Newer tests for the diagnosis of asthma
Impulse Osillometery is a newer
technology that uses small amplitude pressure oscillations to
determine the resistance of the airway. It is largely
independent of effort does not require coordination, but does requires cooperation of the child. To
perform IOS child holds a mouth piece in place over a 30 second
period of time while breathing normally. Sound impulses of
various frequencies from 5 to 35 Hz are applied to the airway
through the mouth piece with total respiratory system resistance
(Rrs) and reactance (Xrs) determined at various frequencies.
Change in Rrs and Xrs is noted after inhalation of a
beta-agonist. Young children with asthma show significant change
in Rrs following beta-agonist inhalation. Till now IOS is mainly
used as a research tool.
Exhaled Nitric Oxide test
NO is produced in discrete concentrations in the healthy human
airway where it is important in physiological functions such as
maintaining airway patency. NO is over produced in asthmatic
individuals. It is responsible for airway inflammation
(swelling) and is also the product of airway inflammation.
Nitric oxide analyzers are used to
measure exhaled nitric oxide (FENO). In 2003 Aerocrine exhaled
nitric oxide monitoring system NIOX was granted clinical
approval by USFDA for age of 4 to 65 years.
The analyzer measures Nitric oxide
by a chemiluminescent reaction with ozone. NO is drawn into the
chamber and is combined with ozone. The reaction yields NO2, O2
and a photon, which is captured by the photomultiplier tube that
analyzes and reports a proportional value of Nitric oxide. FENO
is measured as part per billion (ppb) in asthmatic patient.
Normal values vary with the
patient, but it is considered that 20 to 30 ppb in the steroid
ľnaive patient is indicative of inflammation.
Potential of FENO to predict
exacerbations of asthma was recently examined and levels were
found to be elevated before the fall in lung functions or the
development of clinical symptoms of asthma exacerbations.
There are two methods of
collecting exhaled nitric oxide.
An offline technique, where Nitric
oxide is collected in special reservoir, that allows storage and
subsequent analysis of nitric oxide content.
An online nitric oxide techniques
use continuous sampling and quantification during exhalation for
dynamic measurement and flow analysis.
Ingestion of foods containing
nitrates, smoking status, ambient nitric oxide level,
nasopharyngeal contamination, airway infections and drugs such
as leukotrienes modifiers may effect the actual collection and
quantification of exhaled nitric oxide. Patients are asked to
take nothing by mouth for one hour before sample collection.
Last edited 18-7-2010
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