BACKGROUND: Instillation of normal saline before endotracheal suctioning is thought to facilitate removal of secretions and ultimately to improve the patient's oxygenation status. To date, no studies have used an in vivo measure of oxygenation such as mixed venous oxygen saturation to characterize the effects of instillation of normal saline. OBJECTIVES: To describe the effects of instillation of normal saline into an endotracheal tube before suctioning on mixed venous oxygen saturation in critically ill adult patients. METHODS: In a descriptive, observational study, 35 patients were assigned to either of 2 groups after coronary artery bypass grafting. One group had 5 mL of normal saline instilled at the start of endotracheal tube suctioning; the other group had the same endotracheal tube suctioning procedure without the use of saline. Data on mixed venous oxygen saturation were recorded at 1-minute intervals for a 5-minute baseline period and then throughout the suctioning procedure until mixed venous oxygen saturation returned to baseline levels. RESULTS: The time required for mixed venous oxygen saturation to return to baseline values after suctioning was an average of 3.78 minutes longer when saline was used. CONCLUSIONS: Instillation of normal saline before endotracheal suctioning has an adverse effect on oxygenation as indicated by mixed venous oxygen saturation. This finding contradicts the assumption that instillation of normal saline improves oxygenation status.