What is causing this patient's dyspnea?

oral temperature, 98.1° F (36.7° C); and Spo2, 97% on room air. Her physical examination included heart sounds consistent with her established aortic stenosis and clear breath sounds. Abdomen was nontender to palpation, she had no lower extremity edema, and peripheral pulses and sensation were normal. Diagnostic testing An ECG (Figure 1), chest radiograph, complete blood cell count with differential, comprehensive metabolic panel, cardiac profi le, coagulation studies, B-type natriuretic peptide (pro-BNP), and venous blood gas analysis were ordered. All laboratory tests, including troponin, were negative or consistent with the patient’s expected baseline, except an elevated white blood cell count (WBC) of 11,200 cells/mm3 (normal range, 4,500 to 10,500 cells/mm3) and pro-BNP of 4,152 pg/mL (normal range, less than 450 pg/mL). The chest radiograph did not reveal evidence of acute disease.