Source: Heartware International

What: After reporting that it is investigating the cause of adverse events in patients participating in a trial of its next-generation heart pump, shares in Heartware International (NASDAQ: HTWR) tumbled by over 18% earlier today.

So what: Heartware International manufactures implantable pumps that can take over some or all of the heart's pumping function.

The company's ventricular assist devices (HVAD) are used as either a bridge to transplantation or instead of transplantation, but they're still bigger than the company (and surgeons) would like, and the invasiveness associated with the surgical procedure of implantation limits the number of people who can receive them.

As a result, Heartware is developing a next-generation pump -- the MVAD -- that is one-third the size of its HVAD's.

In September, Heartware reported that it had paused its MVAD trial to address a manufacturing issue and to update the MVAD software, and it appears that during discussions with trial investigators, the company became aware of adverse events in certain patients that, while not uncommon for patients receiving ventricular assist devices, warrant additional research.

Now what: Previously, Heartware had hoped to restart its MVAD trial in November; however, today's revelation casts doubt on that timeline. Instead, investors will have to wait and hope that the company offers up a new timeline for its MVAD trial when it reports third quarter earnings on October 29.

Until we learn what the adverse events were and get insight into when the MVAD trial can restart, investors are left with more questions than answers. Since Heartware is an unprofitable company, investors might be best served patiently waiting on the sidelines for insight, rather than jumping in and buying on this slide.