Hypoxemia in attack free asthmatic children: relationship with lung volumes and lung mechanics

In 113 asthmatic children (mean age +/- SD = 11.0 +/- 2.8 years), the presence of hypoxemia was evaluated during an attack free period. Blood gases were measured in arterialized blood samples. 71% of the patients were hypoxemic. Hypoxemia was defined as partial pressure of O2 (PaO2) below 90% of the mean value for normal children. In a first step, the relationship between hypoxemia and frequency of asthma attacks was examined: frequency of attacks appeared to be a poor predictor of hypoxemia in an attack free period. In a second step, the relationship between hypoxemia and 9 pulmonary function test variables was examined. The static lung volumes considered were: functional residual capacity (FRC: helium dilution technique); thoracic gas volume (TGV: body plethysmography technique) and trapped gases (TG: difference between TGV and FRC). TG was expressed as a percentage of 1) measured FRC (TG/FRCmeas), 2) measured TGV (TG/TGVmeas), 3) predicted FRC (TG/FRCpred). Lung mechanics variables were determined by the oesophageal catheter technique: lung resistance (RL) and dynamic lung compliance (CLdyn). RL and CLdyn were expressed 1) in absolute terms, 2) as a percentage of the index of distension (ID: TGV/FRCpred). All variables are expressed as a percentage of predicted values except for TG/FRCmeas and TG/TGVmeas. Considering lung volumes, hypoxemia was found when TG was present. When lung mechanics were altered, hypoxemia was also present. The relationship between PaO2 and PFT variables was best expressed by the following equation: PaO2 (%) = 85.393 + 0.121 CLdyn/ID--0.151 TG/FRCpred--0.026 RL.(ABSTRACT TRUNCATED AT 250 WORDS)