Can cerebrovascular microemboli induce cognitive impairment in patients with prosthetic heart valves?

Doppler ultrasonography can be used to detect the presence of emboli in the cerebral arterial circulation. Emboli can be produced by different sources and can be of different nature: solid elements as thrombi, platelet aggregates or atheromatous material, or gaseous when they are produced during the decompression phase of diving or during extracorporeal circulation (ECC) procedures. A more recent source of emboli has been found in the mechanical prostheic heart valves (MHV). The emboli generated by MHV are likely of gaseous nature and are found in the middle cerebral artery blood flow at a variable rate, where they are detected by transcranial Doppler sonography. The mechanism of production of these microbubbles may be related to the rapid leaflet motion especially at closure when very high local pressure gradients appear, which may be able to provoke a release of the disolved blood gas. Solid element emboli constitute a major cause of cerebrovascular disease and particularly stroke. Conversely, gaseous emboli coming from ECC or MHV are considered as clinically silent. Nevertheless, cognitive alterations have been reported after ECC. As the MHV carriers are chronically submitted to embolic events, it can be assumed that cognitive impairments may occur also in these patients. A preliminary study was designed to inpatients attention and memory in patients with normally functioning MHV and microemboli, with biological prosthesis and in normal subjects. In the two groups of patients, episodic memory was significantly altered relatively to the control group. In the MHV carriers group, a significant decrease in working memory performance was observed relatively to the two other groups. These results confirm a long term effect of the microembolization occuring during ECC and point out the effect of the chronic exposition to microemboli.