RecruitingPhase 4Drug trialVasoactive Drugs in Intensive Care UnitThe investigators hypothesis is that for ICU patients with shock, the use of the vasoactive drugs phenylephrine and vasopressin will reduce tachydysrhythmias when compared to norepinephrine and epinephrine. To investigate this hypothesis, the investigators are conducting a randomized double blind controlled trial comparing phenylephrine and vasopressin vs. norepinephrine and epinephrine in ICU patients with shock that is not responsive to IV fluids. All patients admitted to the adult intensive care units at the University of Chicago will be screened for eligibility.