Other Medical Gases
Several other medical gases are occasionally used in the ICU.
The most frequently used are helium and nitric oxide.
Figure 75-2
Several types of oxygen delivery facemasks. A,
Simple facemask. B, Partial rebreather facemask.
C, Nonrebreather facemask. D,
High-flow oxygen facemask with "whiskers."
Helium
Helium is an inert, low-density gas, which makes it useful as
a temporizing measure for patients with increased airway resistance. Heliox is a
mixture of helium and oxygen that is less viscous than air. Gas viscosity is most
important in areas of turbulent flow such as in the larger airways and airway branch
points. Heliox is most often used to decrease work of breathing in patients with
upper airway obstructions, such as from tumors or angioedema.[8]
The use of heliox in small airway obstruction, as in asthma, chronic obstructive
pulmonary disease (COPD), and bronchiolitis, is controversial and is not typically
used in the management of these patients.[9]
[10]
[11]
Nitric Oxide
Nitric oxide (NO) is endogenously produced by endothelial cells
and acts through cGMP to promote smooth muscle relaxation leading to vasodilation.
Inhaled NO has a very short half-life and must be given continuously at doses up
to 80 ppm. Inhaled NO has been used in the diagnosis and acute management of pulmonary
hypertension.[12]
It has the advantage of vasodilating
the pulmonary circulation with minimal effect on the systemic circulation. Inhaled
NO is also occasionally used as an adjuvant treatment for severe hypoxemia in patients
with acute respiratory distress syndrome (ARDS), based on the principle of selective
pulmonary vasodilation.[13]
As an inhaled drug,
NO is preferentially delivered to areas of the lung that are well ventilated, leading
to increased pulmonary vasodilation in those areas and thus increased perfusion to
well-ventilated areas of the lung. This improvement in ventilation-perfusion (V̇/
)
matching leads to significant improvement in oxygenation. However, this effect is
short-lived, typically lasting only two to three days.[14]
Clinical trials in patients with ARDS have not demonstrated an improvement in clinically
relevant outcomes.[15]
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