Arterial Oxygen Saturation in Acute Childhood Asthma: Interpretation of Change Following Salbutamol Nebulization

The significance of change in SaO 2 (δSaO 2 ) following initial bronchodilator therapy in acute childhood asthma is not clear. Increase in SaO 2 following initial bronchodilator therapy has been advocated as a measure of improvement in acute asthma. We hypothesized that the initial level of SaO 2 would be inversely related to δSaO 2 and would change very little for most children with mild or moderate asthma. Therefore, we measured SaO 2 before and 30 min after salbutamol inhalation in 135 children (age range 1-14.5 yr) presenting to an emergency room with mild/moderate (SaO 2 > 91%) and severe (SaO 2 ≤ 91%) asthma. δSaO 2 was inversely related to initial SaO 2 ( p < 0.01) with the greatest rise (7%) occurring in children with the lowest initial level (84%). SaO 2 increased more in the severe group than the mild to moderate group—2.3% versus 0.6% respectively ( p < 0.01)—although the change in peak expiratory flow (PEF) was similar for both groups. δSaO 2 expressed as a percent of potential increase increased with decreasing SaO 2 indicating that a small δSaO 2 at a higher initial SaO 2 could not be fully explained by a "ceiling" effect. We postulate that varying contributions of bronchoconstriction and ventilation perfusion inequality could explain this observation. Thus, salbutamol usually improves hypoxia in severe asthma, but SaO 2 is not a reliable guide to response to initial bronchodilator therapy in the majority of children with asthma (SaO 2 ≥91 %) as it usually increases little and does not reflect increase in PEF.