A fluid coupling has been developed for use in securing a vascular outflow graft (cannula) to the outlet of a surgically implanted NASA/DeBakey heart assist pump. The design of the coupling can also be adapted to other applications in which it is necessary to join flexible tubes with rigid ones. A joint formed by use of this coupling is separable, yet free of leaks; this is advantageous in that (1) it is necessary to be able to install or remove a pump in accordance with requirements for surgery, sterilization, and pump maintenance, but (2) seepage of blood from an installed pump/cannula joint cannot be tolerated. Moreover, the coupling provides a smooth transition for flow from the pump outlet to the cannula; this feature helps to prevent clotting, which is triggered by flow-surface discontinuities.
The coupling (see figure) includes tapered and threaded surfaces on the pump outlet tube, a differentially tapered locking collet, a connector nut that mates with the thread on the pump outlet tube, and the affected end portion of the cannula. A volume is provided at the upstream end of the tapered outlet-tube surface to allow excess cannula material (including material extruded by the clamping described below) to expand to form a bulbous collar that helps to retain the cannula in place. This volume also facilitates manufacture and assembly, inasmuch as it desensitizes the joint to slight variations in cannula geometry.
The differential angle between the tapered surfaces of the pump outlet and the locking collet helps to clamp the cannula in place and to provide a seal when the connector nut is tightened. The radial clearance between the locking collet and the connector nut is sufficient to allow the collet to move slightly to align itself for uniform clamping of the cannula. The nut can be tightened to obtain the desired clamping and sealing force, or can be loosened to disconnect the pump from the cannula.
This work was done by Bernard J. Rosenbaum of Johnson Space Center.
This invention has been patented by NASA (U.S. Patent No. 6,050,987). Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to the Patent Counsel, Ames Research Center, (650) 604-5104. Refer to MSC-22865.