Microbubble detection during extracorporeal circulation for open heart surgery

In a review of the scientific literature clinical and experimental evidence is produced to implicate microbubbles as a source of brain damage during extracorporeal circulation for open heart surgery. Direct evidence of tissue damage and definitive, quantitative effects of microbubbles in vivo have yet to be determined. The factors reported to be responsible for the formation and elimination of microbubbles during extracorporeal circulation are examined and consideration is given to the treatment of microbubbles as dynamic entities. An analysis is developed, in non-dimensional form, to relate bubble lifetime to system parameters appropriate to the extracorporeal circuit. Reported techniques for detecting microbubbles during cardiopulmonary bypass are examined and found to be inadequate for quantitative measurements. Doppler and other continuous wave ultrasonic techniques are shown to be inherently less suitable for quantitative microbubble detection than pulsed ultrasonic methods. A design for a flow dependent microbubble detector based upon a consideration of the limitations of previously used techniques and a consideration of physical principles is presented and the instrumentation discussed. Calibration methods have been developed, applied and the flow dependent detector shown to be effective within defined limits. The results of calibration and evaluation tests are presented and discussed. In vitro and in vivo applications of the flow dependent detector are presented and discussed. In particular the influences of blood flow rate, oxygen flow rate, oxygenator reservoir level and arterial line filtration upon microbubble liberation are investigated and the importance of quantification in extracorporeal microbubble studies examined. Recommendations are presented for developing the microbubble detector and for reducing the volume of gas in the form of microbubbles pumped into the patient during cardiopulmonary bypass for open-heart surgery