Respiratory System
Physiology and pulmonary function tests
Obstructive disease: reduced FEV1/FVC (below 0.70) with reduced FEV1 and increased residual volume and total lung capacity; asthma is reversible, COPD is not fully reversible. Restrictive disease: normal or high FEV1/FVC with reduced FVC and total lung capacity - interstitial lung disease, neuromuscular disease, obesity, and chest wall deformity. Diffusing capacity is reduced in emphysema and interstitial disease, normal in asthma, and increased in pulmonary hemorrhage. Hypoxemia from shunt does not correct with supplemental oxygen, unlike V/Q mismatch. The alveolar-arterial gradient is normal in hypoventilation and high-altitude illness and elevated in most other causes of hypoxemia.
Obstructive airway disease
Asthma: episodic wheeze, cough, atopy; treat with inhaled corticosteroids plus a short-acting beta-agonist. COPD: smoking-related; emphysema (pink puffer, low diffusing capacity) versus chronic bronchitis (blue bloater). Supplemental oxygen improves mortality in chronic hypoxemia but must be titrated in CO2 retainers.
Infection, embolism, effusion
Community-acquired pneumonia: Streptococcus pneumoniae most common, with atypical organisms such as Mycoplasma. Tuberculosis: apical cavitary disease with acid-fast bacilli; treat with rifampin, isoniazid, pyrazinamide, ethambutol. Pulmonary embolism: risk factors, tachycardia, hypoxia; D-dimer for low probability and CT angiography for high probability; massive embolism with hypotension requires thrombolysis. Pleural effusion: transudate (heart failure, cirrhosis, nephrotic syndrome) versus exudate (infection, malignancy, embolism) using Light criteria.
Rule of thumb
Light criteria: an effusion is exudative if pleural fluid to serum protein ratio is above 0.5, pleural fluid to serum LDH ratio is above 0.6, or pleural fluid LDH is more than two-thirds the upper limit of normal for serum.