Respiratory Medicine
Core concepts
Start with spirometry: obstruction is a reduced FEV1/FVC ratio (<0.7), restriction a reduced FVC with preserved or raised ratio. Asthma shows variable, reversible airflow obstruction with diurnal variation; COPD shows persistent obstruction with a smoking or occupational history, graded by FEV1 percentage predicted (GOLD) and assessed for exacerbation frequency. Blood gases: type 1 is hypoxia with normal or low PaCO2; type 2 adds hypercapnia. Acidosis matters more than the absolute CO2 - acidotic COPD (pH 7.25–7.35, raised CO2) is the indication for non-invasive ventilation. Learn long-term oxygen therapy criteria: PaO2 <7.3 kPa, or <8 kPa with polycythaemia, pulmonary hypertension or peripheral oedema, used at least 15 hours a day. Pneumonia: severity by CURB-65 and clinical judgement. PE: use Wells or PERC before D-dimer, then CTPA; anticoagulate for at least three months, longer for unprovoked or cancer-associated events. Pleural fluid: Light's criteria define exudate (protein ratio >0.5, LDH ratio >0.6, or LDH above two-thirds the upper limit of normal). Interstitial lung disease: UIP pattern (basal, subpleural, honeycombing), connective tissue disease, hypersensitivity pneumonitis; sarcoidosis gives bilateral hilar lymphadenopathy, erythema nodosum and hypercalcaemia.
Traps
In a COPD patient target saturations of 88–92%; uncontrolled high-flow oxygen is the harm, not oxygen itself. Salbutamol causes tachycardia, hypokalaemia, tremor and a raised lactate. CURB-65 does not include oxygen or comorbidity. D-dimer is unhelpful in pregnancy - go to Doppler or CTPA.
Rule of thumb
A parapneumonic effusion with a pleural pH below 7.2, or frank pus, needs a chest drain - antibiotics alone will not do.