BOSTON (MarketWatch) -- Shares of HeartWare International HTWR -15.28% plunged almost 20% to $68.39 in early action Monday. Late Friday, HeartWare released mixed study results for its HeartWare Ventricular Assist System heart pump in the treatment of patients with end-stage heart failure. HeartWare's negative news boosted shares of rival Thoratec Corp. THOR +14.84% , which markets a similar heart-pump called HeartMate II. Shares of Thoratec rallied 14% in early trading.