An apparatus for determination of a recruiatable volume in a lung has a control system connectable to a pneumatic unit and to a measurement system and operable to, at a first point in time, regulate the pneumatic unit to generate a first test breath having predetermined parameters with respect to flow and/or pressure of the gas flow, register volume and pressure in the lung during the first test breath, measured by the measurement system and at a second point in time, to regulate the pneumatic unit to generate a second test breath identical to the first test breath, register volume and pressure in the lung during the second test breath, measured by the measurement system, compare the registered volume and pressure for the first test breath and the second test breath, and determine a recruiatable volume based on the comparison.