Extracorporeal bubbles: a word of caution

Gaseous microemboli (GME) are a cause of neurocognitive deficit postcardiac surgery with cardiopulmonary bypass. However, the measurement of microbubbles during cardiopulmonary bypass is not easy, because blood is an opaque fluid and contains microparticles. Recently, two new microbubble counters, the Gampt BC200 and the emboli detection and classification (EDAC) quantifier, were developed for use during cardiac surgery. The accuracy of both devices was validated against industrial standards.

A clear blood analogue (30%/70% glycerol-water) was pumped, by means of a rollerpump out of a partially filled arterial line screen filter with a nominal pore size of 20 microm. Downstream the pump, all test sensors and an optical cuvette, were inserted in a vertically mounted 3/8 inch tubing. Measurements were taken at flows of 405, 1080, 3000 and 6000 ml/min. Backlight shadowgraphy and optical counting were used as industrial reference techniques for measuring size and counts.

On average the EDAC quantifier underestimates average bubble diameter by 35+/-1%, 13+/-8%, 71+/-7% and 33+/-4% at 405, 1080, 3000 and 6000 ml/min, respectively. The Gampt BC200 has good sizing agreement at 1080 ml/min (+5+/-17%) and 3000 ml/min (+33+/-6%) but overestimates average diameter by 220+/-40% and 295+/-49% at 405 and 6000 ml/min, respectively. Both devices only partially count the number of microbubbles at higher flows. At 3 l/min the EDAC counts 38%, the Gampt 18% of total counts and at 6 l/min both the EDAC and Gampt only count 3% of total counts.

Both the EDAC and Gampt can be used in a clinical setting for monitoring basal GME production. However, both devices have some major limitations when used for studying 'worst case' scenarios. One should take great caution when correlating measured data with neurocognitive outcome. Finally, results obtained by one device in a first study cannot be compared nor exchanged with results from the other device in a second study.