Cordis Orbis Sigma
control devices (ASDs) can be incorporated in series with the valve. The device, which is placed subcutaneously in series with the valve, houses a mobile membrane that moves in response to a pressure change across it. The outer surface is theoretically at atmospheric pressure. When the pressure within the shunt falls, the membrane moves to occlude the shunt lumen. ASDs are available as separate components to insert below the valve in existing shunts; alternatively they may be incorporated into a valve as in the Delta valve (P.S. Medical Corporation, California, USA) which combines a diaphragm valve and a siphon control device in one.
A different approach to the siphon problem is seen in the Orbis-Sigma valve (Cordis Corporation). By contrast to differential pressure valves, which are pressure regulating, this valve is designed to be a flow-regulating device, permitting a relatively constant flow rate over a wide range of differential pressures.
Unfortunately, at present, there are insufficient data to support many manufacturers' claims of superior function of particular shunt valves. Choice of valve design continues to be, for the most part, a matter of surgical preference, no clear advantages have been consistently demonstrated for any individual model .
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