programmed for example to relieve pressure at 10 minute intervals at these intervals valve 8 will open for a specified period allow the pressure to equalize at the wound site and then close to restore the suction it will be appreciated that when the constant suction or negative pressure is being applied to the wound site valve 8 remains closed and there is no leakage from atmosphere in this state it is possible to maintain negative pressure at the wound site without running the pump continuously but only from time to time to maintain a desired level of negative pressure i e a desired pressure below atmospheric which is detected by the transducer 105 this saves power and enables the appliance to operate for long periods on its battery power supply instead of running two separate tubes to the wound site it is preferable to contain tubes 106 and 101 in a single tube which is connected through the canister thus for example tubes 103 and 101 may comprise an internal tube surrounded by an annular space represented by tube 106 this is illustrated in fig5 a to 5f and in a modified form of fig6 e in an alternative embodiment the multi lumen tube may be constructed as shown in fig6 e in this embodiment the internal bore 606 comprises the line 101 see fig1 and is used to extract fluids from the wound site air flow represented by line 106 in fig1 passes down conduits 607 located within the walls of the.