Cavitation phenomena in mechanical heart valves: the role of squeeze flow velocity and contact area on cavitation initiation between two impinging rods.

In this study, the closing dynamics of two impinging rods were experimentally analyzed to simulate the cavitation phenomena associated with mechanical heart valve closure. The purpose of this study was to investigate the cavitation phenomena with respect to squeeze flow between two impinging surfaces and the parameter that influences cavitation inception. High-speed flow imaging was employed to visualize and identify regions of cavitation. The images obtained favored squeeze flow as an important mechanism in cavitation inception. A correlation study of the effects of impact velocities, contact areas and squeeze flow velocity on cavitation inception showed that increasing impact velocities results in an increase in the risk of cavitation. It was also shown that for similar impact velocities, regions near the point of impact were found to cavitate later for those with smaller contact areas. It was found that the decrease in contact areas and squeeze flow velocities would delay the onset and reduce the intensity of cavitation. It is also interesting to note that the squeeze flow velocity alone does not provide an indication if cavitation inception will occur. This is corroborated by the wide range of published critical squeeze flow velocity required for cavitation inception. It should be noted that the temporal acceleration of fluid, often neglected in the literature, can also play an important role on cavitation inception for unsteady flow phenomenon. This is especially true in mechanical heart valves, where for the same leaflet closing velocity, valves with a seat stop were observed to cavitate earlier. Based on these results, important inferences may be made to the design of mechanical heart valves with regards to cavitation inception.

Topical glyceryl trinitrate and eutectic mixture of local anaesthetics in children. A randomised controlled trial on choice of site and ease of venous cannulation.

One hundred and four children aged between 1 and 11 years were studied in a double-blind randomised controlled trial of glyceryl trinitrate ointment versus placebo, when used in addition to standard eutectic mixture of local anaesthetics cream. Each child received glyceryl trinitrate ointment on one hand and placebo on the other, and thus acted as his/her own control. A group of 30 children who received only the eutectic mixture on both hands (60 measurements) was also studied. The choice of site and ease of cannulation was scored. Skin colour and venous dilatation under the eutectic mixture were scored on a visual analogue scale. The addition of topical glyceryl trinitrate ointment to the standard eutectic mixture positively affected venous dilatation (p less than 0.01), choice of cannulation site (p less than 0.001), and ease of cannulation (p less than 0.001) of topical anaesthetic-treated skin.