 Methods-Patients: Non surgical patients admitted in a single medical intensive care unit, under mechanical ventilation for an expected duration 2 days, with decreased concentrations of nasal NO 60 ppb, without severe sepsis or septic shock, will be enrolled. On admission before treatment, the severity will be evaluated SAPS II and SOFA score together with an assessment of plasmatic L-arginine, cytokines IL-6, IL-17, MDSC, and expression of HLA-DR by monocytes. The same evaluation will be repeated on day 4 during treatment and on day 7 after treatment. The enrolment of 50 patients is statistically enough to demonstrate an increased expression of HLA-DR in the L-arginine group as compared to the placebo group on day 4. 