Platelet aggregation and high-intensity transient signals (HITS) in a sheep model of mitral valve replacement.
OBJECTIVE
The composition of microemboli detected as 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 either a mechanical or a biological mitral valve prosthesis in place. The effect of ICI 170809, a 5HT2a antagonist, on the frequency of HITS and on platelet aggregates, counted in arterial blood smears per nine high-power fields, was assessed at three and six months after valve implantation. The mitral transvalvular gradient was measured by transthoracic echocardiography at three and six months.


RESULTS
Data are expressed as median and interquartile range. At three months, there were 36 (20-114) HITS/h in the mechanical group, and 0 (0-15) HITS/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 ICI 170809, or by duration of implant in either group. Platelet aggregate counts were higher with the mechanical than with the biological valve at three months, but not at six months. ICI 170809 reduced platelet aggregate counts in both valve types; the reduction was not significant in the bioprosthetic valve group. The pressure gradient across the bioprosthesis increased during the study from 2 (2-3) mmHg to 7.5 (6-10) mmHg, but was unchanged in the mechanical valve.


CONCLUSIONS
(i) It was confirmed that the frequency of HITS is higher with the mechanical prosthesis than the bioprosthesis; (ii) circulating platelet aggregates in the bioprosthetic valve group tended to increase as structural valve deterioration occurred; (iii) the frequency of HITS was not influenced by either an increase or a decrease in circulating platelet aggregates; and (iv) HITS detected in patients with prosthetic valves are unlikely to be due to circulating platelet aggregates.
Raco
L
L
Department of Cardiac Surgery, University of Glasgow, UK.
Belcher
P R
PR
Sim
I
I
McGarrity
A
A
Bernacca
G M
GM
Wheatley
D J
DJ
D000818:Animals; D001705:Bioprosthesis; D004452:Echocardiography; D004617:Embolism / Q000097:blood / Q000000981:diagnostic imaging* / Q000209:etiology; D019918:Heart Valve Prosthesis Implantation / Q000009:adverse effects*; D008943:Mitral Valve / Q000601:surgery*; D010974:Platelet Aggregation / Q000187:drug effects; D011804:Quinolines / Q000494:pharmacology; D012702:Serotonin Antagonists / Q000494:pharmacology; D012756:Sheep; D018608:Ultrasonography, Doppler
D011804:Quinolines; D012702:Serotonin Antagonists; C060632:ICI 170809