In our previous study, we found that low dose (50mg/2h) recombinant tissue plasminogen activator (rt-PA) regimen had less bleeding tendency than the 100mg/2h regimen (3% vs.10%), it is worthwhile to reveal whether low dose rt-PA plus Low Molecular Weight Heparin (LMWH) can rapidly reverses RV pressure overload in PE, but not increase bleeding and other adverse events.