Interrupter resistance: What's feasible?
Section snippets
Background
Interrupter resistance was described for the first time in 1927 by Von Neergard1, 2 but was reassessed during the late eighties when rapid progresses in equipment and computer analysis allowed a reappraisal of the technique. This technique is based on the assumption that during a brief airway occlusion performed during tidal breathing, alveolar pressure (Palv) rapidly equilibrates with mouth pressure (Pmo). Rint is calculated as the ratio of Pmo, measured after occlusion, and flow, measured
Technical aspects
To perform Rint measurements, a flowmeter, a flow interruption system (valve), and a pressure measurement device are needed. The closure of the valve must be very rapid (<10 ms) to ensure an instantaneous airway and prevent air leak during occlusion. The distance between the valve and the pressure transducer must be minimal, to avoid blunting of the Pmo changes. The dead space of the device should be low and must be specified. The range of pressure measurement within the device must be adapted
How to measure Rint
The child should feel as comfortable as possible, be in a seated position (if necessary on parent's knees), and get used to the procedure before beginning measurements. The neck of the child has to be in a neutral position, neither too extended nor flexed. The child breathes through a facemask with an integral mouthpiece or through a mouthpiece with a nose-clip on, and cheeks supported from behind by an adult/parent. The use of a facemask rather than a mouthpiece can be more convenient in very
How to calculate Rint
The method used to estimate Palv is important (cf Background section) and the later during the occlusion the Pmo value is selected, the higher Rint measurement will be. The commonly used two-point linear back-extrapolation (from 70 and 30 ms to 15 or 0 ms after interruption) method estimates Pinit. Rint measured using this method is higher than Raw but has less variability that Rint measured using other Pmo values (curvilinear, oscillation, end interruption methods). It has therefore been
What is Rint variability?
Within occasion variability includes intra-subject measurement variability assessed by the coefficient of variation (CV)(i.e. 100 × SD/mean of at least 5 measurements) and short term repeatability assessed by the coefficient of repeatability (CR)(i.e. 2 SD of the mean difference between two sets of measurements few minutes apart). Reports of Rint CV have been quite variable and may depend on the method used to estimate Palv in the different studies, but on average it is usually 10–13% and does
References values
Several reference equations based on relatively large groups of healthy children have been published3, 4, 5, 6, 7 (Fig. 2). The variability of prediction (RSD) is given for each equation, allowing the calculation of Z score for any Rint measurement: where Z score = (observed value − predicted value)/RSD; among a healthy population 95% Z scores will lay between −2 to +2. Pulmonary function values expressed as Z scores are useful to follow lung function of children throughout different ages,
Clinical applications
Rint is very easily performed in young children (under 6 years of age). The percentage of young children able to perform Rint measurement is generally above 85% and portable device allows measurement in field conditions. Since reference values and short term variability have been published, potential applications of Rint technique could include epidemiologic investigations of early determinants of pulmonary function, diagnostic assistance in wheezy young children or in those with non specific
References (9)
- et al.
Evaluation of airway interruption technique as a method of measuring pulmonary air-flow resistance
J Appl Physiol
(1954) - et al.
A reevaluation of the interrupter technique for airway resistance measurement
J Appl Physiol
(1974) - et al.
Airway resistance measured by the interrupter technique: normative data for 2-10 year olds of three ethnicities
Arch Dis Child
(2002) - et al.
Measurements of interrupter resistance: reference values for children 3–13 yrs of age
Eur Respir J
(2002)