Abiomed shares sold off 20% Monday after two retrospective, propensity-matched data sets were presented comparing the outcomes of the company's Impella and intra-aortic balloon pumps at the American Heart Association meeting on Sunday, William Blair analyst Margaret Kaczor tells investors in a research note. These data sets suggested that Impella, when compared to IABP, was associated with higher death and bleeding, did not improve clinical outcomes, and was a more costly treatment, explains the analyst. Kaczor does not think these data sets accurately capture the proper use of, and subsequently outcomes with, Impella, but she sees them as "likely another hurdle that Abiomed will need to overcome as it looks to reaccelerate adoption and utilization of its products." She continues to believe in Impella's "unique" clinical advantages and believes management is pursuing appropriate initiatives to drive long-term adoption and utilization of Impella. The analyst keeps an Outperform rating on the shares.