A. A. is a 27-year-old woman with asthma who presents to the emergency room with cough and dyspnea. The cough began approximately 2 weeks ago after the onset of cold symptoms. Her symptoms (sore throat, runny nose, and congestion) have resolved, but the cough has persisted. Since last night, she has felt increasing shortness of breath and tightness in her chest. She is breathing at a rate of 28 breaths per min and her oxygen saturation is 97 % while breathing room air. Auscultation of the chest reveals diffuse expiratory wheezing. Electrolytes and complete blood cell counts are within normal limits, and the chest X-ray shows no pulmonary opacities. Analysis of the arterial blood gas reveals: pH 7.5, pCO2 25 mmHg, and HCO 3- 20 mEq/L.