Respiratory Medicine

Physiology and PFT

• Hypoventilation gives a normal A-a gradient; V/Q mismatch, shunt and diffusion limitation raise it.
• Dead space = ventilation without enough perfusion; shunt = perfusion without enough ventilation.
• FRC = ERV + RV; it is the resting oxygen reservoir between breaths.
• Simple spirometry cannot measure RV, FRC or TLC.
• Low FVC with preserved ratio suggests restriction, but low TLC confirms it.
• DLCO is low in emphysema, ILD, pulmonary vascular disease and anaemia; asthma often has normal or high DLCO.
• Inspiratory loop flattening suggests variable extrathoracic obstruction; expiratory flattening suggests variable intrathoracic obstruction; both limbs flattened suggests fixed large-airway obstruction.

Obstructive and immune airway disease

• Asthma = variable symptoms plus variable expiratory airflow limitation; SABA-only care is unsafe for adults/adolescents.
• Life-threatening asthma clues include silent chest, drowsiness, exhaustion, inability to speak and rising PaCO2.
• ABPA requires Aspergillus sensitization, total IgE at least 500 IU/mL and supportive features.
• HP depends on exposure history plus HRCT, BAL/pathology and multidisciplinary judgement; serum IgG alone does not diagnose it.
• COPD requires persistent post-bronchodilator FEV1/FVC below 0.70 in clinical context.
• COPD care targets smoking/biomass exposure, vaccination, inhaler technique, pulmonary rehabilitation, bronchodilators, exacerbations and comorbidities.

Infection, vascular and critical care

• Stable low/intermediate probability PE can use D-dimer; high-probability or unstable PE needs imaging/escalation without D-dimer reassurance.
• Hypercapnia-risk oxygen target is commonly 88–92% pending ABG and reassessment.
• Upfront rapid molecular testing with resistance detection is central for suspected active TB in India.
• ARDS can be recognized in non-intubated patients on HFNO when global criteria are met; manage trigger plus lung-protective strategy if ventilated.
• Pleural pus, positive Gram/culture, loculation or low pH needs antibiotics plus drainage/source control.
• PH is mPAP >20 mmHg on right-heart catheterization; pre-capillary disease also needs PAWP ≤15 and PVR >2 Wood units.
• HCAP is retired; pneumonia broadening depends on validated resistance risks, severity, cultures and local antibiogram.

Emergency anchors

• Unstable PE, quiet/drowsy asthma, massive haemoptysis, tension pneumothorax, empyema and ARDS trajectory need monitored escalation.
• Do not withhold oxygen from a hypoxaemic COPD patient; titrate and check ABG.
• Massive haemoptysis kills by asphyxia: protect airway, position bleeding side down if known and arrange embolization pathway.
• Long-term oxygen in COPD improves survival only in chronic severe resting hypoxaemia after stable reassessment.