Administration of gas in medical care
Medical gas therapy
is a treatment involving the administration of various gases. It has been used in medicine since the use of
oxygen therapy
.
[1]
Many other gases, collectively known as
factitious airs
, were explored for medicinal value in the late eighteenth century.
Nitric oxide
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Nitric oxide
is a gas that is inhaled.
[1]
It was initially described in 1987 as an "endothelial-derived relaxing factor" and has since been used to treat pulmonary disorders.
[2]
It works by relaxing
smooth muscle
to widen (dilate)
blood vessels
, especially in the lungs.
[1]
Nitric oxide is used together with a
mechanical ventilator
to treat
respiratory failure
in
premature infants
.
[1]
Helium and oxygen
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In medicine,
heliox
generally refers to a mixture of 21% O
2
(the same as
air
) and 79% He, although other combinations are available.
Heliox generates less airway resistance than air and thereby requires less mechanical energy to
ventilate
the lungs.
[3]
"Work of Breathing" (WOB) is reduced. It does this by two mechanisms:
- increased tendency to
laminar flow
;
- reduced resistance in
turbulent flow
.
Heliox has a similar
viscosity
to air but a significantly lower
density
(0.5 g/L versus 1.2 5g/L at
STP
). Flow of gas through the
airways
comprises
laminar flow
, transitional flow and
turbulent flow
. The tendency for each type of flow is described by the
Reynolds number
.
Heliox
's low density produces a lower
Reynolds number
and hence higher probability of laminar flow for any given airway.
Laminar flow
tends to generate less resistance than turbulent flow.
In the small airways where flow is laminar, resistance is proportional to gas
viscosity
and is not related to
density
and so
heliox
has little effect. The
Hagen?Poiseuille equation
describes laminar resistance. In the large airways where flow is turbulent, resistance is proportional to density, so
heliox
has a significant effect.
Heliox has been used medically since the early 1930s. It was the mainstay of treatment in acute
asthma
before the advent of
bronchodilators
. Currently, heliox is mainly used in conditions of large airway narrowing (upper airway obstruction from tumors or foreign bodies and
vocal cord dysfunction
). There is also some use of heliox in conditions of the medium airways (
croup
, asthma and
chronic obstructive pulmonary disease
).
Patients with these conditions may develop a range of symptoms including
dyspnea
(breathlessness),
hypoxemia
(below-normal oxygen content in the arterial blood) and eventually a weakening of the respiratory muscles due to
exhaustion
, which can lead to
respiratory failure
and require
intubation
and
mechanical ventilation
. Heliox may reduce all these effects, making it easier for the patient to breathe.
[4]
Heliox has also found utility in the weaning of patients off mechanical ventilation, and in the nebulization of inhalable drugs, particularly for the elderly.
[5]
Research has also indicated advantages in using helium?oxygen mixtures in delivery of
anaesthesia
.
[6]
References
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