Mechanical heart valve cavitation: valve specific parameters.

Several aspects of mechanical heart valve cavitation, in particular of "severe" vapor cavitation, have been investigated in order to describe the phenomenon of cavitation itself and to classify various mechanical heart valves with respect to their tendency to cavitation. Furthermore, following the results of the measurements, a model for determination of time-dependent physical properties and dynamics of cavitation bubbles, such as size, pressure and temperature was developed. In order to classify the cavitation tendency of mechanical valves, a pulsatile hydraulic-driven circularly mock loop was used. Besides measurements of the relevant hemodynamic parameters, the leaflet velocities of the valves were also determined. In addition, numerous high-resolution pressure measurements, in particular the pressure drops necessary for the initiation of cavitation (local atrial pressure drop), were performed. For the investigation of bubble dynamics, a second pulsatile electro-magnetically-driven tester was used. The influence of density, viscosity and temperature of the fluid on the onset of cavitation was investigated. Cavitation events were recorded with a digital high-speed video camera (up to 40,500 frames/sec) for all investigated heart valves and under different conditions. A critical local upstream pressure drop (located within the model atrium after valve closure) of 450 mmHg was found for all valves as well as a valve specific correlation between left ventricular pressure gradient and local upstream pressure drop. Also, a valve dependent correlation between left ventricular pressure gradient and the local upstream pressure drop was provided. Finally, valve specific parameters were found to predict the cavitation tendency for a specific heart valve. The implementation of a suitable theoretical model allowed conclusions on bubble physics. High pressures (up to 800 bar) and temperatures (up to 1,300 degrees C) at bubble collapse have been determined. The influence of fluid parameters such as density, viscosity and temperature on the onset of cavitation is negligible within physiological range. Critical regions for cavitation for all mechanical heart valves were detected. All mechanical heart valves investigated show cavitation under different conditions (dp/dt) associated with high pressures and temperatures at bubble collapse. Cavitation bubble occurrence depends on valve design and location.

Selective control by corticosterone of serotonin1 receptor capacity in raphe-hippocampal system.

The effect of short-term bilateral adrenalectomy (ADX) and corticosterone or aldosterone replacement was investigated on serotonin1 (5-HT1) receptor density in rat brain regions. One hour after ADX the 5-HT1 receptor density was increased in subiculum, molecular layer of the dentate gyrus, substantia nigra, and dorsal raphe nucleus as shown by in vitro autoradiography and computerized densitometric analyses of the film images. In subiculum, dentate gyrus, and dorsal raphe nucleus the 5-HT1 receptor density was restored (decreased) towards the level observed in sham-operated rats after administration of a low dose of corticosterone (CORT) at the time of ADX. CORT replacement decreased the 5-HT1 receptor number also in hippocampal pyramidal cell layer CA1, presubiculum, and other dentate gyrus subregions. The 5-HT1 receptor density was not affected by ADX or CORT replacement therapy in cerebral cortex, central grey, CA3, ventral hippocampus, and median raphe nucleus. Aldosterone administered under the same experimental conditions did not change the 5-HT1 receptor number in any of the hippocampus or raphe regions. The dose of CORT (30 micrograms/100 g body weight, s.c.) gave physiological plasma levels and maintained almost complete occupation of CORT receptors (mineralo-corticoid-like receptors) in hippocampus which was also the case in the sham-operated control animals. CORT replacement did not maintain occupancy of glucocorticoid receptors in hippocampus. When the binding of the selective glucocorticoid agonist, 3H-RU 28362, was taken as 100% at 1 h after ADX, CORT replacement and sham ADX left 75 and 50% of these sites available respectively.(ABSTRACT TRUNCATED AT 250 WORDS)