First, etiology
The causes of lung abscess are various pyogenic bacteria, mycobacteria, fungi, or parasitic infections, the most common pathogen being anaerobic bacteria (anaerobe), such as Peptostreptococcus, Bacteroides, Melaninogenes, Bacteroides fragilis, Porphyromonas, etc.; followed by various Gram-negative and Gram-positive bacteria, such as Escherichia coli, Klebsiella pneumoniae, Enterobacter cloacae, Citrobacter freundii, Acinetobacter lophotrichus, Haemophilus influenzae, Haemophilus parainfluenzae, Legionella, Staphylococcus aureus, Streptococcus, Enterococcus faecalis, less common bacteria include Pseudomonas aeruginosa, Streptococcus pneumoniae, etc.; in addition, in patients with diabetes and immunosuppressed individuals, certain fungal infections can cause cavities, such as Nocardia, Actinomycetes; parasites such as Entamoeba histolytica, an important but not common pathogen of lung abscess, mainly causing abscesses in the lower lobe base; mycobacteria such as Mycobacterium tuberculosis, Mycobacterium avium, and intracellular mycobacteria can also cause lung abscess. In lung abscesses acquired outside the hospital, anaerobic infections account for60％～85percent, and most are mixed infections of several anaerobic bacteria. Recent research shows that among the isolated anaerobic bacteria,10percent to15percent of vulnerable Bacteroides,6percent of non-vulnerable Bacteroides and4percent of梭形杆菌 can produce β-lactamase, so penicillin-resistant anaerobic infections have become an issue that cannot be ignored. In hospital-acquired lung abscesses, it is often a mixed infection of Gram-negative bacilli and Gram-positive cocci.
Second, pathogenesis
Clinically, it is divided into3types.
1, aspiration lung abscess
due to the aspiration of contents from the oropharynx and nose. In lung abscess caused by anaerobic bacterial infection,85percent to9percent of cases have aspirated oral contents or have risk factors such as periodontal disease. Disorders of consciousness (anesthesia, alcohol intoxication, use of sedatives, cranial injury, cerebrovascular accidents, and grand mal seizures), swallowing disorders caused by various reasons, gastroesophageal reflux, degenerative or acute neurological diseases, after the destruction of the normal defense system of the airway, the suppression of the cough reflex, and other situations, can become the cause of aspiration, and the change in the quality of oral contents in patients with periodontal disease can also become a cause of lung abscess, but also10percent to15percent of patients have no obvious periodontal disease or aspiration risk factors.
Toxins move with gravity and block the bronchus of a lung segment, so the location of the lung abscess caused by aspiration is related to the anatomical characteristics of the bronchus. Since the angle of the left main bronchus is greater than that of the right, and the bronchus is wider, the probability of lung abscess in the right lung is higher than that in the left lung.2倍。在仰卧位时，肺脓肿好发于下叶背段或上叶后段，发生于这两个部位的肺脓肿占全部肺脓肿病例的75％。在重症监护病房的重症患者，由于服用抗酸剂组胺H2-受体拮抗药预防应激性溃疡，使口腔革兰阴性杆菌的定植增加，故这类患者易发生革兰阴性杆菌感染引起的肺脓肿。
2、血源性肺脓肿败血症
时或脓毒病灶中的细菌或栓子可经血液循环到达肺部，引起肺小动脉栓塞，形成血源性肺脓肿。多见于葡萄球菌败血症、急性化脓性骨髓炎、化脓性阑尾炎、中耳炎、产后子宫内膜炎及亚急性细菌性心内膜炎，也可见于面部及皮肤的化脓性感染。
3、继发性肺脓肿
某些细菌性肺炎如葡萄球菌性肺炎、肺炎杆菌肺炎、流感嗜血杆菌肺炎及军团菌肺炎可发展形成空洞病变出现继发性肺脓肿。当肿瘤或异物阻塞支气管时，可造成远端分泌物滞留，继发细菌感染引起肺脓肿。
