Study Notes · Asthma

Why Asthma Gets Less PEEP Than COPD on BiPAP, Despite Both Being Obstructive

It's a reasonable assumption that two obstructive airway diseases should be ventilated the same way. On BiPAP, they aren't — and the reason comes down to what's actually holding the airway closed in each condition, not just how closed it is.

Asthma and COPD get lumped together constantly as "obstructive lung disease," and for a lot of purposes that's a fair grouping. Non-invasive ventilation settings are where the grouping stops being useful, because the two conditions respond to externally applied pressure in opposite ways.

The Instinct That Doesn't Transfer

In COPD, expiratory positive airway pressure (EPAP) is doing real physiological work: COPD patients generate intrinsic PEEP because their airways collapse during exhalation, and applying extrinsic PEEP via the ventilator helps stent those airways open, counteracting the intrinsic pressure and making the next breath easier to trigger and take. It's a genuinely helpful intervention matched to the specific mechanical failure in COPD.

Bring that same instinct into an asthma resuscitation and it stops helping. As the Asthma course puts it directly: keep EPAP intentionally low in asthma — unlike COPD, extrinsic PEEP does not improve gas exchange and increases the risk of dynamic hyperinflation and hemodynamic compromise. Typical initial BiPAP settings for severe asthma call for EPAP of just 3–5 cmH2O, deliberately low, specifically to avoid stacking additional pressure on top of the intrinsic PEEP the patient is already generating from severe bronchospasm.

Why the Same Number Means Something Different Here

The difference isn't the diagnosis category — it's the mechanism of obstruction. COPD's airway collapse is largely a structural, expiratory-flow problem that external pressure can mechanically oppose. Asthma's obstruction is bronchospasm and mucus plugging distributed throughout small airways, and a patient in status asthmaticus is often already breath-stacking and auto-PEEPing badly on their own. Adding meaningful extrinsic EPAP on top of that doesn't stent anything open — it adds to a hyperinflation problem that's already building, pushing the patient closer to the dynamic hyperinflation and hemodynamic compromise that make severe asthma so dangerous to ventilate.

The rule underlying the rule: EPAP helps when it's counteracting a passive, structural expiratory collapse. EPAP hurts when the patient is already trapping air via active bronchospasm, because now it's adding to the trap instead of opposing it. Same setting, same device, opposite physiological role — which is exactly why "obstructive disease" isn't a specific enough diagnosis to set a ventilator by.

The Takeaway

Treating asthma and COPD as interchangeable "obstructive lung disease" for NIV settings is a mistake that a settings template won't catch for you. The question that actually matters isn't whether the airway is obstructed — it's whether that obstruction is a passive structural collapse that pressure can prop open, or an active, ongoing process that pressure will make worse. Asthma is the second kind, and the low-EPAP strategy exists because of that distinction specifically.