The effect of the aortic valve orientation on cavitation.

When implanting a mechanical aortic valve the annulus orientation is important with respect to turbulence. However, the effect on cavitation has not yet been investigated. The aim of this study was to investigate how cavitation is influenced hereof in vivo. Three pigs were included in the study. An Omnicarbon 21mm valve equipped with a rotating mechanism enabling controlled rotation of the valve was implanted in aortic position. Under stable hemodynamic conditions, measurements were performed using a hydrophone positioned at the aortic root. The valve was rotated from 0-360° in increments of 30°. From the pressure fluctuations recorded by the hydrophone the root mean square of the 50 kHz high pass filtered signal as well as the non-deterministic signal energy was calculated as indirect measures of cavitation. Various degrees of cavitation were measured but no relationship was found between either of the two cavitation measures and the valve orientation. Hemodynamics varied during the experiments for all pigs (3.9-5.7 l/min; 5.0-7.2 l/min; 3.1-7.5 l/min). Changes in cavitation quantities seemed to be caused by changes in hemodynamics rather than valve angular position. In conclusion, these results do not favor any position over another in terms of cavitation potential.

A method for the measurement of left ventricular overload for aortic valve insufficiency.

BACKGROUND: The degree of left ventricular overload in patients with aortic valve insufficiency (AI) plays an important role in determining the need and timing of surgical intervention. Because hemodynamic evaluation of AI may potentially predict the effects of an insufficient valve on the ventricle before they occur, it would be useful to guide valve surgery with such a diagnostic tool. The purpose of this study was to test the performance of a new hemodynamic index based on mechanical energy loss for the measurement of the effects of insufficiency on ventricular workload. METHODS AND RESULTS: An intact and subsequently perforated aortic bioprosthesis was tested within an in vitro model of the left heart, varying cardiac output, diastolic aortic pressure, and the size of perforation. Regurgitant orifice area (ROA), regurgitant volume (RV), regurgitant fraction (RF), and energy loss index (ELI) were measured for each experimental condition and plotted against the increase in workload per unit volume net forward flow (DeltaWPV) due to perforation. ROA, RV, and RF showed good correlations with DeltaWPV, but the relationship between these variables and DeltaWPV became ambiguous as their magnitudes increased. ELI had a near perfect linear relationship with DeltaWPV (slope=1.00, r(2)=0.98) independent of the experimental condition. CONCLUSIONS: RV, RF, and ROA do not by themselves fully describe the increase in difficulty the ventricle has in moving the blood across an insufficient valve. ELI, in contrast, was found to be a very good measure of the decrease in pump efficiency due to aortic valve insufficiency.

A method for the measurement of left ventricular overload for combined aortic valve pathology.

BACKGROUND AND AIM OF THE STUDY: Patients with combined aortic valve pathology (stenosis and insufficiency) are often evaluated as if they had only a single pathology, because a means of evaluating the detrimental effects of combined insufficiency and stenosis does not yet exist. The study aim was to test the performance of a new hemodynamic index based on mechanical energy loss to measure the effects of combined valve disease on ventricular workload. METHODS: An intact and subsequently perforated and sutured aortic bioprosthesis was tested in an in vitro model of the left heart, varying cardiac output, average diastolic aortic pressure, and the type and combination of valve lesion. The regurgitant fraction (RF), systolic transvalvular pressure gradient (Deltaps), and energy loss indices of forward flow (LPVf), regurgitant flow (LPVr), and the sum of the two (LPVc), were measured for each experimental condition and compared with the increase in work per unit volume net forward flow (DeltaWPV) due to perforation and suturing. RESULTS: Deltaps was found to underestimate LPVf when the valve was perforated. LPVc had an excellent linear relationship with DeltaWPV (slope = 0.98, r2 = 0.97) that was independent of valve lesion or flow and pressure conditions. CONCLUSION: Deltaps does not describe the increase in ventricular workload, or even the forward flow portion of it, when valve insufficiency is present. LPVc was found to be a very good measure of the decrease in pump effectiveness due to aortic valve insufficiency or combined valve pathology.

The effect of gap width on viscous stresses within the leakage across a bileaflet valve pivot.

BACKGROUND AND AIM OF THE STUDY: Stresses of leakage flow may contribute to the increased tendency for thromboembolic complications in patients with mechanical valves. In bileaflet valves, leakage occurs primarily in the pivots, and the width of the pivot gap influences viscous stress magnitudes. The present study was conducted to investigate the effects of gap width on viscous stresses within the pivots of a bileaflet mitral valve during the leakage phase. METHODS: A computational model of a bileaflet valve was created and inserted between models of the left atrium and ventricle. Three simulations were performed, varying gap width between the leaflet and housing in the pivot region. To validate these calculations, steady leakage across a scaled in-vitro model of a single pivot was initiated, and velocity measurements at specific locations within flow the pivot were obtained using one-component laser Doppler velocimetry. RESULTS: The average viscous stresses on the housing surface of the pivot increased from 198 to 299 Pa, and on the leaflet surface from 242 to 271 Pa, as gap width was increased from 100 to 300 microm. These stresses were similar in magnitude to the maximum turbulent stresses reported within the pivots in previous studies. Velocities measured experimentally were even larger than those estimated computationally. CONCLUSION: These experiments suggest that viscous stresses in leakage flow across a bileaflet mitral valve increase with gap width, and may contribute more to blood damage and increased risk of thromboembolic complications in patients with such valves than would turbulence.

Changes in turbulence with rotation of the omnicarbon prosthesis.

This study was performed to determine whether annular plane orientation of the Omnicarbon aortic valve influences forward flow turbulence. The Omnicarbon prostheses was modified to allow in situ manual rotation of the valve when implanted in the aortic position of eight 90 kg pigs. Pulsed Doppler ultrasound was used to acquire velocity measurements at 17 locations within the cross-sectional area of the ascending aorta. In each animal, 12 valve rotations were tested in this manner. Reynolds normal stresses were estimated from the velocity measurements. High Reynolds normal stresses were concentrated between left and posterior-right sides of the aortic wall for all orientations studied. No trends in mean or maximum Reynolds normal stresses with respect to valve rotation were consistent in the experiments. Unlike previous experiments with the Medtronic-Hall tilting disc valve, these experiments showed no notable changes in Reynolds normal stress with respect to orientation of the Omnicarbon valve. This suggests that the tendency of turbulent stresses to change with tilting disc valve orientation may be dependent on valve design.

In-vivo turbulent stresses of bileaflet prosthesis leakage jets.

BACKGROUND AND AIM OF THE STUDY: Previous studies of leakage jet turbulence have been carried out in vitro, using a Newtonian fluid to simulate blood and large, rigid approximations to the chambers of the heart. The study aim was to apply an in-vivo method of quantifying leakage jet turbulence to a variety of bileaflet mechanical heart valves, and thereafter to determine the effects of exercise and valve design on turbulent shear stresses within leakage flow. METHODS: Bileaflet prostheses sewn to a manual traversing device were implanted in the mitral position of 29 pigs of body weight ca. 90 kg. Pulsed Doppler ultrasound was used to acquire velocity measurements within the leakage jets detected 1 mm upstream of the housing. Analytical techniques were used to estimate peak velocities and maximum turbulent shear stresses from these velocity measurements. RESULTS: Maximum turbulent shear stress was found to rise with increasing ventricular pressure. No leakage turbulence was found from a valve with relatively small leakage gap widths. The Medtronic Parallel valve was found to have considerable significant leakage flow disturbance, even under low ventricular pressure conditions. Similar maximum turbulent shear stress magnitudes were estimated in the leakage jets of the St. Jude Medical, CarboMedics and Sorin Bicarbon valves at medium ventricular pressure conditions. The maximum turbulent shear stresses estimated in these experiments were lower than those found in previous in-vitro measurements. CONCLUSION: Exercise raises the turbulent shear stresses of leakage flow substantially. Hinge design and leakage gap width also affect the magnitudes of these stresses. Leakage flow turbulence may be less damaging to the blood than was previously thought, and is considerably less damaging than forward-flow turbulence.

An in vivo method for measuring turbulence in mechanical prosthesis leakage jets.

This work introduces a method for the in vivo measurement and analysis of turbulence within the leakage of a mechanical heart valve. Several analysis techniques were applied to ultrasound measurements acquired within the atrium of a pig, and error associated with these techniques was analyzed. The technique chosen applies cyclic averaging to mean and maximum velocity measurements within small, normalized phase windows to calculate Reynolds normal stresses in the direction of the ultrasound beam. Maximum shear stresses are estimated from these normal stresses using an analytical technique. The stresses observed were smaller than those reported from previous in vitro simulations.

Cavitation caused by mechanical heart valve prostheses--a review.

Heart valve dysfunction often necessitates surgical implantation of a mechanical heart valve (MHV). Although implantation of a MHV is a life-saving procedure, the patient still faces potentially complications such as thromboembolic events and material failure. These complications may be caused by cavitation, which can occur during valve closure. Cavitation is an erosive phenomenon that can be generated in fluids when the pressure locally drops below the vapor pressure. This paper reviews the literature on cavitation and MHVs and particular features of the valve and closing conditions that potentially increase the intensity of cavitation. Techniques for detecting cavitation will be discussed. Of these, an acoustic approach will be emphasized, since it is currently the only technique able to detect and quantify cavitation in vivo.

An analysis of turbulent shear stresses in leakage flow through a bileaflet mechanical prostheses.

In this work, estimates of turbulence were made from pulsatile flow laser Doppler velocimetry measurements using traditional phase averaging and averaging after the removal of cyclic variation. These estimates were compared with estimates obtained from steady leakage flow LDV measurements and an analytical method. The results of these studies indicate that leakage jets which are free and planar in shape may be more unstable than other leakage jets, and that cyclic variation does not cause a gross overestimation of the Reynolds stresses at large distances from the leakage jet orifice.