In order to improve the accuracy of the diagnosis of pulmonary pathogens and reduce the adverse impact of excessive BAL volume on patients, this study intends to explore the most optimal lavage volume in the middle lobe and the lower lobe of critical patients as well as seeking for the best way to manage BALF samples by means of detecting alveolar proteins and bacterial composition in BALF samples. The hypothesis is that the optimal lavage volume in the middle lobe and the lower lobe might be different. And to sample BALF separately through sequential lavage might be a better way to improve the accuracy of the diagnosis of pneumonia pathogens.

This is a self-control study consisted of three parts. Firstly, searching for the optimal lavage volume in the middle lobe and the lower lobe of critical patients by exploring the differences between the concentration of alveolar proteins among BALF samples of every participant. Several BALF samples are seperately collected from every single participant through sequential lavage(an initial 20 ml saline lavage at main bronchus and 5 aliquots of 20 ml saline lavage at the subsegment of the right middle lobe or 6 aliquots of 20 ml saline lavage at the lower lobe) . The concentration of Alveolar proteins including Surfactant protein B (SP-B), Surfactant protein D (SP-D) and Human typeⅠprotein (HTⅠ-56) will be determined by enzyme-linked immunosorbent assay (ELISA). Meanwhile, the amount of living cells as well as the proportion of squamous cells and columnar cells in BALF samples will be counted. Secondly, to confirm the optimal lavage volume through quantitative bacterial culture of BALF and sputum samples of participants. And BALF samples will also be tested by next generation sequencing（NGS） to identify microorganism. Thirdly, to observe the effect of BAL on respiratory and cardiovascular systems. The Vital signs, arterial gas analysis, ventilator parameters and respiratory mechanical parameters of patients before and within 24 hours after the BAL procedure will be recorded.

Inclusion Criteria:

• Admitted to intensive care unit

• Mechanically ventilated patient

• 18 years old or above

• Pneumonia diagnosed by one of 1 - 4 plus 5

  1. purulent endotracheal secretions or increasing oxygen requirements;
  2. body temperature exceeds 38.0 ℃;
  3. potentially pathogenic bacteria be isolated from the endotracheal secretions;
  4. leukocyte count exceeds 10×10^9 per liter or less than 4×10^9 per liter;
  5. new or persistent radiographic features of pneumonia without another obvious cause.

Exclusion Criteria:

• considered to be unsuitable for bronchoscopy by attending physician;
• underwent bronchoalveolar lavage within the last 48 hours;
• medical history of lobectomy
• airway bleeding or pulmonary edema
• refuse to sign the informed consent

The study population is selected from the Departement of Critical Care Medicine of the First Affiliated Hospital of Sun Yat-sen University.