Chest
ReviewsPlethysmographic Measurements of Specific Airway Resistance in Young Children
Section snippets
Physics
sRaw is a measure of airway caliber-comprising effects of lung size. sRaw reflects the overall dimensions of the airway, including the effect of lung expansion on the caliber of the airway. Since sRaw is the product of Raw and TGV, it cannot distinguish if improvements are due to either of its two components. Breathing at different levels of lung volume might give the same sRaw, although the relationship between Raw and TGV differs. Likewise, other principles of lung function measurements do
Measurements of sRaw
The child breathes though a flowmeter (pneumotachograph) recording the flow ( ) while seated inside a sealed, volume-constant box equipped with a pressure transducer that records changes of pressure inside the plethysmograph (Fig 1). This enables the measurement of the variations of the pressure in the plethysmograph due to the compression of the lung gas volume during expiration and decompression during inspiration through the breathing cycle, ie, the effort driving the flow is
Equipment
The constant-volume, whole-body plethysmograph is in principle a sealed cabin. The flow may be measured by a pneumotachograph or other flowmeters. Commercially available plethysmographs are built for adults and are typically approximately 800 L. The price of the equipment is approximately $50,000 (US dollars). Commercially available ple-thysmographs are available in most secondary and tertiary care units, which allows a widely disseminated use of this method, although their size and price
Practical Aspects
Standard commercial equipment and software is basically sufficient. However, some adaptations to standard, whole-body plethysmography may be required, including entertainment (video) to attract the child's interest during the test and to distract him/her from potential discomfort from the closed box, the seat, and the adapted facemask.6 Also, a dedicated, trained, and committed pulmonary function technician is important.
Data Analysis
On-line display of the flow-pressure loops allows deletion of artifacts such as abnormal patterns caused by swallowing, vocalization, breath-hold, coughing, or leakage around the facemask. The decision whether to accept or reject a measurement should be made on-line during measurements, not allowing subsequent censoring of the data.
Pressure builds up in the box while the door is closed due to rising temperature. This constant drift in the box pressure is compensated for in most commercially
Acceptance
The acceptability of measurements of sRaw is good. Since no active cooperation is required, sRaw measurements should be feasible at a younger age than methods requiring active cooperation. Clearly, acceptance increases with age, but many children from 2 years of age can perform acceptable measurements. Particularly, children familiar with the use of facemasks are successful from an early age.
At the age of 2 years, 57% of untrained healthy children (n = 28) completed the measurement.10 At the
Reference Values
sRaw is independent of height and gender,52930 facilitating the interpretation of measurements carried out longitudinally in individual children. We reported measurements of sRaw in 121 healthy 2- to 7-year-old white children (61 boys and 60 girls), with approximately 20 youngsters at each year of age. The participants in this study had no history of recurrent lower respiratory symptoms, no history of eczema and no atopic first-degree relatives, no significant exposure to tobacco smoke, and no
Short-term Repeatability (Within-Day)
Repeatability is slightly dependent on the chosen estimate for sRaw. The total SD for sRaw is in the range of 0.20 to 0.21 kPa/s, with its constituents the within-subject SD (SDw) and between-subject SD (SDb) ranging from 0.086 to 0.109 kPa/s and 0.19 to 0.20 kPa/s, respectively. Correcting SDb for the SDw gives SDb in the range from 0.172 to 0.176 kPa/s. The within-subject coefficient of variation is in the range of 8 to 11%, and the short-term intraclass correlation coefficient (ICC) within
Response to Bronchodilators
Two provisos should be made when evaluating the response to β-agonist as an indication of asthma. First, there is no standard for estimating the response to a β-agonist. It may be calculated as the absolute change, the percentage change from baseline, the number of SDw units, or the percentage change of the predicted value.1731 The latter expresses an estimate of the absolute β-agonist response independent of baseline lung function, age, and height, and may therefore be recommended.1731
Challenge Tests
Bronchial responsiveness to direct stimuli such as methacholine and histamine can be estimated safely and reliably in young children from 2 years of age by measurement of sRaw.92532 Dosimetry is recommended to reduce the variability of the dose of the drug to the lung. The day-to-day repeatability studied in eight young children showed a mean repeatability of 0.7 (SD, 0.6) doubling doses, comparable to that reported in school children and adults.9 Another study32 showed good correlation between
Discrimination Between Health and Disease
Studies in children suggest that sRaw is as efficient as Raw and TGV in distinguishing between asthmatic and healthy children. In a study24 of children aged 5 to 8 years, the single-step sRaw measurement discriminated more accurately between healthy and asthmatic children than did the measurement of Raw.
sRaw was significantly increased compared to healthy control subjects in random11 and selected groups of 2- to 6-year-old asthmatics.1314151724 sRaw was reported in 110 consecutive 2- to
Future Directions
Improvements of equipment and software are needed for more reliable measurement of sRaw in young children. Improvements in the equipment should pay special attention to the seating and footrest. A facemask with a large cushion and a built-in noncompressible yet soft tube should be made available. Software improvements should specifically focus on the algorithms for thermal correction of the inspired volumes. The software should include an algorithm for automatic selection of acceptable and
Conclusions
Raw measurements are feasible in children from 2 years of age. The reliability and feasibility compares favorably with alternative methods. sRaw allows clinical monitoring and research during this critical period of growth and development in early life. sRaw measurements promise to bridge the gap of lung function measurements between infancy and school age.
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