case continues:

The patient was sent to the emergency department of the local hospital where he was noted to have a pulse ranging in the 130-140's. He was admitted and underwent several changes in his medical regimen. A diagnosis of multi-focal atrial tachycardia was made and the patient was started on atenolol for his rhythm disturbance. The next morning when his physician went on hospital rounds, he noted the patient to be sitting bolt upright with moderately severe wheezing. His pulse was 80 and he had a respiratory rate of 40. His blood pressure was not 160/100 mm Hg and he had developed mild pedal edema. A pulmonary specialist was consulted.

What is the most probable cause of this patient's decline overnight?