Lung Compliance vs Airway Resistance for EDAIC

Compliance describes volume change for a pressure change. Resistance describes the pressure difference needed to produce flow. A high peak airway pressure may contain an increased elastic component, an increased resistive component, or both; the peak alone cannot separate them.

The numerical exercises below use a passive, simplified respiratory-system model for exam learning. They are not ventilator-setting recommendations.

Name the conditions before using the equations

Assume volume-controlled ventilation with constant inspiratory flow, no spontaneous effort, no leak, and a valid zero-flow inspiratory hold. Use total PEEP, including any intrinsic component. Under these assumptions:

Static respiratory-system compliance = VT ÷ (Pplat − total PEEP).

Inspiratory resistance = (Ppeak − Pplat) ÷ flow.

Plateau pressure is measured when inspiratory flow has stopped; the difference between peak and plateau helps isolate the flow-related component. MSD describes the elastic and resistive contributions.

Compliance here belongs to the respiratory system, including chest wall as well as lung. Airway pressure alone does not isolate lung transpulmonary pressure. Also remember that the calculated resistance includes relevant resistive elements between the pressure measurement and alveoli; it is not automatically a pure measure of small-airway calibre.

Worked example: reconstruct the pressure

Use fictional values:

• VT = 450 mL.
• Total PEEP = 6 cmH₂O.
• Plateau pressure = 21 cmH₂O.
• Peak pressure = 29 cmH₂O.
• Constant inspiratory flow = 0.5 L/s.

The elastic pressure difference is 21 − 6 = 15 cmH₂O. Compliance is 450 ÷ 15 = 30 mL/cmH₂O, equivalent to 0.030 L/cmH₂O.

The resistive pressure difference is 29 − 21 = 8 cmH₂O. Resistance is 8 ÷ 0.5 = 16 cmH₂O·s/L.

Reconstruct the peak to check the calculation: 6 baseline + 15 elastic + 8 resistive = 29 cmH₂O. All three terms are pressures, so the addition is dimensionally valid. The Society of Mechanical Ventilation's teaching material provides these calculation relationships.

Now inspect how the same peak could arise from a different combination. If baseline remains 6, elastic pressure is 19 and resistive pressure is 4, peak is still 29. A peak value without the other measurements cannot distinguish these examples.

Change resistance while keeping compliance fixed

In the original model, double resistance from 16 to 32 cmH₂O·s/L, leaving flow at 0.5 L/s. The resistive pressure becomes 16 cmH₂O. Elastic pressure and plateau remain 15 and 21 cmH₂O respectively, so peak becomes 6 + 15 + 16 = 37 cmH₂O.

The teaching pattern is a higher peak with the same plateau, provided the other assumptions remain valid. It suggests a larger resistive component; it does not establish a specific cause from pressure values alone. OpenAnesthesia contrasts peak and plateau behaviour.

Change compliance while keeping resistance fixed

Instead halve compliance from 30 to 15 mL/cmH₂O while keeping VT 450 mL, resistance 16 and flow 0.5 L/s. Required elastic pressure becomes 450 ÷ 15 = 30 cmH₂O. Plateau becomes 6 + 30 = 36 cmH₂O, and peak becomes 36 + 8 = 44 cmH₂O.

Here both plateau and peak rise. Again, the exercise identifies a mechanical component rather than diagnosing a disease. The model does not tell you whether the altered respiratory-system compliance arises predominantly from lung, chest wall, positioning or another factor.

These two controlled changes make better revision cards than a long list of causes. Write “only R changed” or “only C changed” above the card so that the predicted pattern has a clear basis.

Common mistakes that change the answer

Using L/min where the equation expects L/s. A flow of 30 L/min is 0.5 L/s, not 30 L/s. The resistance calculation would be wrong by a factor of sixty.

Using set PEEP when intrinsic PEEP is present. The compliance denominator requires the appropriate total baseline pressure in this model. Omitting a component changes the calculated pressure difference.

Calling dynamic compliance a pure elasticity measure. The common calculation VT/(Ppeak − PEEP) includes a pressure affected by resistance and flow. It cannot be substituted for the static calculation without changing its meaning.

Ignoring effort or a failed hold. A number labelled plateau is not automatically a valid zero-flow passive measurement. The conditions of measurement are part of the interpretation.

Original self-test

1. The original example has compliance 30 mL/cmH₂O. True: 450/15 = 30.
2. Its resistance is 16 cmH₂O·s/L. True: 8/0.5 = 16.
3. Raising inspiratory flow can raise resistive pressure without changing compliance. True within the stated model: pressure across resistance depends on flow.
4. A high peak pressure alone proves reduced compliance. False: resistance contributes too.
5. These airway measurements isolate lung compliance from chest-wall compliance. False: they describe the combined respiratory system under the stated assumptions.

Return to the physics and measurement guide for the wider topic map. When using the EDAIC question bank, annotate which variable a stem holds constant before judging each statement.

Sources

• MSD Manual: Overview of Mechanical Ventilation.
• OpenAnesthesia: Peak vs. plateau airway pressure.
• Society of Mechanical Ventilation: Mechanical Ventilation, Part 1 Answers.

Prepared with AI assistance for examination revision; this article is not a patient-care protocol or a substitute for current primary guidance. Sources checked 7 October 2026.