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mediated via the cardiac autonomic nervous system. Xe
is also capable of preventing activation of the adrenal
medullary system3 and inhibits adrenal secretion,3,12
which might result in decreased HR in vivo.
In conclusion, 50% Xe produced cardiac effects
identical to those of N2O or N2 in isolated rat hearts
under conditions of decreased O2 delivery. Xe itself
lacks cardiodepressant effects and may become a use-
ful alternative to N2O.
Many years of clinical experience with N2O indicate
that it possesses mild cardiovascular depressant effects
that may be counterbalanced by a simultaneous,
reflexly mediated increase in sympathetic tone. These
studies demonstrated small increases,22 small decreas-
es,23 or no change24 in indices of contractility. A few in
vitro investigations regarding the effects of N2O on
myocardial muscle have indicated that N2O depressed
contractility similar to, or more than that in con-
trols.25–27 Regarding sympathetic effects, N2O, but
not Xe, has been reported to augment its outflow. The
sympathetic activation by N2O has been shown by ele-
vated plasma catecholamine concentrations23 and by
microneurography.28,29 This sympatho-activating
property of N2O can complicate evaluation of its car-
diovascular effects and direct effects on cardiac func-
tion. In an isolated global heart study, Stowe et al.
reported that N2O caused only small but significant
depression of indices of cardiac contractility.11 Our
findings, however, showed that 50% N2O produced
no cardiodepressant effects compared with an identi-
cal concentration of N2, which is inconsistent with
their report.11 No statistically significant difference
between Xe, N2O and N2, could be shown but N2O
tended to be cardiodepressant. The number of hearts
studied in the current laboratory investigation may
not have been sufficient to confirm the cardiodepres-
sant effects of N2O. Meanwhile, Xe did not depress
cardiac function more than N2O, at least in the
absence of autonomic influence or changes in preload
and afterload.
Acknowledgements
The authors wish to thank Air Water Inc. for their
generous donation of Xe. The authors also thank Ms.
Yumi Isaka and Mr. Yasuyuki Baba for their technical
assistance.
References
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The effect of the restriction of O2 delivery is a lim-
itation of the current investigation. The hearts were
solely dependent on the crystalloid solution from
which to extract dissolved O2. Exposure to 50% Xe
(i.e., exposure to 45% O2) decreased O2 delivery with,
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lated hearts11 (Table I). Even under such conditions,
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As mentioned initially, Xe surpasses N2O in several
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(0.115–0.14),6,7 higher MAC (71% in humans),6,8 and
innocuousness to the environment, because Xe is pre-
pared by fractional distillation of atmospheric air.
Therefore, the replacement of N2O with Xe may
become appropriate when delivery systems (i.e., closed
circuit) become available with efficient techniques for
recycling the gas to decrease the cost of Xe anesthesia.30
10 Mazoit JX, Orhant EE, Boïco O, Kantelip J-P, Samii K.
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