Microemboli produced during extracorporeal circulation are a recognised cause of increasing morbidity and mortality in cardiac surgery procedures [3, 4]. We agree that TCD can be helpful in assessing and monitoring the quality of extracorporeal perfusion in regard of blood flow velocity and emboli detection, allowing the best perfusion strategy, not only during cardiopulmonary bypass, but also during ECMO.
In literature there aren't papers about the brain emboli detection monitoring with TCD during ECMO. There are few reports about brain emboli monitoring with TCD on patients treated with left ventricular assist device (LVAD) [5, 6]. One paper reports that in Novacor LVAD patients the amount of MES is directly correlated with clinical thromboembolism [7]; the pathogenesis of microembolization in LVAD-patients seem to be still unknown.
Our experience sustains the hypothesis that patient treated with high blood flow ECMO can be at high risk for brain and systemic embolization from venous gas embolism. Mini extracorporeal circuits and the membrane oxygenator filters seem to be not efficient enough to remove microemboli. We believe that the 50% ECMO assistance is not associated with microembolic signals because the microemboli enter more easily into the pulmonary circulation. The bubbles test performed with a mixture of 1 ml of air and 9 ml of crystalloid solution, injected into a central venous catheter sustains our hypothesis.
In the last years Doppler technology has made possible to differentiate not only gaseous but also solid microemboli [8]. The solid component of the embolic load that we detected is probably related to platelets aggregation on gas microbubbles, as documented by others papers [9]. The different proportion of solid emboli in four patients treated with ECMO 100%, could be explained with a difference in the coagulation status of the patients, despite activated clotting time was in the same range. Gaseous and solid microemboli can produce tissue ischemia and subsequent tissue damage through the obstruction of the microcirculation. On account of this we hypothesize that the pharmacological anticoagulation requested to maintain the extracorporeal circulation may be a risk factor of subsequent brain haemorrhage [2].
Our study supports the possible role of microembolization in worsening the patients outcome since the total embolic load can be enormous during several days of ECMO assistance.
Moreover in two of our patients the emboli count is underestimated because they had showers of MES (patient 5 and 6) not countable by the software for the high numbers of microbubbles that occur at once. Some authors recently proposed a radio-frequency based TCD analysis to overcome this limitation and better correlate the neurologic outcome to cerebral embolic load [10].
To reduce the probability of microembolization on ECMO we tested an air filter device (0.2 micron), high efficient in removing air bubbles from venous infusion lines (Figure 2 and additional file 4). The use of this device has made possible to erase microemboli during crystalloid solution infusion. This investigation is still open during fast infusion of fresh frozen plasma and red packed cells, because a bigger filter capacity is required. Moreover there is evidence, in patients with mechanical heart valves, that gas microbubbles may be reduce by administering 100% oxygen through the mechanism of blood de-nitrogenation [11]. This protective effect could be utilize to reduce number and size of air microbubbles also during extracorporeal perfusion.
