Plethysmographic specific airway resistance
Section snippets
Practical aspects
Flow (V′) in the airways and variations of the pressure in the box are simultaneously measured as the child breathes through the pneumotachograph using a mouthpiece and nose clip or specially adapted face-mask with a large cushion and a built-in flexible tube, while seated inside the box. Volume and pressure in the box are calibrated routinely before measurements whereby changes in box volume (ΔVbox) are measured through changes in box pressure.1 The simultaneous recordings of V′ and ΔVbox are
Quality control of measurements
On-line display of the loops allows detection of artifacts. The decision whether to accept or reject a measurement can be made during and after the measurements. sRaw is usually reported as the mean of the median from at least two sets of at least 5 consecutive and technically satisfactory loops.
Reliability
The standard deviation of sRaw has been reported as 0.20 kPa s and the within-subject SD (SDw) as 0.11 kPa s. CVw% is 8–11%.4, 7, 9 Repeatability appears to be independent of age9 and measurements of sRaw with and without an accompanying adult do not alter repeatability.7
Normative data
sRaw is independent of height, age and gender,9 provided it is calculated as pressure/flow changes between points of maximum pressure (sRawTOT). Considerable caution is required when interpreting results in young children, since prediction equations in commercially available equipment usually default to values based on historic data collected at least 20 years ago with heated rebreathing systems. These tend to give predicted values approximately half those obtained when using electronic
Acceptance
Measurement is accepted in >80% of healthy 2–7 year old children, but success rate increases by age.9 In a birth cohort study of 766 children, 66% completed measurements at age 3.10
Asthma: discrimination between health & disease
sRaw was significantly increased in selected groups of preschool asthmatics compared to healthy controls and in randomly selected groups of preschool asthmatics.1 In fact, sRaw discriminated more accurately between healthy and asthmatics than Raw.11 sRaw was significantly higher in 3 year old children who wheezed at least once during the first 3 years of life than in those who had never wheezed.10
Asthma: bronchial challenge & Bronchial Hyperresponsiveness (BHR)
Cold air challenge (CACh) is feasible in 2 to 5 year old children.2, 12 BHR to CACh, defined as an
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