Dynamic Hyperinflation & Auto-PEEP
How auto-PEEP (intrinsic PEEP) develops
Expiratory airflow limitation + ↓ elastic recoilThe lung cannot empty fully before the next breath
Part of each tidal volume is retained — "breath-stacking"Progressive hyperinflation; end-expiratory lung volume rises above FRC
Positive alveolar pressure at end-expiration = auto-PEEP (PEEPi)↑ work of breathing / trigger load, ↓ venous return & LV filling → hypotension; barotrauma risk (bullae)
Detecting air-trapping intra-op: capnograph CO₂ that up-slopes and never plateaus before the next breath; an expiratory flow trace that doesn't return to zero before the next breath; a rising measured PEEP; and a fall in BP as hyperinflation worsens.
Lung-Protective Ventilation in COPD
🎯 Goals — reduce air-trapping, avoid hypercapnia's harm, maintain oxygenation
- Low tidal volume 6–8 mL/kg (ideal body weight)
- Prolonged expiratory time — I:E ~1:2.5 to 1:3 (or longer); a higher inspiratory flow shortens inspiration and gives more time to exhale
- Low respiratory rate (avoid dynamic hyperinflation)
- FiO₂ titrated to SpO₂ > ~90%; monitor auto-PEEP
- Plateau pressure < 30 cmH₂O (barotrauma/bullae → pneumothorax risk)
- Accept permissive hypercapnia (as long as pH > ~7.25)
- Pressure-controlled (decelerating flow) ventilation often gives lower peak pressures & better distribution
How to Counter Auto-PEEP
Ventilator manoeuvres
- Lower TV & RR; allow more expiratory time (I:E to 1:3–1:5)
- Shorten inspiratory time (↑ peak flow) to free up exhalation time
- Treat bronchospasm at its source
Role of external PEEP
- Counteracts intrinsic PEEP — matches the hidden pressure the patient must overcome to trigger, ↓ trigger load & muscle fatigue
- Splints open unstable airways in expiration → lets more air empty, ↓ hyperinflation, improves synchrony & gas exchange
The "70% rule" for external PEEP
Use low external PEEP (~3–5 cmH₂O), kept to about 70–85% of the measured intrinsic PEEP. Too much external PEEP adds to hyperinflation rather than relieving it → higher pressures & hypotension. Titrate & monitor
Intra-op Bronchospasm
Recognise: ↑ peak airway pressure, rhonchi/absent breath sounds, prolonged expiration, a "shark-fin" up-sloping capnograph, ↓ tidal volume & SpO₂. Differentials: light anaesthesia, airway manipulation, endobronchial intubation, ETT obstruction/kink/cuff herniation, aspiration, pneumothorax, pulmonary oedema/embolism, and anaphylaxis.
| Step | Action |
|---|---|
| Immediate | Deepen anaesthesia; 100% O₂; if severe, ask surgeon to stop & remove stimulus; check tube & circuit (block/misplacement) |
| 1st line | Salbutamol — MDI ≥ 6–8 puffs (repeat) or nebulised 5 mg |
| 2nd line | IV hydrocortisone 2 mg/kg; nebulised ipratropium 0.5 mg; magnesium ~5 mg/kg (≈2 g) over 20 min; ketamine bolus/infusion |
| If anaphylaxis | Adrenaline first-line — IV 10–100 µg titrated (or nebulised 1:1000) |