The use of Doppler ultrasound in the assessment of microemboli during cardiac surgery

Arterial microemboli may contribute to bypass (CPB) related cerebral dysfunction and might be reduced by arterial line filtration. Doppler ultrasonic techniques have been used to detect such emboli in the bypass circuit since 1965. Attention has recently focussed on detecting microemboli which actually reach the cerebral circulation. We evaluated a transcranial Doppler system (TC2-64, EME) on an extracorporeal circuit and confirmed that microemboli can be detected and quantified. In the operating theatre, we measured middle cerebral artery blood velocity and the incidence of microembolic events (MEE) during cardiopulmonary bypass. Twenty patients (median age 56 yrs, range 50-64) with arterial line filters (40 micron, Pall ECplus) and 20 without (56 yrs, 43-70) have been studied. Bypass technique was standardized and a bubble oxygenator (Harvey H 1700) used in all cases. MEE occurred during aortic cannulation and at inception of bypass. On bypass, patients with filters had fewer MEE (0-10 per 30 minutes) than nonfiltered patients (30->250 per 30 minutes). We conclude that the TC2-64 transcranial Doppler system can detect and quantify microemboll of greater than 40 microns diameter in the middle cerebral artery during cardiac surgery. It cannot differentiate particulate from gaseous microemboli. We have also identified high risk periods for cerebral microembolisation.