Pathology of ARDS and pneumonia

Question 1

Characteristics of ARDS

Answer 1

Sever diffuse damage to alveolar capillary wall
Rapid onset of severe life-threatening respiratory insufficiency
Sometimes, progresses to multi-organ failure

Question 2

Direct lung injuries that cause ARDS

Answer 2

Diffuse pulmonary infections
Inhaled irritants, e.g. oxygen toxicity
gastric aspiration

Question 3

Systemic conditions that cause ARDS

Answer 3

Septic shock
Shock associated with trauma (Vietnam lung), burns, pancreatitis, complex abdominal surgery, narcotic OD, paraquat poisoning
Near-drowning

Question 4

Gram-negative sepsis model for pathogenesis of ARDS

Answer 4

Endotoxin: induces release of TNFalpha (from monocytes and macrophages)
Upregulates adhesion molecules on endothelial cells, release of cytokines
Activates alternate complement pathway to generate C5a, which upregulates adhesion molecules on neutrophils
“Sticking” of neutrophils to endothelium & sequestration of neutrophils in lung capillaries

Activated neutrophils: release O radicals, proteases, leukotrienes, PGs
Damage endothelium, increase permeability, trigger coagulation
–> necrosis of endothelial & epithelial cells, diffuse alveolar damage

Question 5

Pathological findings of ARDS - macroscopic

Answer 5

Lungs are heavy, firm rather than spongy, red

There may be superimposed pneumonia

Question 6

Pathological findings of ARDS - microscopic

Answer 6

Early exudative phase: interstitial and intra-alveolar edema, cell necrosis, hyaline membrane formation
Later: organizing/healing phase charcterized by alveoli lined by regenerating type 2 pneumocytes with intra-alveolar and interstitial fibrous tissue formation

Question 7

Classification of pneumonia

Answer 7

Lobar pneumonia
bronchopneumonia
interstitial pneumonia

Question 8

Predisposing factors to pneumonia

Answer 8

Suppression of cough reflex
Impaired systemic immunity
Impaired mucociliary apparatus 
impaired alveolar macrophage function
pulmonary edema
general debility

Question 9

Lobar pneumonia - characteristics

Answer 9

Uniform consolidation in a part of/all of a lobe
Commonest type of CAP
Majority due to S. pneumoniae
Lobar distribution is a function of the virulence and vulnerability

Question 10

Pathological stages of lobar pneumonia

Answer 10

Stage I: congestion (intra-alveolar fluid, neutrophils, numerous bacteria)
Stage II: Red hepatization (massive intra-alveolar neutrophils with RBCs and fibrin)
Stage III: gray hepatization (macrophages replace neutrophils, ingest debris)
Stage IV: resolution (but complications may ensue)

Question 11

Bronchopneumonia - characteristics

Answer 11

Patchy infective consolidation in a predominantly lobular distribution
Common, especially in hospitalized patients and as a terminal event complicating systemic illness
Usually bilateral
Pre-existing infective bronchitis spreads to cause bronchiolitis and extends to involve adjancet lung parenchyma

Question 12

Complications of pneumonia

Answer 12

Bacterial dissemination
Lung abscess formation
Empyema
Death

Question 13

Pneumonia in an immunocompromised host

Answer 13

Usual bacterial organisms  plus
mycobacterium
fungal
PJP
CMV