Improved lower-limb negative-pressure equipment for the reduction of cerebral blood pressure in neurosurgery.

Abstract

1. Negative-Pressure Chambers. This unit comprises two Perspex cylinders (Fig. 1), one fitting over each leg. I t is designed this way for lightness, to facilitate setting up, and to provide complete and unobstructed visibility of the legs. The open end of each tube is sealed by a Celeron plastic end-plate, especially machined to take a short rubber sleeve with which the seal around the legs is effectcd. The tubes are 9 inches in diameter to give comparative freedom of foot movement, and for convenience of manufacture are not tapered. The thickness of the wall is 88 inch and the over-all length is 3 feet. For purposes of design these tubes can be treated as short thinwalled cylinders. Thus with the ultimate tensile strength of plasticised Pcrspex at 3 tons/sq, in. and the tubes subject to a maximum vacuum (15 lbs./sq, in.) the factor of safety is well in excess of 6, the normal factor of safety for alternating load applications. The longitudinal joint is tongued, grooved and cemented, and the closed end of the tube is domed for extra strength. The tubes are strengthened at the points of connection by cemented bosses. The points of connection can be seen in Fig. ~, the tubes being interconnected by a preformed rubber bend. In each tube a special port is provided, designed and positioned to facilitate rapid application of a drip set to either leg. ~. Connectors. The connectors are screwed-brass rubber-tube adaptors of conventional type. They are tightened by hand in the connector bosses, an 0 ring, recessed into the adaptor, effecting the seal against the face of the boss. The connectors and the vacuum gauge adaptor are readily interchangeable in the bosses. The dimension of the tube adaptor permits an easy-push fit for 9/16 inch I.D. thick-walled rubber vacuum hose. 3. Tube and Leg Sealing End Plates (Fig. ~). These end plates are machined from 89 inch thick Celeron, a fabric-reinforced plastic sheet. They spigot into the Perspex cylinders, the seal at this point being effected by a sponge-rubber ring, of rectangular cross section, cemented to the plate. They are fastened to the cylinders by two diametrically opposed quick-acting toggle clips. In Fig. 4 it may be seen that the aperture in the plate for the leg is machined off-centre. This is to enable the leg to be more comfortably positioned as each plate can be rotated to offer the least amount of pressure against the leg. This rotation does not upset retention of the clip, for the groove in the plate in which the clip engages is continuous. There are three sizes of end plates--5, 6 and 7 inch--and in each size the aperture for the leg is offset so that the distance between the peripheries of the aperture for the leg and the end plate remains the same.