTRATIONS OF NITROUS OXIDE
Miroljub Jakovljević 2 & Igor B. Mekjavić 1
1 Department of Automation, Biocybernetics and Robotics, Jozef Stefan Institute &
2 University of Ljubljana, University College of Health Studies & Ljubljana, Slovenia
Contact person: miroljub.jakovljevic@vsz.uni-lj.si
INTRODUCTION.
Nitrogen narcosis is a significant danger to divers, as it enhances the risk of accidents
and diminishes a diver’s ability to cope with an emergency. The present study
investigated the effect of inert gas narcosis on psychomotor performance of males and
females. The aim was to quantify any gender differences in the susceptibility to
subanesthetic concentrations of nitrous oxide, a behavioural analogue for hyperbaric
nitrogen. Namely, it has previously been demonstrated that hyperbaric N2 and
normobaric nitrous oxide (N2O) exert similar effects on cognitive performance and
behaviour. Therefore normobaric N2O is an adequate analogue for hyperbaric N2,
allowing the study of inert gas narcosis without the confounding effect of pressure.
N2O is a nonvolatile, gaseous, inhaled anesthetic with a modest analgesic effect at
subanesthetic (i.e., FN2O = 0.2 to 0.3) concentrations (Maze and Fujinaga, 2000;
Quock and Vaughn, 1995). Human behavioural studies indicate that N2O, in this same
concentration range, depresses psychomotor function (Korttila et al., 1981), cognition
(McMenemin and Parbrook, 1988), learning, and memory (Block et al., 1988).
Several studies have reported an acute tolerance to N2O inhalation. Acute tolerance
occurs when a drug’s effectiveness diminishes during the course of a single
administration (Kalant et al., 1971). Experiments on humans suggest that acute
tolerance develops with N2O’s hedonic effects (Zacny et al., 1996), but not to its
subjective, cognitive, or psychomotor effects (Yajnik et al., 1996; Zacny et al., 1996).
The majority of studies to date have included only male subjects. There is a lack of
data regarding gender differences in the susceptibility, and acute tolerance to N2O,
and consequently to nitrogen narcosis.
METHODS.
Twenty four healthy (12 females and 12 males) students participated in the present
study. Prior to participating in the study, subjects attended a screening interview,
during which their medical status was assessed to determine, whether there were any
contraindications to their participation in the study. None of the subjects had any
previous experience with N2O, neither were any of the female subjects pregnant.
Their mean (SD) age was 21.6 (1.8 years) (females: 21.4 (2.3) years; males: 21.8 (1.3)
years), and body mass index 23.5 (3.4) kgm -2 (females: 22.2 (3.0) kgm -2 ; males: 24.8
(3.4) kgm -2 ). Computerized Visual Simple Reaction Time (VSRT) is a sustained
attention task, measuring attention and response speed to an easily discriminated, but
temporally uncertain visual signal. The task is to press a key on the mouse as quickly
as possible when a red circle is presented on the display. A total of five stimuli were
presented within one session and the average was the result. Psychomotor speed and
executive control were assessed with the Trial Making Test part A (TMT-A) and part
B (TMT-B) (Reltan, 1959). The task of TMT-A was to connect lines to 25 circled
numbers in sequence, in TMT-B each circle contains either a letter or a number and
the task is to draw lines alternating from number to letter, consecutively (e.g. 1-A-2-
B…). VSRT and the TMT were measured under two experimental conditions;
breathing air (Air trial) and breathing normoxic mixture of 30% N2O (N2O trial) in the
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