Mechanical heart valve cavitation

Cavitation was first directly related to mechanical heart valves in the mid 1980s after a series of valve failures observed with the Edwards-Duromedics valve. The damages observed indicated that cavitation could be responsible. Later, several in vitro studies visualized the bubble formation and collapse of cavitation at mechanical heart valves. It was suggested that cavitation could also cause damage to the formed elements of blood and thereby enhance the risk of thromboembolic complications seen in mechanical heart valve patients. Therefore, an applicable technique for in vivo detection of cavitation is required. This article reviews techniques developed for in vivo detection of cavitation and suggests focus for future studies.

A misinterpreted case of aorta prosthesis endocarditis: remember the phenomenon of microbubbles

A 17-year-old male with a history of newly implanted mechanical valve at the aortic position, presented with fever, rigors, and painful cutaneous abscesses on his lower extremities and was suspected for infective endocarditis. Transthoracic echocardiography (TTE) showed a vegetation-like structure following the movement of the mechanical heart valve (MHV), which eventually proved to be a product of degassing microbubbles (MB).

Effects of the driving condition of a pneumatic ventricular assist device on the cavitation intensity of the inlet and outlet mechanical heart valves

Our group is currently developing a pneumatic ventricular assist device (PVAD), and in previous studies, we reported the mechanical heart valve (MHV) cavitation intensity at the inlet valve in the PVAD only. In this study, we investigated the effect of the running conditions on the cavitation intensity both for the inlet and outlet valve in the PVAD using an acoustic signal. A 23-mm Medtronic Hall valve with an opening angle of 70 degrees was mounted in the inlet and outlet port of the PVAD after removing the sewing ring. A mini pressure sensor with high frequency was mounted 15 mm downstream from the inlet valve and downstream from the outlet valve. The pressure signal was band-pass filtered between 35 and 500 kHz using a digital filter. The band-pass filtered root mean squared (RMS) pressure was used as an index of the cavitation intensity. The RMS pressure of the inlet valve was higher than that of the outlet valve. Even if the outlet valve has a lower RMS pressure than the inlet valve, cavitation occurs. In case of a full-filling and full-ejection condition, the RMS pressure of the inlet valve was higher than that of the partial-filling and partial-ejection condition. This means that a partial-filling and partial-ejection condition is best to prevent the hemolysis caused by the cavitation phenomenon and the damage to the valve surface in our PVAD system.

Effect of systolic duration on mechanical heart valve cavitation in a pneumatic ventricular assist device: using a monoleaflet valve

The cavitation intensity of a mechanical heart valve (MHV) may differ according to the geometry of the blood pump and driving mechanism. Our group is currently developing a pneumatic ventricular assist device (VAD), and the effects of different operating conditions on MHV cavitation in our pneumatic VAD were investigated. Tests were conducted under physiological pressure at heart rates ranging from 60 to 90 beats/min and at a systolic duration ranging from 38% to 43%. The valve-closing velocity was measured using a charge-coupled device (CCD) laser displacement sensor, and images of MHV cavitation were recorded using a high-speed video camera. A miniature pressure sensor was mounted 10 mm away from the inlet valve surface. The data were stored at a 1-MHz sampling rate using a digital oscilloscope. The pressure signal was band-pass filtered between 35 and 200 kHz using a digital filter. The cavitation bubbles were concentrated at the inlet valve stop, and were caused mainly by the squeeze flow. The band-pass filtered root mean squared (RMS) pressure and cavitation cycle duration increased with the closing velocity of the inlet valve. At a low heart rate and low systolic duration, the inlet valve closed before the outlet valve opened, which caused no cavitation bubbles to form around the valve stop.

Estimation of mechanical heart valve cavitation in a pneumatic ventricular assist device

In this study, we investigated the possibility of estimating the mechanical heart valve (MHV) cavitation intensity using the slope of the driving pressure (DP) just before valve closure in a pneumatic ventricular assist device. We installed a 23-mm Medtronic Hall valve at the inlet of our pneumatic ventricular assist device (VAD). Tests were conducted under physiologic pressures at heart rates ranging from 60 to 90 beats/min and cardiac outputs ranging from 4.5 to 6.7 l/min. The valve-closing velocity was measured with a CCD laster displacement sensor, and the images of MHV cavitation were recorded using a high-speed video camera. The cavitation cycle time (equal to the observed duration of the cavitation bubbles) was used as the MHV cavitation intensity. The valve-closing velocity increased as the heart rate increased. Most of the cavitation bubbles were observed near the valve stop, and the cavitation intensity increased as the heart rate increased. The slope of the DP at 20 ms before valve closure was used as an index of the cavitation intensity. There were differences in the slope of the DP between low and high heart rates, but the slope of the DP had a tendency to linearly increase with increasing valve-closing velocity.

Hydrodynamic characteristics of the Edwards MIRA bileaflet valve in a pneumatic ventricular assist device

In the present study, we used a bileaflet valve in our pneumatic ventricular assist device (PVAD). To estimate the effects of the orientation angle of a bileaflet valve on mechanical heart valve cavitation in the PVAD, the valve was rotated from 0 degrees to 90 degrees on an inclined horizontal plane. Tests were conducted under physiological pressure with heart rates of 80 bpm and a systolic ratio of 43%. A 23-mm Edwards MIRA bileaflet valve was installed in the inlet position of the PVAD, and the valve-closing velocity was measured with a closed circuit digital laser displacement sensor. Images of mechanical heart valve cavitation bubbles were recorded with the use of a high-speed video camera. The closing delay time between the two leaflets ranged from 0.88 +/- 0.41 to 0.50 +/- 0.27 ms, which was the largest at a valve orientation angle of 0 degrees . Cavitation bubbles were concentrated along the leaflet tip and were caused by the initial valve closure, valve rebound, and the second valve closure. Even when the valve-closing velocity was slow, stronger cavitation bubbles were observed at the second valve closure and valve rebound. The cavitation event ratio differed from the valve orientation angle, which resulted from the high initial valve closure.

Observation and quantification of cavitation on a mechanical heart valve with an electro-hydraulic total artificial heart

In previous studies, we investigated the cavitation phenomenon in a mechanical heart valve using an electro-hydraulic total artificial heart. With this system, a 50% glycerin solution kept at 37 degrees C was used as the working fluid. We reported that most of the cavitation bubbles were observed near the valve stop and were caused by the squeeze flow. However, in these studies, the effect of the partial pressure of CO(2) on the mechanical heart valve cavitation was neglected. In this study, in order to investigate the effect of the partial pressure of CO(2) on mechanical heart valve cavitation using an electro-hydraulic total artificial heart, we controlled the partial pressure of the CO(2) in vitro. A 25-mm Medtronic Hall valve was installed in the mitral position of an electro-hydraulic total artificial heart. In order to quantify the mechanical heart valve cavitation, we used a high-speed camera. Even though cavitation intensity slightly increased with increases in the PCO(2) at heart rates of 60, 70 and 100 bpm, throughout the experiment, there was no significant difference between the PCO(2) and cavitation intensity.

Estimation of mechanical heart valve cavitation in an electro-hydraulic total artificial heart

The purpose of this study was to establish a method for estimating mechanical in vitro heart valve cavitation in an electro-hydraulic total artificial heart (EHTAH). The variations in the left driving pressure (LDP) slope of the EHTAH were used as an index of the cavitation intensity. The LDP slope was controlled by changing the stroke volume of the EHTAH. The stroke volume was changed from full-filling and full-eject to partial-filling and partial-eject conditions. A 25-mm Medtronic Hall valve was installed in the mitral position of the EHTAH. Cavitation bubbles were concentrated on the valve stop; the major cause of these cavitation bubbles was determined to be squeeze flow. The valve-closing velocity was found to be proportional to increases in the LDP slope and the stroke volume of the left blood pump. The cavitation intensity and the cavitation event rate increased with increases in the stroke volume of the EHTAH. A consistent correlation was observed between the valve-closing velocity and the cavitation intensity. The LDP slope of the EHTAH may play an important role in estimating the mechanical heart valve cavitation intensity.

Björk-Shiley Convexoconcave Valves: Susceptibility Artifacts at Brain MR Imaging and Mechanical Valve Fractures

PURPOSE

To assess the relationship between heart valve history and susceptibility artifacts at magnetic resonance (MR) imaging of the brain in patients with Björk-Shiley convexoconcave (BSCC) valves.

MATERIALS AND METHODS

MR images of the brain were obtained in 58 patients with prosthetic heart valves: 20 patients had BSCC valve replacements, and 38 had other types of heart valves. Two experienced neuroradiologists determined the presence or absence of susceptibility artifacts in a consensus reading. Artifacts were defined as characteristic black spots that were visible on T2*-weighted gradient-echo MR images. The statuses of the 20 explanted BSCC valves-specifically, whether they were intact or had an outlet strut fracture (OSF) or a single-leg fracture (SLF)-had been determined earlier. Number of artifacts seen at brain MR imaging was correlated with explanted valve status, and differences were analyzed with nonparametric statistical tests.

RESULTS

Significantly more patients with BSCC valves (17 [85%] of 20 patients) than patients with other types of prosthetic valves (18 [47%] of 38 patients) had susceptibility artifacts at MR imaging (P =.005). BSCC valve OSFs were associated with a significantly higher number of artifacts than were intact BSCC valves (P =.01). No significant relationship between SLF and number of artifacts was observed.

CONCLUSION

Susceptibility artifacts at brain MR imaging are not restricted to patients with BSCC valves. These artifacts can be seen on images obtained in patients with various other types of fractured and intact prosthetic heart valves.