An apparatus for treating wound with negative pressure. The apparatus including a wound cover (1), a first pump (9) for providing said negative pressure at a chosen pressure level to the wound, a canister (6), a first conduit (8) between the wound cover and the canister, which first conduit (8) connects the wound with the canister (6) during use of the apparatus, first means for measuring the pressure within the canister, and a second conduit (10) which connects the canister with the first pump. A circulating pump (12) is arranged to intermittently transport a gas volume from the canister (6) to the wound region via a third conduit (13) and back to the canister via the first conduit (8) in order to press out possible exudates from the first conduit to the canister. A second means is arranged to analyse the pressure curve over time of said pressure in the canister and a third means is arranged to indicate by a visible and/or audible alarm when said pressure curve differs from a normal cyclical form. The invention further relates to a method for controlling the negative in a wound with use of said apparatus.
