Fill in the blanks with the suitable (active or passive) form of the verbs in brackets:
Once a valve…………. (damage) beyond repair, as happens with recurrent attacks of a rheumatic fever, the only way to save the patient, …………(be) to replace it. Almost all replacements …………(do) today using mechanical devices. Among the earliest …………(be) the ball-and-cage valve which …………(invent) in 1961 by Albert Sterr, a surgeon of Portland, Oregon, USA and his collaborators M. Lowell Edwards. It …………(have) a plastic ball in a stainless steel or titanium cage. But the device ……………(have) several disadvantages. The presence of the ball…….. (cause) severe turbulence and cavitation which also ………………(cause) damage to red blood cells leading to the risk of clot formation. It ……………… (be) therefore necessary to put the patient on a long term anticoagulant therapy.