Piper Jaffray analyst Matt O'Brien noted that Abiomed hosted a conference call on its STEMI safety and feasibility study that was released yesterday and he is "surprised" by the stock's pullback following the call. This was only supposed to be a feasibility study with a small number of patients and Impella was used in both arms, which will not be the case in the pivotal study, O'Brien points out. Without Impella in the pivotal study, he expects a more pronounced benefit to patients with mechanical support once it is done, O'Brien added. He would be using the weakness to buy Abiomed and keeps an Overweight rating on the shares with a $480 price target.