This is the most common manifestation of a community-acquired pneumonia and typically results from a pneumococcal (Streptococcus pneumoniae) infection after airborne transmission. The localized inflammatory reaction causes the exudate to spill into the adjacent alveoli. This process spreads from the distal air spaces via the pores of Kohn, and may progress to occupying a whole lobe. As the central airspaces are patent despite the distal ones being compromised, the typical radiographic appearance is an air bronchogram without significant volume loss. In patients with a potential malignancy, a central obstructing lesion needs to be considered as being suspicious (Fig. 24.2).

This is the result of the inflammatory exudate localizing around inflamed airways but respecting septal boundaries. Some acini are involved while others are spared. The exudates initially originate around the terminal and respiratory bronchioles rather than the distal airspaces in lobar pneumonia. Several foci of infection usually develop resulting in the radiographic appearance of multifocal nodular or patchy consolidation. Air bronchograms and volume loss are usually absent. Numerous organisms can result in bronchopneumonic patterns (Fig. 24.3).

This predominantly involves the supporting interstitium between the airspaces/alveoli. Bronchial and bronchiolar walls become infiltrated and edematous with further extension from the peribronchial tissues into the interlobular septa. The inflammatory process can also progress into the terminal air spaces and alveoli. The usual radiographic appearance is a reticulonodular pattern reflecting the compromised peribronchial and interstitial tissue. The infective organisms tend to be viral or Mycoplasma pneumoniae (Fig. 24.4).

This often rapidly spreading, destructive process is usually caused by Staphylococcus aureus, gram-negative bacteria, fungal infections or Mycobacterium tuberculosis. Pneumatoceles may mimic cavitations and are defined as air cysts more than 1 cm in size and a wall thickness of less than 4 mm (Fig. 24.5a, b). If there is co-existing emphysema with bullae, consolidated lung can give a similar appearance to cavitation.

VAP is the most common healthcare associated infection in ICU and may account for up to half the antibiotic use. It occurs in up to 30 % of ventilated patients, its incidence is quoted as 5 cases per 1,000 ventilator days. It carries a mortality of 9 %.

In the absence of a consensus definition it is generally accepted that VAP is a clinical diagnosis. Table 24.1 details the criteria for diagnosis as proposed by the Centre for Disease Control and Prevention.