Spirometry and arterial blood gases in acute severe asthma

Background: We studied in a group of patients admitted for acute severe asthma (ASA) arterial blood gases at admission and it relationship with airway obstruction. We also compared arterial gases of a subgroup of patients unable to perform spirometry because their poor clinical status with those able to complete the test. Methods. Data from arterial gases and spirometry were obtained from two previously published series of patients admitted for ASA to a State Hospital (Hospital María Ferrer, Buenos Aires, Argentina). Blood was drawn for arterial gases on arrival to the Emergency Department (ED) in 314 asthma patients. PaO2, PaCO2 and pH were measured using standard electrodes. Measurements of serum electrolytes were made in 250 of the patients. Anion gap was calculated as Na(Cl + HCO3 ). Acid-base disturbances at presentation were classified according to commonly accepted criteria. Forced expiratory volume (FEV1) was measured on ED arrival and expressed as % of theoretical values (FEV1%). Forty eight patients presented poor clinical condition based on physician evaluation and were unable to carry out spirometry. Correlation was analyzed between FEV1% and arterial gases and different acid base variables on arrival. PaO2,PaCO2, pH and other acid base data were compared between patients completing spirometry vs. and those unable to do so (n = 266 vs. 48). Results. Significant correlation (p < 0.05) was observed between FEV1% vs PaO2 (r=0.21), FEV1% vs PaCO2 (r=-0.33), pH vs PaCO2 (r=-0.86), PaCO2 vs PaO2 (r=-0.36), lactic acid vs anion gap (r=0.58), lactic acid vs base excess (r=-0.50) and anion gap vs Bic (r=-0.38). Significant differences were found between PaCO2, pH and base excess, when comparing patients who could perform spirometry vs. those who could not. No significant differences were found between bicarbonate, anion gap and lactic acid. Comparison of patients able or unable to perform spirometry indicated: respiratory alkalosis (either alone or as a part of a mixed disturbance) in 56.6 vs. 37.8% of cases (p< 0.03), respiratory acidosis (either alone or as a part of a mixed disturbance) in 22.8 vs 67.5% (p< 0.001) and metabolic acidosis (either alone or as a part of a mixed disturbance) in 41.6 vs 51.3% (NS). Conclusions. Our findings confirm the existence of a weak correlation between gas exchange, acid base status and magnitude of airway obstruction. We also observed that patients in poor clinical conditions who were unable to perform spirometry tests also had more severe acid base imbalance.