 The investigators will conduct a double-blind, randomized controlled trial comparing the safety and effectiveness of ecallantide to conventional therapy. A rescue cross-over design will be used such that patients failing to improve on standard therapy will additionally be treated with ecallantide. Therefore, a historical control cohort will be enrolled for analysis of secondary endpoints. In addition, since some patients treated with conventional therapy may improve rapidly and therefore not be eligible for inclusion in the study, the investigators will enroll these patients as an observational arm to enable the conduct of sensitivity analysis. 