Background and Purpose: With recent transcranial Doppler ultrasonography (TCD) techniques, high intensity transient signals (HITS) can be detected automatically. However, the significance of automatically detected HITS without associated sound is unclear. We attempted to determine whether HITS detected automatically during veno-arterial extracorporeal membrane oxygenation (V-A ECMO) reflect the state of blood coagulability. Methods : In an animal study, V-A ECMO was performed in 11 adult pigs by aortic and pulmonary cangulation using a roller pump and membrane oxygenator. HITS were counted automatically during bypass before and after administration of anticoagulants. In a clinical study of 6 patients with V-A ECMO after cardiac surgery, HITS were detected during and after removal of ECMO. Results: In the animal study, the majority of HITS recorded during V-A ECMO had no associated sound. The HITS count before and after ECMO was 0 and 49±35/10-min (n=11), respectively. The HITS count after administration of heparin was 4±4/10-min (n=5), and that after administration of argatroban was 7±7/10-min (n=6) . HITS counts were significantly reduced after administration of either anticoagulant (p<0.001) . In the clinical study, HITS were detected frequently during ECMO (158±99/15-min (n=6) ), but no HITS were detected after its removal. The HITS count rate was negatively correlated with activated clotting time. Conclusions: Automatically detected HITS without associated sound may reflect the degree of coagulation during V-A ECMO. Therefore, inaudible HITS may be a useful indicator of the level of anticoagulation during V-A ECMO.