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

Abstract

The significance of change in SaO₂ (δSaO₂) following initial bronchodilator therapy in acute childhood asthma is not clear. Increase in SaO₂ following initial bronchodilator therapy has been advocated as a measure of improvement in acute asthma. We hypothesized that the initial level of SaO₂ would be inversely related to δSaO₂ and would change very little for most children with mild or moderate asthma. Therefore, we measured SaO₂ before and 30 min after salbutamol inhalation in 135 children (age range 1-14.5 yr) presenting to an emergency room with mild/moderate (SaO₂ > 91%) and severe (SaO₂ ≤ 91%) asthma. δSaO₂ was inversely related to initial SaO₂ (p < 0.01) with the greatest rise (7%) occurring in children with the lowest initial level (84%). SaO₂ 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. δSaO₂ expressed as a percent of potential increase increased with decreasing SaO₂ indicating that a small δSaO₂ at a higher initial SaO₂ 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 SaO₂ is not a reliable guide to response to initial bronchodilator therapy in the majority of children with asthma (SaO₂≥91 %) as it usually increases little and does not reflect increase in PEF.