EFFECT OF NITROUS OXIDE ON CLOSED GAS SPACES
Volume Changes in Highly Compliant Spaces
During nitrous oxide administration, appreciable volumes of nitrous
oxide can move into closed gas spaces within the body. This transfer does not influence
FA/FI but may have
important functional consequences. There are two types of closed gas spaces in the
body: those enclosed by
Figure 5-12
In dogs, when only the right lung was ventilated, the
rise of the very soluble anesthetic methoxyflurane in arterial blood was normal (i.e.,
did not deviate from control), but the rise for the poorly soluble anesthetic cyclopropane
was significantly slowed. (Adapted from Stoelting RK, Longnecker DE: Effect
of right-to-left shunt on rate of increase in arterial anesthetic concentration.
Anesthesiology 36:352–356, 1972.)
compliant walls and those enclosed by noncompliant walls. The former (e.g., bowel
gas, pneumothorax, or pneumoperitoneum) are subject to changes in volume secondary
to the transfer of nitrous oxide into these spaces.[54]
These spaces normally contain nitrogen (from air), a gas whose low solubility (blood-gas
partition coefficient of 0.015) limits its removal by blood. The entrance of nitrous
oxide (whose solubility permits it to be carried to the space in substantial quantities)
is not countered by an equal loss, and the result is an increase in volume. The
theoretical limit to the increase in volume is a function of the alveolar nitrous
oxide concentration, because at equilibrium this concentration must be achieved in
the closed gas space. The partial pressure of nitrous oxide in the closed gas space
must equal its partial pressure in the alveoli. An alveolar concentration of 50%
may double the volume of the gas space, and a 75% concentration may produce a fourfold
increase.
These theoretical limits may be approached rapidly in patients
with pneumothorax or gas emboli. Administration of 75% nitrous oxide in the presence
of a pneumothorax may double the pneumothorax volume by 10 minutes and may triple
it by 30 minutes[54]
( Fig.
5-13
). This increase in volume may seriously impair cardiorespiratory
function,[55]
and the use of nitrous oxide is contraindicated
in the presence of a significant pneumothorax.
Figure 5-13
The volume of a pneumothorax created by air injection
is affected little when oxygen subsequently is breathed (lower
two curves). However, if 75% nitrous oxide is breathed, the volume doubles
in 10 minutes and triples in 0.5 hour (upper three curves).
(Adapted from Eger EI II, Saidman LJ: Hazards of nitrous oxide anesthesia
in bowel obstruction and pneumothorax. Anesthesiology 26:61–66, 1965.)
A still more rapid expansion of volume occurs if air inadvertently
enters the bloodstream in a patient anesthetized with nitrous oxide. Expansion may
be complete in seconds rather than minutes. Munson and Merrick[56]
demonstrated that breathing nitrous oxide rather than air decreased the lethal volume
of an air embolus in animals ( Fig.
5-14
). The difference could be entirely explained by expansion of the
embolus in the animals breathing nitrous oxide (i.e., the predicted total volume
of air plus nitrous oxide in the embolus equaled the volume of air needed to produce
death in animals breathing only air). These studies suggest caution in the use of
nitrous oxide for procedures in which air embolization is a risk (e.g., posterior
fossa craniotomies, laparoscopy). They also suggest that nitrous oxide administration
should be immediately discontinued if air embolization is suspected. Conversely,
a nitrous oxide "challenge" may be used to test whether air embolization has occurred.
[57]
The tracheal tube cuff normally is filled with air and is susceptible
to expansion by nitrous oxide.[58]
The presence
of 75% nitrous oxide surrounding such a cuff can double or triple the volume of the
cuff. This results from a more rapid diffusion of nitrous oxide than nitrogen across
the cuff; the more rapid diffusion occurs because of the greater solubility of nitrous
oxide. The result may be an unwanted increase in pressure exerted on the tracheal
mucosa. Similarly, nitrous oxide may expand the cuffs of balloon-tipped (e.g., Swan-Ganz)
catheters[59]
[60]
when the balloons are inflated with air. The expansion is rapid, and a doubling
of volume may occur within 10 minutes.
Figure 5-14
An air embolus equaling 0.55 mL/kg killed 50% of rabbits
breathing oxygen. If the inspired gas mixture contained 75% nitrous oxide, only
0.16 mL/kg was required to kill one half of the animals. (Adapted from Munson
ES, Merrick HC: Effect of nitrous oxide on venous air embolism. Anesthesiology
27:783–787, 1966.)
