Newer tests for the diagnosis of asthma

Impulse osillometery (IOS)
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 –naive 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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