Lung Compliance: Static vs Dynamic
Respiratory · 7 min read
The compliance gap tells you where the problem is. Static low → parenchyma. Dynamic only low → airway. Use it to titrate PEEP + diagnose at the bedside.
Compliance — the math
COMPLIANCE = ΔV / ΔP — how much volume you get for the pressure you apply. STATIC compliance = TV / (Pplat − PEEP), where Pplat is the inspiratory hold (no flow). It reflects parenchymal + chest-wall stiffness. DYNAMIC compliance = TV / (Ppeak − PEEP), measured during flow. Includes BOTH parenchyma + airway resistance. NORMAL static compliance ~70-100 mL/cmH2O in the supine adult; lower in obesity, pregnancy, supine, GA (15-20% drop).
The plateau-pressure trick
On the ventilator, click the inspiratory hold. The plateau pressure is what the lung 'sees' once flow stops. Compare Ppeak vs Pplat: NORMAL gap ~3-5 cmH2O (resistance through the ETT + airways). Big gap (Ppeak much higher than Pplat) = airway resistance problem (bronchospasm, kinked ETT, mucus plug, mainstem). Both Ppeak AND Pplat elevated = parenchymal problem (pulmonary edema, pneumonia, atelectasis, fibrosis, ARDS, abdominal compartment).
Bedside diagnosis from compliance
Patient's TV is dropping at constant pressure. Click the hold. If Pplat is normal but Ppeak rose: bronchospasm or mechanical airway issue. Albuterol + check ETT cuff/position/secretions. If both Ppeak + Pplat rose: parenchymal — recruit (PEEP), suction, consider chest X-ray for atelectasis vs pneumothorax vs fluid overload. If compliance fell suddenly during a case: think pneumothorax, mainstem migration (turn patient + listen + check ETT depth), gastric distention pushing diaphragm.
Driving pressure — the mortality target
DRIVING PRESSURE = Pplat − PEEP. Amato et al, NEJM 2015: in ARDS patients, driving pressure was the strongest predictor of mortality — stronger than tidal volume per kg PBW or plateau pressure alone. Target driving pressure <13-15 cmH2O. If your TV is 6 mL/kg PBW but driving pressure is 18, the lungs are still over-stressed — drop TV or raise PEEP (assuming PEEP improves compliance). If raising PEEP raises driving pressure, you're past the optimal point — back off.
Compliance-titrated PEEP
Set PEEP at 5, measure compliance. Raise PEEP to 7 — does compliance improve? Raise to 10 — still better? At some point, raising PEEP starts overdistending alveoli + dropping compliance. The PEEP that maximizes compliance (or minimizes driving pressure for a fixed TV) is the lung-protective sweet spot. ARDSnet PEEP-FiO2 ladder is a reasonable starting protocol; compliance + driving pressure are how you fine-tune it.
Special cases
OBESE patient: chest-wall component drops static compliance even when lung is normal. Higher PEEP often needed (10-15+) to keep alveoli open. PRONE: improves compliance in ARDS by recruiting dorsal lung. PREGNANCY: large gravid uterus + cephalad diaphragm + reduced FRC = lower compliance baseline. ABDOMINAL compartment syndrome: dropped compliance from below; treat the abdomen, not just the vent.
Pearl — measure, don't guess
When a vented patient becomes harder to ventilate, the question 'is it the airway or the lung?' is solved in one click of the inspiratory hold button. Make this a reflex. Bronchospasm gets albuterol; pneumothorax gets a chest tube; ARDS gets PEEP titration. Same symptom (harder to ventilate), three different fixes — and compliance tells you which.
References
- · Amato MBP, NEJM 2015 (driving pressure)
- · ARDSnet NEJM 2000
- · Miller's Anesthesia 9e Ch 13