Forty critical patients with respiratory failure in the intensive care unit were randomly divided into two groups,Group A with administration of parenteral nutritino(PN) and Group B,no parenteral nutrition given.Blood gas analysis and respiratory monitoring showed that the respiratory rate,pH、PaO2、PaCO2 and HCO3- had no marked difference between the two groups.But in Group A there was a slight decrease of Pao2/FiO2 and a marked increase of A-aDO2 and the pulmonary shunt.This study indicates that the content of fat emulsion and hydrocarbon in PN may be the main factor that affects the respiratory function.