Platelet aggregation and high-intensity transient signals (HITS) in a sheep model of mitral valve replacement

Background and aims of the study: The composition of microemboli detected a s high-intensity transient signals (HITS) by Doppler ultrasound in patients with prosthetic heart valves is still debated. Here, platelet aggregation and HITS were investigated in a sheep model. Methods: Insonation of the carotid artery was performed in 20 sheep with ei ther a mechanical or a biological mitral valve prosthesis in place. The eff ect of ICI 170809, a 5HT(2a) antagonist, on the frequency of HITS and on pl atelet aggregates, counted in arterial blood smears per nine high-power fie lds, was assessed at three and six months after valve implantation. The mit ral transvalvular gradient was measured by transthoracic echocardiography a t three and six months. Results: Data are expressed as median and interquartile range. At three mon ths, there were 36 (20-114) HITS/h in the mechanical group, and 0 (0-15) HI TS/h in the biological group. At six months, there were 21 (0-82) and 0 (0- 2) HITS/h, respectively. The occurrence of HITS was unaffected by either IC I 170809, or by duration of implant in either group. Platelet aggregate cou nts were higher with the mechanical than with the biological valve at three months; but not at six months. ICI 170809 reduced platelet aggregate count s in both valve types; the reduction was not significant in the bioprosthet ic valve group, The pressure gradient across the bioprosthesis increased du ring the study from 2 (2-3) mmHg to 7.5 (6-10) mmHg, but was unchanged in t he mechanical valve. Conclusions: (i) It was confirmed that the frequency of HITS is higher with the mechanical prosthesis than the bioprosthesis; (ii) circulating platele t aggregates in the bioprosthetic valve group tended to increase as structu ral valve deterioration occurred; (iii) the frequency of HITS was not influ enced by either an increase or a decrease in circulating platelet aggregate s; and (iv) HITS detected in patients with prosthetic valves are unlikely t o be due to circulating platelet aggregates.