What is the most important reason to control the conditions of an experiment

A92-year-old man with a 45-year history of chronic obstructive pulmonary disease is intubated in the icu because of a bout of viral pneumonia that fails to improve after 72 hours of antibiotics. although the inspired fraction of oxygen is 100%, the patient's po2 remains at 57 mmhg. positive-end expiratory pressure (peep) is added to allow the inspired fraction of oxygen. twelve hours after the introduction of peep the patient suddenly become hypotensive. at the same time, his oxygen saturation drops from 92% to 61%. on physical examination, his bp is 80/50 mmhg and his pulse is 124/min. he has distended neck veins and distant heart sounds. which of the following would also most likely be seen on this patient's physical examination? - absence of breath sounds in the right hemithorax- high amplitude carotid artery upstroke- a pleural friction rub- pulsus alternans- splenomegaly