Patients being hospitalized for acute heart failure and already receiving standard therapy will be randomized to receive either tolvaptan or placebo, based on the level of copeptin measured in their bloodstream. Patients with high copeptin levels will be able to participate in the trial, patients with low levels will be excluded. Patients being admitted to the observation unit for acute heart failure and already receiving standard therapy will be randomized to receive either tolvaptan or placebo without consideration of the copeptin level. The hypothesis is that patients receiving tolvaptan will have better improvement of shortness of breath than those receiving placebo, within 9 hours of drug administration.