Application of Lung Ultrasound-Guided Bronchoalveolar Lavage in Patients with Severe Pneumonia

INTERVENTION: Experimental Group:Bronchoalveolar Lavage Guided by Lung Ultrasound;Control Group:Bronchoalveolar Lavage; CONDITION: Severe pneumonia PRIMARY OUTCOME: Lung Ultrasound Score; SECONDARY OUTCOME: Procedure related indicators;Duration of mechanical ventilation;Blood Gas Analysis; INCLUSION CRITERIA: 1. Age >= 18 years; 2. Clinical and radiological diagnosis of severe pneumonia; 3. Requirement for invasive mechanical ventilation; 4. No history of hospitalization for pulmonary infection within 3 months prior to enrollment.