BACKGROUND: With the advent of second harmonic imaging in echocardiography, microbubbles have been observed during opening and closure of mechanical prosthetic valves. The single phenomenon of cavitation, an extremely short event described in the literature, cannot explain the persistence of microbubbles during several hundred milliseconds. Therefore, in vitro we reproduced two distinct phenomena created by a local depression occurring during the closure and/or opening of prosthetic valves: Cavitation and degassing. METHODS: We used a water circuit system enriched with CO(2) that passes through a Venturi tube in order to create variable pressure gradients. Three types of observations were performed: (1). the dimensions of the bubbles as a function of pressure, (2). calibration of the echocardiograph, and (3). comparison and illustrations of the difference between bubble formation by cavitation (vaporization) and degassing (liberation of CO(2)). RESULTS: According to the different pressures exerted, the dimensions of the bubbles only vary by several microns, not measurable in practice. Second, the calibration of the echocardiograph reveals that the dimensions of the bubbles measured by ultrasound are greater by a factor of 1.75. Finally, the observed cavitation is a short phenomenon (several milliseconds) and happens under a great local pressure gradient. The degassing produces microbubbles lasting up to as long as > 1 second under much lower pressure. CONCLUSION: This in vitro study suggests that microbubbles observed during several hundred milliseconds after the opening of prosthetic cardiac valves are the result of degassing of CO(2) in blood rather than a cavitation phenomenon as suggested in the literature.