Ventilatory inequalities, pulmonary function and blood oxygenation in advanced states of cystic fibrosis

In 36 patients with cystic fibrosis (CF), aged 6.2-26.4 years, the relationship between functional lung deterioration and gas exchange characteristics was studied by backward stepwise discriminant analysis of 22 dependent variables, within 3 clinically defined severity degrees of the disease. The stratification was based on a general clinical score, a chest X-ray score and the relative underweight of the patients. Lung function testing included whole body plethysmography and multibreath nitrogen washouts (MBNW). Blood gas analyses were performed under room air (FiO2 = 0.21) and 100% oxygen breathing (FiO2 = 1.0). The clinical score correlated best with vital capacity (r = 0.776) and PaO2 at room air (r = 0.829); the X-ray score correlated best with PaO2 at room air (r = 0.768). Impairment of oxygenation characteristics was closely related to the degree of ventilation inequalities (MBNW) and the amount of trapped gases, both increasingly present from severity group I to III. In addition the calculated arterial-alveolar oxygen ratio (group I 0.47; group II 0.38; group III 0.29) as an estimate of intrapulmonary gas exchange deficiency revealed that only patients with mild lung involvement (group I) fulfil the functional conditions to achieve a paO2 of 70 mm Hg at room air, whereas patients with severer lung involvement (groups II and III) would need a FiO2 of 26.0 or 39.6, respectively, in order to have a sufficient oxygenation.