A 28-year-old G1, PO 26-wk pregnant woman is seen in the OB clinic. She has a past history of bronchial asthma that has been well controlled for the last year by inhaled steroids. She states that she has noted increasing shoness of breath for the last 3 days. On examination, she appears tachypneic and moderately uncomfoable. On physical examination, she has a pulse of 110 bpm; normal temperature; respirations 32/min; blood pressure 160/90 mm Hg. Hea exam: NSR without any gallop. A grade 2/6 systolic murmur in the pulmonic area is heard. Lung exam is clear to auscultation; abdomen exam confirms a 26-wk gravid uterus. Laboratory data: Hb 12 g/dL; Hct 36%; WBCs 7.0/uL with normal differential; BUN 23 mg/dL; creatinine 0.9 mg/dL; sodium 136 mEq/L; potassium 4.2 mEq/l. ABGs on room air: pH 7.34; PCO2 34 mm Hg; PO2 68 mm Hg. PEFR 450 L/min. Chest x-rays are shown.The most likely diagnosis is