Body Plethysmograph Measurements Before and After Methacholine Challenge in Early Childhood

In young children that cannot perform adequately spirometry methacholine challenge test (MCT) is continued until any of the following clinical signals occurred: appearance of audible wheeze, a fall of >5% in O2-saturation, or an increase of >50% in respiratory rate and/or heart rate. In our previous study [1] we assessed the feasibility of determining PC20-FEV1 in relation to audible wheeze detection during MCT in asthmatic preschool children. We found that at end of test (EOT), the reduction in flows of the forced expiratory flow volume (FEFV) curves was accompanied by reduction in forced vital capacity (FVC) though reductions in flows were higher than the reduction in volume. We hypothesized that this finding is associated with trapped-air rather than the children's loss of cooperation or concentration. We have also thought that determining lung volume by whole body plethysmograpy, has the advantage of measuring airway resistance in relation to FRC. Hence, plethysmographic measurements of lung volumes during MCT may shed light on the physiological relation between airway obstruction and hyperinflation at the end of MCT when audible wheeze is detected. However, plethysmography requires the patient's cooperation and coordination to perform brief panting maneuvers against closed airways. So far, only airway resistance has been measured in the plethysmograph [10], as it is believed that young children will not comply with the occlusions needed for lung volume measurements within the plethysmograph.

The aims of the present study were twofold: a) to test if young children are capable of performing plethysmographic maneuvers required for FRC measurements; and b) to investigate if the reduction in vital capacity at maximal reduction in flows of the FEFV curves, found at wheeze detection during MCT, is due to true elevation of FRC and air trapping, or is artificially related to patients' loss of motivation/cooperation.

Methods:

Lung volumes and airway resistance will be measured in a commercial, whole body ZAN500 constant volume body plethysmograph (ZAN-Messgeraete GmbH, Germany). The plethysmograph is equipped with a graphic display for preschool children to increase cooperation. The chair can be adjusted so that the child can comfortably sit up straight with a nose-clip, and reach the mouthpiece without flexing the neck.

The plethysmography procedure will be performed according to recommendations. The test events will be explained to the child in detail outside of the plethysmograph. After that, the measurements will be carried out in the same sequence that was explained to the child. FRC will be measured by allowing for 2-3 breathing attempt series of superimposed straight lines (separated only by thermal drift). The best test out of two technically accepted series was analyzed.