Pulmonary function tests (PFTs) classify respiratory disease into obstructive and restrictive patterns. Spirometry measures: FVC (total volume forcefully exhaled), FEV1 (volume in first second), and the FEV1/FVC ratio. Obstructive disease (asthma, COPD, bronchiectasis): FEV1/FVC < 0.70 (or below lower limit of normal) — air trapping due to airway narrowing; FEV1 reduced proportionally more than FVC. Severity grading by FEV1: GOLD 1 (≥ 80%), GOLD 2 (50-79%), GOLD 3 (30-49%), GOLD 4 (< 30%). Bronchodilator responsiveness (≥ 12% AND ≥ 200 mL improvement in FEV1) differentiates asthma (reversible) from COPD (fixed/partially reversible). Restrictive disease (pulmonary fibrosis, chest wall disease, neuromuscular): FEV1/FVC ratio normal or elevated (both reduced proportionally) with reduced TLC (total lung capacity — requires full PFTs/plethysmography to confirm restriction). DLCO (diffusion capacity for carbon monoxide) is reduced in diseases affecting the alveolar-capillary membrane (fibrosis, emphysema) but normal in chest wall restriction and neuromuscular disease. ABG interpretation uses a systematic approach: (1) Assess pH (acidosis < 7.35, alkalosis > 7.45), (2) Identify primary disorder (PaCO2 for respiratory, HCO3 for metabolic), (3) Assess compensation (is the other system responding appropriately?), (4)...