Respiratory failure and ARDS | Respiratory system | Pathology

Respiratory failure is failure of gas-exchange and can either be hypoxemic or hypercapnic. Acute respiratory failure may be type I or type II. In some patients, both types may co-exist.

Types of acute respiratory failure

Type	Features
Type I or hypoxemic	PaO2 <60 mmHg; PaCO2 normal or low; most common type; presence of V/Q mismatch and shunt; increased A-a gradient; hypoxemia can be corrected by 100% oxygen in V/Q mismatch but not in shunts; seen in primarily lung diseases like pulmonary edema, pneumonia, pulmonary hemorrhage, COPD.
Type II or hypercapnic	PaCO2 >50 mmHg; PaO2 low; Normal A-a gradient; seen in chest wall and muscle diseases like myasthenia gravis, poliomyelitis, Guillain-Barre syndrome, asthma, kyphoscoliosis, flail chest, COPD, drug overdose, CNS depression.

Acute deterioration in a patient with chronic respiratory failure is termed acute-on-chronic respiratory failure. It is seen in COPD in the presence of respiratory infections, bronchoconstriction, non-compliance to therapy etc.

ABG and history will help to differentiate between acute and chronic causes of respiratory failure.

Acute respiratory distress syndrome (ARDS)

Development of severe, acute respiratory failure within 1 week of an inciting lung injury, not due to cardiac causes
CxR shows bilateral, diffuse opacities
Characteristically, PCWP* <18 mmHg
PaO2/FiO2 <300 mmHg in the presence of PEEP of >5cmH2O
Mild, moderate or severe types
Mild: PaO2/FiO2 200-300 mmHg
Moderate:PaO2/FiO2 100 to <200 mmHg
Severe: PaO2/FiO2 <100 mmHg
Causes: pneumonia, inhalation of toxic fumes, lung contusion, aspiration of gastric contents, drowning, multiple transfusions, burns, pancreatitis, sepsis, drug overdose
Increased vascular permeability and endothelial damage seen causing fluid buildup in alveoli
Apoptosis of alveolar epithelial cells seen
Presents with tachypnea, dyspnea, hypoxemia, cyanosis
Extracorporeal membrane oxygenation and higher PEEP helps severe cases.

*PCWP or pulmonary capillary wedge pressure of > 18mmHg suggests cardiac origin of pulmonary edema and excludes ARDS (in some cases both conditions may co-exist).