A method to compensate for a non-zero mixed venous partial pressure of inert blood soluble gas when determining physical parameter(s) of a patient in successive inert gas rebreathing tests, said method comprising the steps of: - obtaining by a gas analyzer the partial pressure of said inert blood soluble gas inhaled and/or exhaled by said patient during a period prior to said successive rebreathing test; - obtaining by a gas analyzer the partial pressure of said inert blood soluble gas inhaled and/or exhaled by said patient in said successive rebreathing test; - determining said physical parameter(s) in an iterative procedure by processing means, said processing means performing the steps of: ∘ extracting a first parameter using said obtained partial pressure of said inert blood soluble gas inhaled and/or exhaled by said patient in said successive rebreathing test; ∘ extracting a second parameter using said first parameter and said obtained partial pressure of said inert blood soluble gas inhaled and/or exhaled by said patient during said period prior to said successive rebreathing test; ∘ re-extracting said first parameter using said second parameter and said obtained partial pressure of said inert blood soluble gas inhaled and exhaled by said patient in said successive rebreathing test; ∘ determining said physical parameter(s) using said first parameter, said second parameter and/or said re-extracted first parameter. - obtaining by a gas analyzer the partial pressure of said inert blood soluble gas inhaled and/or exhaled by said patient during a period prior to said successive rebreathing test; - obtaining by a gas analyzer the partial pressure of said inert blood soluble gas inhaled and/or exhaled by said patient in said successive rebreathing test; - determining said physical parameter(s) in an iterative procedure by processing means, said processing means performing the steps of: ∘ extracting a first parameter using said obtained partial pressure of said inert blood soluble gas inhaled and/or exhaled by said patient in said successive rebreathing test; ∘ extracting a second parameter using said first parameter and said obtained partial pressure of said inert blood soluble gas inhaled and/or exhaled by said patient during said period prior to said successive rebreathing test; ∘ re-extracting said first parameter using said second parameter and said obtained partial pressure of said inert blood soluble gas inhaled and exhaled by said patient in said successive rebreathing test; ∘ determining said physical parameter(s) using said first parameter, said second parameter and/or said re-extracted first parameter. ∘ extracting a first parameter using said obtained partial pressure of said inert blood soluble gas inhaled and/or exhaled by said patient in said successive rebreathing test; ∘ extracting a second parameter using said first parameter and said obtained partial pressure of said inert blood soluble gas inhaled and/or exhaled by said patient during said period prior to said successive rebreathing test; ∘ re-extracting said first parameter using said second parameter and said obtained partial pressure of said inert blood soluble gas inhaled and exhaled by said patient in said successive rebreathing test; ∘ determining said physical parameter(s) using said first parameter, said second parameter and/or said re-extracted first parameter.