Respiratory Medicine: Airways Disease, Infection, Pleura and Pulmonary Vascular Disease
Core concepts
Respiratory medicine questions test asthma and COPD, pneumonia, pleural disease, pulmonary embolism, interstitial lung disease, tuberculosis, obstructive sleep apnoea and respiratory failure. Understand spirometry (obstructive ratio below 0.70, restrictive pattern), ABG interpretation, oxygen targets and severity scores such as CURB-65. Asthma management follows a stepwise inhaled corticosteroid-based pathway; COPD needs smoking cessation, bronchodilators, vaccination and pulmonary rehabilitation. Pleural fluid analysis uses Light criteria to separate exudate from transudate. PE diagnosis starts with pretest probability; D-dimer is useful only in low or intermediate probability.
Traps
A normal oxygen saturation does not exclude early asthma or PE. Hypercapnia in asthma is a late, dangerous sign of fatigue. In COPD, target SpO2 88-92% because uncontrolled oxygen can worsen hypercapnia. Do not use CURB-65 alone to decide admission in every patient; social factors and comorbidity matter. Empyema and complicated parapneumonic effusion need drainage, not just antibiotics. TB isolation and notification are mandatory when suspected.
Worked example / rule of thumb
Pleural fluid: protein 45 g/L, LDH 400 U/L, serum LDH 200 U/L, glucose 3.0 mmol/L, pH 7.10. The fluid is an exudate by Light criteria (pleural protein/serum protein above 0.5; pleural LDH/serum LDH above 0.6). A pH below 7.20 in a parapneumonic effusion indicates need for chest drainage. Rule: low pleural glucose, low pH and high LDH suggest complicated infection or malignancy, not a simple transudate.