The patient monitoring apparatus determines a parameter representing an intrathoracic volume compartment, such as extravascular lung water, and normalises the determined parameter on the basis of input biometric data, such as body height, using an algorithm selected from a plurality of algorithms depending on a category a patient is allocated to. Preferably, the parameter is normalised (S6) by dividing it by a predicted (ideal) body weight which is calculated from the patients body height. A memory of the patient monitoring apparatus has stored therein a program for selectively executing one of several algorithms for determining a predicted weight (S51, S52, S53, S54, S55). Before determining normalised values, the operator is prompted to input height and gender of the patient and whether the patient is an adult or a pediatric patient (S1).
