Dynamic Compliance: Formula, Interpretation, and Clinical Use

Dynamic compliance is an important concept in respiratory care because it helps explain how easily a tidal volume is delivered during active airflow.

In mechanically ventilated patients, it reflects both the elastic properties of the lungs and chest wall and the resistance created by the airways, artificial airway, and ventilator circuit. This makes dynamic compliance useful for bedside assessment, ventilator troubleshooting, and identifying changes in airway resistance.

For respiratory therapy students and clinicians, understanding dynamic compliance is essential for interpreting peak pressure, comparing it with static compliance, and recognizing problems during mechanical ventilation.

What is Dynamic Compliance?

Dynamic compliance (Cdyn) is the measurement of how easily the lungs and thorax expand during active airflow. It describes the relationship between the delivered volume and the pressure required to deliver that volume while gas is moving through the airways.

In simple terms, dynamic compliance shows how much volume enters the lungs for each unit of pressure applied during an actual ventilator breath. A higher dynamic compliance means the ventilator can deliver volume with less pressure. A lower dynamic compliance means more pressure is required to deliver the same volume.

Dynamic compliance is different from static compliance because it is measured during airflow. Since gas is moving, the pressure used in the calculation includes more than just the elastic properties of the lungs and chest wall. It also includes the pressure needed to overcome airway resistance.

This means dynamic compliance reflects the total pressure cost of delivering a breath. It is affected by lung stiffness, chest wall stiffness, airway narrowing, secretions, bronchospasm, artificial airway resistance, flow rate, and circuit problems.

Because of this, dynamic compliance is useful at the bedside. It gives the respiratory therapist a practical look at how difficult it is for the ventilator to move gas into the patient during normal breath delivery.

Why Dynamic Compliance Matters

Dynamic compliance matters because it helps clinicians evaluate ventilator mechanics during real-time ventilation. Unlike static compliance, which requires a pause maneuver, dynamic compliance can be followed breath by breath on many ventilators.

This makes it useful for monitoring trends. A single dynamic compliance value can provide information, but the trend is usually more important. If dynamic compliance is decreasing, the patient may be developing increased airway resistance, worsening lung stiffness, or both. If dynamic compliance is improving, airway resistance may be decreasing, lung mechanics may be improving, or treatment may be effective.

Dynamic compliance is especially useful because many ventilated patients develop airway problems. Bronchospasm, retained secretions, mucus plugging, airway edema, water in the circuit, a kinked endotracheal tube, or patient biting can all increase resistance. When resistance rises, peak inspiratory pressure rises, and dynamic compliance falls.

However, dynamic compliance does not identify the exact cause by itself. Since it includes both airway resistance and elastic resistance, a low value could reflect airway obstruction, stiff lungs, poor chest wall movement, or a combination of these problems. This is why dynamic compliance should be compared with static compliance and plateau pressure whenever possible.

Dynamic Compliance Formula

Dynamic compliance is commonly calculated using tidal volume divided by the difference between peak inspiratory pressure and PEEP.

The formula is:

Dynamic compliance = Tidal volume / Peak inspiratory pressure – PEEP

Or written another way:

Cdyn = VT / (PIP − PEEP)

In this formula:

• Cdyn means dynamic compliance
• VT means tidal volume
• PIP means peak inspiratory pressure
• PEEP means positive end-expiratory pressure

The result is usually expressed in mL/cmH₂O.

The denominator, PIP minus PEEP, represents the pressure used to deliver the tidal volume above baseline pressure during active airflow. PEEP is subtracted because the patient is already starting from that baseline pressure at end-expiration.

For example, if a patient receives a tidal volume of 500 mL, has a peak inspiratory pressure of 30 cmH₂O, and has PEEP of 10 cmH₂O, the pressure difference is:

30 − 10 = 20 cmH₂O

Then:

500 ÷ 20 = 25 mL/cmH₂O

Note: This dynamic compliance value is below the normal adult range often used in mechanical ventilation, suggesting increased pressure is required to deliver the tidal volume.

Normal Dynamic Compliance

Normal dynamic compliance is often listed as approximately 30 to 40 mL/cmH₂O in mechanically ventilated adults. However, normal values can vary depending on patient size, disease state, artificial airway size, ventilator settings, flow rate, and measurement method.

Because dynamic compliance is influenced by airway resistance, it is usually lower than static compliance. Static compliance uses plateau pressure, which is measured when airflow has stopped. Dynamic compliance uses peak inspiratory pressure, which is measured while airflow is occurring. Since peak pressure includes resistive pressure, the denominator is usually larger, and the compliance value is usually lower.

For example, if a patient has a tidal volume of 500 mL, a peak pressure of 30 cmH₂O, a plateau pressure of 20 cmH₂O, and PEEP of 5 cmH₂O, the dynamic and static values would differ.

Dynamic compliance:

500 ÷ (30 − 5) = 500 ÷ 25 = 20 mL/cmH₂O

Static compliance:

500 ÷ (20 − 5) = 500 ÷ 15 = 33 mL/cmH₂O

The dynamic compliance is lower because peak pressure includes the pressure needed to move gas through the airways.

For clinical purposes, the trend is often more useful than the isolated number. A dynamic compliance of 28 mL/cmH₂O may be concerning in some patients, but if it improved from 18 mL/cmH₂O after suctioning or bronchodilator therapy, that trend may be clinically meaningful.

Peak Inspiratory Pressure and Dynamic Compliance

Peak inspiratory pressure is central to dynamic compliance because it is the pressure used in the calculation.

Peak inspiratory pressure, or PIP, is the highest pressure reached during inspiration. It reflects the pressure required to move gas through the ventilator circuit, artificial airway, and patient’s airways, as well as the pressure required to expand the lungs and chest wall.

This makes PIP a combined pressure measurement. It includes two major components:

• The pressure needed to overcome airway resistance
• The pressure needed to overcome elastic resistance

Airway resistance includes anything that makes it harder for gas to flow through the airways. Examples include bronchospasm, secretions, mucus plugging, airway edema, a small endotracheal tube, biting on the tube, a kinked tube, water in the circuit, or high inspiratory flow.

Elastic resistance refers to the stiffness of the lungs and chest wall. Conditions such as ARDS, pneumonia, pulmonary edema, atelectasis, fibrosis, pleural effusion, pneumothorax, abdominal distention, or chest wall restriction can increase elastic resistance.

Note: Because peak pressure includes both components, dynamic compliance is broad. It is useful for detecting that the pressure-volume relationship has worsened, but it cannot always identify whether the problem is airway resistance or compliance without additional information.

Corrected Tidal Volume and Circuit Compression

For a more accurate dynamic compliance calculation, the therapist may need to account for compressed volume in the ventilator circuit.

During positive-pressure ventilation, not all of the set tidal volume reaches the patient. Some of the volume is lost because gas is compressed in the circuit and the ventilator tubing expands under pressure. This lost portion is called compressed volume.

When compressed volume is considered, the formula becomes:

Dynamic compliance = Exhaled tidal volume – compressed volume / Peak pressure – PEEP

Or:

Cdyn = (Exhaled VT − compressed volume) / (PIP − PEEP)

Some modern ventilators can automatically compensate for circuit compliance, but respiratory therapists still need to understand how compressed volume affects manual calculations and exam questions.

To calculate compressed volume for dynamic compliance, multiply the tubing compliance factor by the pressure above PEEP. For dynamic compliance, the relevant pressure is peak pressure minus PEEP.

For example, suppose a patient has an exhaled tidal volume of 600 mL, peak pressure of 36 cmH₂O, PEEP of 10 cmH₂O, and a tubing compliance factor of 4 mL/cmH₂O.

First, calculate the pressure above PEEP:

36 − 10 = 26 cmH₂O

Next, calculate compressed volume:

4 × 26 = 104 mL

Then subtract compressed volume from the exhaled tidal volume:

600 − 104 = 496 mL

Finally, calculate dynamic compliance:

496 ÷ 26 = 19 mL/cmH₂O

Note: This value is low, suggesting that the patient requires increased pressure to receive the delivered volume.

Causes of Decreased Dynamic Compliance

Dynamic compliance decreases when more pressure is needed to deliver a given tidal volume during active airflow. This may occur because airway resistance has increased, lung or chest wall compliance has decreased, or both.

Common causes related to increased airway resistance include:

• Bronchospasm
• Retained secretions
• Mucus plugging
• Airway edema
• Biting on the endotracheal tube
• Kinked endotracheal tube
• Kinked ventilator tubing
• Water in the ventilator circuit
• Small artificial airway
• Airway tumor or obstruction

In these cases, gas has more difficulty moving through the airway. Peak inspiratory pressure rises because the ventilator must generate more pressure to deliver flow. Since dynamic compliance uses peak pressure, the calculated value decreases.

Dynamic compliance can also decrease when the lungs or chest wall become stiffer. In this situation, both dynamic and static compliance may fall. Common causes include:

• ARDS
• Pneumonia
• Pulmonary edema
• Atelectasis
• Pulmonary fibrosis
• Lung consolidation
• Pleural effusion
• Pneumothorax
• Hemothorax
• Chest wall restriction
• Abdominal distention
• Advanced pregnancy
• Circumferential burns

Note: When these conditions are present, the respiratory system is harder to expand. More pressure is required to deliver the same tidal volume. Because peak pressure increases, dynamic compliance decreases.

Dynamic Compliance vs. Static Compliance

Dynamic compliance and static compliance are closely related, but they are not identical.

Dynamic compliance is measured during active airflow and uses peak inspiratory pressure. It reflects both airway resistance and the elastic properties of the lungs and chest wall.

Static compliance is measured when airflow is absent and uses plateau pressure. It primarily reflects the elastic properties of the lungs and chest wall.

The formulas are:

Cdyn = VT / (PIP − PEEP)

Cst = VT / (Pplat − PEEP)

The key difference is the pressure used in the denominator. Dynamic compliance uses peak pressure. Static compliance uses plateau pressure.

This difference matters because peak pressure is affected by airway resistance, while plateau pressure is measured during an inspiratory pause when airflow has stopped. Since no gas is moving during the plateau maneuver, airway resistance has less influence on the measurement.

Dynamic compliance is usually lower than static compliance because it includes the pressure needed to overcome airway resistance.

Note: Comparing the two values helps clinicians determine whether a ventilated patient’s problem is related mainly to airway resistance, decreased lung compliance, or both.

Interpreting Peak and Plateau Pressure Changes

One of the most important uses of dynamic compliance is interpreting changes in peak and plateau pressures.

If peak pressure rises but plateau pressure remains unchanged, the problem is usually increased airway resistance. In this case, dynamic compliance decreases, but static compliance remains stable. The lungs and chest wall have not necessarily become stiffer. Instead, gas is having more difficulty moving through the airways.

Possible causes include bronchospasm, secretions, mucus plugging, airway edema, a kinked endotracheal tube, water in the circuit, or patient biting the tube.

If both peak pressure and plateau pressure rise, the problem is more likely decreased lung or thoracic compliance. In this situation, dynamic compliance decreases and static compliance also decreases. The respiratory system has become harder to inflate.

Possible causes include worsening ARDS, pneumonia, pulmonary edema, atelectasis, pneumothorax, pleural effusion, fibrosis, or chest wall restriction.

If both peak and plateau pressures rise and the difference between them also increases, then both problems may be present. The patient may have decreased compliance and increased airway resistance at the same time.

This comparison is clinically important because the treatments differ. Increased airway resistance may require suctioning, bronchodilator therapy, checking for tube obstruction, correcting a kinked tube, draining water from the circuit, or improving patient-ventilator synchrony. Decreased compliance may require evaluation for worsening lung disease, atelectasis, pulmonary edema, pneumothorax, changes in PEEP, tidal volume adjustment, or other ventilator changes.

Common Dynamic Compliance Patterns

Several common patterns can help respiratory therapists interpret ventilator data.

The first pattern is decreased dynamic compliance with stable static compliance. This means peak pressure has increased, but plateau pressure has not changed. The likely problem is increased airway resistance. The therapist should assess for bronchospasm, secretions, mucus plugging, tube obstruction, circuit obstruction, or patient biting.

The second pattern is increased dynamic compliance with stable static compliance. This means peak pressure has decreased, but plateau pressure remains unchanged. The likely explanation is decreased airway resistance. This may occur after suctioning, bronchodilator therapy, clearing a mucus plug, or correcting a kinked tube.

The third pattern is decreased dynamic compliance with decreased static compliance. This means both peak pressure and plateau pressure have increased. The likely problem is reduced lung or chest wall compliance. If the peak-to-plateau pressure difference remains the same, airway resistance has not significantly changed.

The fourth pattern is decreased dynamic compliance and decreased static compliance with an increased peak-to-plateau pressure difference. This suggests both decreased compliance and increased airway resistance. For example, a patient may have pneumonia with retained secretions or ARDS with bronchospasm.

The fifth pattern is increased dynamic compliance with increased static compliance. This means both peak and plateau pressures are decreasing. The respiratory system is easier to inflate. If the peak-to-plateau difference also decreases, airway resistance has improved too.

Note: These patterns are especially important for exam preparation because they connect ventilator pressure changes with the underlying cause.

Dynamic Compliance and Airway Resistance

Dynamic compliance is strongly influenced by airway resistance. Airway resistance is the opposition to airflow through the conducting airways. It depends greatly on airway diameter, airflow pattern, lung volume, artificial airway size, and the presence of obstruction.

When airway resistance increases, more pressure is needed to move gas through the airways. This raises peak inspiratory pressure. Since dynamic compliance uses peak pressure, dynamic compliance decreases.

Bronchospasm is a common example. When smooth muscle in the airways constricts, the airway diameter narrows. Gas flow becomes more difficult, peak pressure rises, and dynamic compliance falls. Static compliance may remain relatively unchanged if lung stiffness has not worsened.

Secretions can produce a similar pattern. Retained secretions or mucus plugs narrow the airway lumen and increase resistance. Suctioning may reduce resistance, lower peak pressure, and improve dynamic compliance.

Artificial airway problems can also affect dynamic compliance. A small endotracheal tube increases resistance because gas must move through a narrower lumen. A kinked tube, biting on the tube, or partial obstruction can sharply increase peak pressure and reduce dynamic compliance.

Note: The ventilator circuit can also contribute. Water in the circuit, kinked tubing, or increased circuit resistance can make gas delivery more difficult and lower dynamic compliance.

Dynamic Compliance and Lung Stiffness

Although dynamic compliance is strongly affected by airway resistance, it is also affected by lung and chest wall stiffness.

When static compliance decreases, dynamic compliance often decreases as well. This is because the ventilator must generate more pressure to expand the respiratory system. Peak pressure rises, and the dynamic compliance value falls.

For example, in ARDS, the lungs become inflamed, heavy, and less aerated. The available lung volume is reduced, and the remaining open lung units may be exposed to higher stress. Static compliance decreases, plateau pressure rises, peak pressure rises, and dynamic compliance decreases.

In pneumonia, lung tissue may become consolidated and less expandable. In pulmonary edema, fluid accumulation reduces lung compliance. In atelectasis, collapsed alveoli reduce available lung volume. In pulmonary fibrosis, scar tissue increases stiffness. All of these conditions can reduce dynamic compliance because they increase the pressure required to deliver the tidal volume.

Note: Chest wall problems can also reduce dynamic compliance. Abdominal distention, advanced pregnancy, chest wall deformity, circumferential burns, or other restrictive conditions may limit expansion of the thorax. This increases pressure requirements during ventilation.

Dynamic Compliance and Ventilator Modes

Dynamic compliance can be observed in different ventilator modes, but interpretation depends on how the breath is delivered.

In volume-control ventilation, the ventilator delivers a set tidal volume. If airway resistance increases or lung compliance decreases, the pressure required to deliver that volume rises. Therefore, worsening dynamic compliance often appears as an increase in peak inspiratory pressure.

In pressure-control ventilation, the ventilator delivers a set pressure. If dynamic compliance worsens, the delivered tidal volume may decrease because the same pressure produces less volume. In this mode, falling tidal volume may be the main clue that compliance or resistance has worsened.

This difference is important. In volume-control ventilation, pressure changes often reveal changes in mechanics. In pressure-control ventilation, volume changes often reveal changes in mechanics.

Note: Regardless of the mode, dynamic compliance should be interpreted with the full clinical picture, including ventilator waveforms, breath sounds, patient effort, oxygenation, carbon dioxide levels, and hemodynamics.

Dynamic Compliance and Flow Rate

Flow rate can influence peak inspiratory pressure and therefore dynamic compliance.

Because dynamic compliance is measured during active airflow, any factor that increases the pressure needed to move gas can affect the value. A higher inspiratory flow rate may increase resistive pressure, especially in patients with airway obstruction. This can raise peak pressure and lower calculated dynamic compliance.

For example, if two measurements are taken at different flow settings, the dynamic compliance values may not be directly comparable. A patient may appear to have worse dynamic compliance simply because a higher flow rate increased peak pressure.

This is one reason trends should be interpreted carefully. When comparing dynamic compliance over time, it is best to consider whether the ventilator mode, flow pattern, tidal volume, PEEP, inspiratory time, and patient effort were similar.

Note: Dynamic compliance is useful, but it is affected by how the breath is delivered.

Dynamic Compliance and Time Constants

Dynamic compliance is connected to the concept of the time constant. A time constant is calculated by multiplying resistance by compliance.

Time constant = Resistance × Compliance

The time constant describes how quickly a lung unit fills or empties after a pressure change. A lung unit with a short time constant fills and empties quickly. A lung unit with a long time constant fills and empties more slowly.

Patients with obstructive lung disease often have increased airway resistance, which lengthens the time constant. This means the lungs need more time to empty. If the respiratory rate is too high or expiratory time is too short, the patient may not fully exhale before the next breath begins. This can lead to air trapping and auto-PEEP.

Auto-PEEP can worsen patient-ventilator interaction because the patient must overcome additional pressure to trigger a breath. It can also increase intrathoracic pressure and reduce venous return.

Note: Dynamic compliance may be affected in these situations because air trapping, resistance, and uneven emptying can increase the pressure cost of ventilation. Understanding time constants helps the respiratory therapist adjust inspiratory time, expiratory time, flow, and respiratory rate.

Dynamic Compliance and Uneven Ventilation

Real lungs are not single uniform compartments. Different lung units may have different resistances, compliances, and time constants. This means some lung regions may fill quickly while others fill slowly.

In a patient with airway obstruction, some units may have longer time constants because resistance is increased. In a patient with stiff or consolidated lung regions, some units may have lower compliance. During active ventilation, these differences affect gas distribution and pressure measurements.

Dynamic compliance can change with ventilator rate, inspiratory time, expiratory time, flow pattern, patient effort, and disease process. If the breath is too short, slower-filling lung units may not receive adequate volume. If exhalation is too short, slower-emptying units may retain air.

This uneven behavior explains why dynamic compliance is not always a fixed value. It can change depending on breath timing and ventilator settings. Static compliance may provide a clearer view of elastic properties, but dynamic compliance shows what is happening during actual breath delivery.

Dynamic Compliance and Ventilator Graphics

Ventilator graphics can help clinicians interpret dynamic compliance. Pressure, volume, and flow waveforms show how the breath is being delivered and how the patient is responding.

A rising peak pressure may suggest worsening dynamic compliance. If the plateau pressure remains stable, increased airway resistance is likely. If plateau pressure also rises, decreased static compliance is likely.

Flow waveforms can help detect air trapping. If expiratory flow does not return to baseline before the next breath begins, the patient may have incomplete exhalation and auto-PEEP. This is common in obstructive disease and can affect dynamic mechanics.

Pressure-volume loops can also provide information. A flatter pressure-volume curve may suggest poor compliance. A widened loop may suggest increased airway resistance because more pressure is needed during active flow.

Note: Dynamic pressure-volume loops are affected by both airflow and resistance, so they are not the same as static pressure-volume loops. Still, they are useful for bedside monitoring because they show the pressure-volume relationship during actual ventilation.

How to Calculate Dynamic Compliance

To calculate dynamic compliance, the therapist needs the tidal volume, peak inspiratory pressure, and PEEP.

The steps are:

• Identify the tidal volume
• Identify the peak inspiratory pressure
• Identify the PEEP
• Subtract PEEP from peak inspiratory pressure
• Divide tidal volume by that pressure difference
• Express the result in mL/cmH₂O

For example, if VT is 500 mL, PIP is 30 cmH₂O, and PEEP is 10 cmH₂O:

30 − 10 = 20 cmH₂O

500 ÷ 20 = 25 mL/cmH₂O

The dynamic compliance is 25 mL/cmH₂O.

If compressed volume must be considered, subtract it from the exhaled tidal volume before calculating compliance.

For example, if the exhaled tidal volume is 600 mL, compressed volume is 100 mL, PIP is 35 cmH₂O, and PEEP is 10 cmH₂O:

Corrected tidal volume:

600 − 100 = 500 mL

Pressure difference:

35 − 10 = 25 cmH₂O

Dynamic compliance:

500 ÷ 25 = 20 mL/cmH₂O

Note: This indicates reduced dynamic compliance.

Example Dynamic Compliance Calculations

Example 1

A patient has a tidal volume of 500 mL, peak inspiratory pressure of 30 cmH₂O, and PEEP of 10 cmH₂O.

Pressure difference:

30 − 10 = 20 cmH₂O

Dynamic compliance:

500 ÷ 20 = 25 mL/cmH₂O

Note: This value is below the expected range and suggests decreased dynamic compliance.

Example 2

A patient has a tidal volume of 600 mL, peak inspiratory pressure of 40 cmH₂O, and PEEP of 5 cmH₂O.

Pressure difference:

40 − 5 = 35 cmH₂O

Dynamic compliance:

600 ÷ 35 = 17.1 mL/cmH₂O

Note: Rounded, the dynamic compliance is 17 mL/cmH₂O. This is significantly reduced.

Example 3

A patient has a tidal volume of 450 mL, peak inspiratory pressure of 25 cmH₂O, and PEEP of 5 cmH₂O.

Pressure difference:

25 − 5 = 20 cmH₂O

Dynamic compliance:

450 ÷ 20 = 22.5 mL/cmH₂O

Note: This value is low and should prompt further assessment.

Example 4

A patient has an exhaled tidal volume of 600 mL, peak pressure of 36 cmH₂O, PEEP of 10 cmH₂O, and compressed volume of 104 mL.

Corrected tidal volume:

600 − 104 = 496 mL

Pressure difference:

36 − 10 = 26 cmH₂O

Dynamic compliance:

496 ÷ 26 = 19 mL/cmH₂O

Note: This suggests decreased dynamic compliance.

Clinical Response to Low Dynamic Compliance

When dynamic compliance is low or suddenly decreases, the respiratory therapist should assess both airway resistance and lung compliance.

A rapid decrease may be caused by airway obstruction. The therapist should check for secretions, mucus plugging, bronchospasm, patient biting, tube kinking, circuit obstruction, or water in the tubing. Breath sounds, ventilator graphics, suction catheter passage, and patient assessment can help identify the cause.

If the patient has wheezing and increased peak pressure with stable plateau pressure, bronchospasm is likely. Bronchodilator therapy may be indicated. If secretions are present, suctioning may improve airflow and increase dynamic compliance. If the endotracheal tube is kinked or the patient is biting, correcting the obstruction may reduce peak pressure.

If both peak and plateau pressures are rising, the clinician should assess for decreased compliance. This may require evaluating for worsening pneumonia, pulmonary edema, ARDS, atelectasis, pneumothorax, pleural effusion, or chest wall restriction.

Note: The response should be based on the cause. Dynamic compliance is a clue, not a complete diagnosis.

Common Mistakes When Interpreting Dynamic Compliance

One common mistake is assuming low dynamic compliance always means stiff lungs. Dynamic compliance may fall because of increased airway resistance even when static compliance is unchanged.

Another mistake is ignoring plateau pressure. Without plateau pressure, it is difficult to separate airway resistance from decreased lung compliance. Comparing peak and plateau pressure gives a clearer picture.

A third mistake is comparing dynamic compliance values without considering ventilator settings. Changes in flow rate, tidal volume, PEEP, mode, inspiratory time, or patient effort can affect the value.

A fourth mistake is forgetting that artificial airway problems can lower dynamic compliance. A kinked tube, small tube, mucus plug, or patient biting can increase peak pressure and reduce the calculated value.

Another mistake is failing to trend the value. A single dynamic compliance measurement is less useful than serial measurements. Trends can show whether the patient is improving, worsening, or responding to treatment.

Note: Dynamic compliance should not be interpreted without the patient assessment. Breath sounds, secretions, oxygenation, CO₂ clearance, hemodynamics, ventilator graphics, and imaging all matter.

Dynamic Compliance in Exam Preparation

For respiratory therapy exams, dynamic compliance is important because it combines formula knowledge with clinical interpretation.

Students should know the formula:

Cdyn = VT / (PIP − PEEP)

They should also know that dynamic compliance uses peak inspiratory pressure because it is measured during airflow.

A low dynamic compliance can be caused by increased airway resistance, decreased static compliance, or both. Increased airway resistance may occur with bronchospasm, secretions, mucus plugging, airway edema, a small endotracheal tube, a kinked tube, or water in the circuit. Decreased compliance may occur with ARDS, pneumonia, pulmonary edema, atelectasis, fibrosis, pneumothorax, pleural effusion, or chest wall restriction.

A key exam rule is simple: if peak pressure changes but plateau pressure does not, think airway resistance. If plateau pressure changes, think compliance. If both change, compare the difference between peak and plateau pressure.

Note: Dynamic compliance is usually lower than static compliance because it includes airway resistance.

Clinical Importance of Trending Dynamic Compliance

Dynamic compliance is most useful when trended over time. Because it can be measured during active ventilation, it is often available more easily than static compliance. This makes it valuable for observing changes in airway resistance and overall ventilator mechanics.

For example, if a patient’s dynamic compliance falls from 35 to 20 mL/cmH₂O, the therapist should investigate. The cause may be bronchospasm, retained secretions, airway obstruction, worsening pneumonia, pulmonary edema, atelectasis, or another problem.

If dynamic compliance improves after suctioning, the likely problem was increased airway resistance from secretions. If it improves after bronchodilator therapy, bronchospasm may have been the cause. If static compliance also improves, lung recruitment or improvement in lung stiffness may be occurring.

Note: Trends should be measured under similar conditions whenever possible. If ventilator settings change, the value may change for reasons unrelated to disease progression. Flow rate, PEEP, tidal volume, inspiratory time, circuit resistance, and patient effort should all be considered.

Dynamic Compliance Practice Questions

1. What is dynamic compliance?
Dynamic compliance is the measurement of how easily the lungs and thorax expand during active airflow.

2. What does dynamic compliance reflect?
Dynamic compliance reflects both the elastic properties of the lungs and chest wall and the resistance to airflow through the airways, artificial airway, and ventilator circuit.

3. Why is dynamic compliance measured during active airflow?
Dynamic compliance is measured during active airflow because it evaluates the pressure-volume relationship while gas is actually moving into the lungs.

4. What ventilator pressure is used to calculate dynamic compliance?
Peak inspiratory pressure is used to calculate dynamic compliance.

5. Why does dynamic compliance use peak inspiratory pressure?
Dynamic compliance uses peak inspiratory pressure because PIP is measured while gas is flowing and includes both airway resistance and elastic resistance.

6. What is the formula for dynamic compliance?
Dynamic compliance equals tidal volume divided by peak inspiratory pressure minus PEEP.

7. How is dynamic compliance written as an equation?
Cdyn = VT / (PIP − PEEP)

8. What does Cdyn stand for?
Cdyn stands for dynamic compliance.

9. What does VT represent in the dynamic compliance formula?
VT represents tidal volume.

10. What does PIP represent in the dynamic compliance formula?
PIP represents peak inspiratory pressure.

11. What does PEEP represent in the dynamic compliance formula?
PEEP represents positive end-expiratory pressure.

12. What unit is dynamic compliance usually expressed in?
Dynamic compliance is usually expressed in mL/cm H₂O.

13. Why is PEEP subtracted from peak inspiratory pressure?
PEEP is subtracted because dynamic compliance is based on the pressure change above the patient’s baseline pressure.

14. What does a low dynamic compliance value indicate?
A low dynamic compliance value indicates that more pressure is required to deliver a given tidal volume during active airflow.

15. What is a common normal range for dynamic compliance in mechanically ventilated adults?
A common normal range for dynamic compliance in mechanically ventilated adults is approximately 30 to 40 mL/cm H₂O.

16. Why is dynamic compliance usually lower than static compliance?
Dynamic compliance is usually lower than static compliance because it includes the pressure needed to overcome airway resistance.

17. What is peak inspiratory pressure?
Peak inspiratory pressure is the highest pressure reached during inspiration.

18. What two major pressure components are included in peak inspiratory pressure?
Peak inspiratory pressure includes the pressure needed to overcome airway resistance and the pressure needed to expand the lungs and chest wall.

19. What is one reason dynamic compliance is useful at the bedside?
Dynamic compliance is useful at the bedside because it reflects the total pressure cost of delivering a breath during active ventilation.

20. Why is dynamic compliance less specific than static compliance?
Dynamic compliance is less specific because it includes both airway resistance and lung or chest wall compliance.

21. What happens to dynamic compliance when airway resistance increases?
Dynamic compliance decreases when airway resistance increases.

22. What happens to dynamic compliance when the lungs become stiffer?
Dynamic compliance decreases when the lungs become stiffer.

23. What happens to peak inspiratory pressure when airway resistance increases?
Peak inspiratory pressure usually rises when airway resistance increases.

24. What happens to dynamic compliance when peak inspiratory pressure rises and tidal volume stays the same?
Dynamic compliance decreases when peak inspiratory pressure rises and tidal volume stays the same.

25. Why should dynamic compliance be trended over time?
Dynamic compliance should be trended over time because serial values help show whether the patient’s airway resistance or lung mechanics are improving or worsening.

26. What is one common cause of decreased dynamic compliance related to airway resistance?
Bronchospasm is a common cause of decreased dynamic compliance related to increased airway resistance.

27. How can retained secretions affect dynamic compliance?
Retained secretions can increase airway resistance, raise peak inspiratory pressure, and decrease dynamic compliance.

28. How can mucus plugging affect dynamic compliance?
Mucus plugging can partially obstruct airflow, causing increased peak pressure and decreased dynamic compliance.

29. How can airway edema affect dynamic compliance?
Airway edema can narrow the airway, increase resistance to airflow, and reduce dynamic compliance.

30. How can biting on the endotracheal tube affect dynamic compliance?
Biting on the endotracheal tube can obstruct airflow, increase peak pressure, and lower dynamic compliance.

31. How can a kinked endotracheal tube affect dynamic compliance?
A kinked endotracheal tube can increase airway resistance and cause dynamic compliance to decrease.

32. How can water in the ventilator circuit affect dynamic compliance?
Water in the ventilator circuit can obstruct gas flow, increase resistance, and lower dynamic compliance.

33. How can a small artificial airway affect dynamic compliance?
A small artificial airway increases resistance to gas flow, which can decrease dynamic compliance.

34. What does it suggest if dynamic compliance decreases but static compliance remains stable?
It suggests increased airway resistance rather than a primary lung or chest wall compliance problem.

35. What does it suggest if both dynamic compliance and static compliance decrease?
It suggests decreased lung or chest wall compliance, although airway resistance may also be involved.

36. What pressure pattern suggests increased airway resistance?
An increased peak pressure with an unchanged plateau pressure suggests increased airway resistance.

37. What pressure pattern suggests decreased lung compliance?
An increase in both peak pressure and plateau pressure suggests decreased lung compliance.

38. What does it mean if peak pressure rises but plateau pressure stays the same?
It means gas is having more difficulty moving through the airways, but the lungs and chest wall have not necessarily become stiffer.

39. What does it mean if both peak pressure and plateau pressure rise together?
It usually means the respiratory system has become harder to inflate.

40. What does an increased peak-to-plateau pressure difference suggest?
An increased peak-to-plateau pressure difference suggests increased airway resistance.

41. What does an unchanged peak-to-plateau pressure difference suggest when both pressures rise?
It suggests that airway resistance has not significantly changed and the main problem is decreased compliance.

42. What does it suggest if peak pressure decreases while plateau pressure remains unchanged?
It suggests that airway resistance has improved.

43. What intervention may improve dynamic compliance if secretions are the cause?
Suctioning may improve dynamic compliance if retained secretions are increasing airway resistance.

44. What intervention may improve dynamic compliance if bronchospasm is present?
Bronchodilator therapy may improve dynamic compliance if bronchospasm is increasing airway resistance.

45. How can correcting a kinked ventilator circuit affect dynamic compliance?
Correcting a kinked ventilator circuit can reduce resistance and improve dynamic compliance.

46. Why should plateau pressure be checked when dynamic compliance decreases?
Plateau pressure should be checked to help determine whether the problem is increased airway resistance, decreased lung compliance, or both.

47. How is dynamic compliance different from static compliance?
Dynamic compliance is measured during airflow and includes airway resistance, while static compliance is measured when airflow is paused and mainly reflects elastic resistance.

48. Which pressure is used for static compliance instead of peak inspiratory pressure?
Plateau pressure is used for static compliance.

49. Why does static compliance use plateau pressure?
Static compliance uses plateau pressure because it is measured during an inspiratory pause when airflow has stopped.

50. Why is comparing dynamic and static compliance clinically useful?
Comparing dynamic and static compliance helps identify whether a ventilator problem is due to airway resistance, lung stiffness, or both.

51. What pulmonary condition can decrease dynamic compliance by making the lungs stiff and inflamed?
ARDS can decrease dynamic compliance by making the lungs stiff, inflamed, and harder to inflate.

52. How can pneumonia decrease dynamic compliance?
Pneumonia can decrease dynamic compliance by causing consolidation and reducing the lung’s ability to expand.

53. How can pulmonary edema affect dynamic compliance?
Pulmonary edema can decrease dynamic compliance because fluid in the lungs makes ventilation require more pressure.

54. How can atelectasis decrease dynamic compliance?
Atelectasis decreases dynamic compliance by reducing the amount of open lung available for ventilation.

55. How can pulmonary fibrosis affect dynamic compliance?
Pulmonary fibrosis decreases dynamic compliance because scar tissue makes the lungs stiff and less expandable.

56. How can pleural effusion affect dynamic compliance?
Pleural effusion can decrease dynamic compliance by restricting lung expansion.

57. How can pneumothorax affect dynamic compliance?
Pneumothorax can decrease dynamic compliance by impairing lung expansion and increasing the pressure needed to deliver volume.

58. How can chest wall restriction affect dynamic compliance?
Chest wall restriction can decrease dynamic compliance by limiting thoracic expansion during ventilation.

59. How can abdominal distention affect dynamic compliance?
Abdominal distention can decrease dynamic compliance by limiting diaphragmatic movement and chest expansion.

60. How can circumferential burns affect dynamic compliance?
Circumferential burns can decrease dynamic compliance by restricting chest wall movement.

61. What does dynamic compliance show in volume-control ventilation?
In volume-control ventilation, dynamic compliance helps show how much pressure is required to deliver the set tidal volume.

62. What happens in volume-control ventilation when dynamic compliance worsens?
When dynamic compliance worsens in volume-control ventilation, peak inspiratory pressure usually rises.

63. What does dynamic compliance show in pressure-control ventilation?
In pressure-control ventilation, worsening dynamic compliance may show up as a decrease in delivered tidal volume.

64. Why can delivered tidal volume fall in pressure-control ventilation when dynamic compliance worsens?
Delivered tidal volume can fall because the same set pressure produces less volume when resistance increases or lung compliance decreases.

65. Why can flow rate affect dynamic compliance?
Flow rate can affect dynamic compliance because higher inspiratory flow may increase resistive pressure and raise peak inspiratory pressure.

66. Why should dynamic compliance values be compared under similar ventilator settings?
Dynamic compliance values should be compared under similar settings because changes in flow, tidal volume, PEEP, mode, or inspiratory time can affect the value.

67. How can high inspiratory flow affect peak pressure?
High inspiratory flow can increase resistive pressure and raise peak inspiratory pressure.

68. What is the time constant formula?
Time constant = Resistance × Compliance

69. What does a time constant describe?
A time constant describes how quickly a lung unit fills or empties after a pressure change.

70. How does increased airway resistance affect the time constant?
Increased airway resistance lengthens the time constant, causing lung units to fill or empty more slowly.

71. Why do patients with obstructive lung disease often need more expiratory time?
Patients with obstructive lung disease often need more expiratory time because increased resistance lengthens the time constant and slows exhalation.

72. What can happen if expiratory time is too short in obstructive lung disease?
If expiratory time is too short, the patient may not fully exhale before the next breath, causing air trapping and auto-PEEP.

73. How can auto-PEEP affect patient triggering?
Auto-PEEP can make triggering harder because the patient must overcome additional pressure to initiate a breath.

74. How can auto-PEEP affect venous return?
Auto-PEEP can increase intrathoracic pressure and reduce venous return.

75. Why can dynamic compliance change with ventilator rate?
Dynamic compliance can change with ventilator rate because lung units with different time constants may not have enough time to fill or empty evenly.

76. Why are real lungs described as multiple compartments?
Real lungs are described as multiple compartments because different lung regions can have different resistance, compliance, and time constants.

77. What does uneven ventilation mean?
Uneven ventilation means some lung units fill or empty faster than others because their resistance and compliance characteristics are different.

78. How can airway obstruction affect gas distribution?
Airway obstruction can cause some lung units to fill or empty more slowly, leading to uneven gas distribution.

79. What is pendelluft?
Pendelluft is the movement of gas between lung compartments even when there is no airflow at the airway opening.

80. Why can dynamic compliance vary with inspiratory time?
Dynamic compliance can vary with inspiratory time because slower-filling lung units may not receive enough volume if inspiration is too short.

81. Why can dynamic compliance vary with expiratory time?
Dynamic compliance can vary with expiratory time because slower-emptying lung units may retain air if expiration is too short.

82. How can patient effort affect dynamic compliance?
Patient effort can affect dynamic compliance by changing airway pressure and volume delivery during active ventilation.

83. How can ventilator graphics help assess dynamic compliance?
Ventilator graphics help assess dynamic compliance by showing pressure, volume, and flow changes during breath delivery.

84. What might a rising peak pressure on the ventilator suggest?
A rising peak pressure may suggest increased airway resistance, decreased lung compliance, or both.

85. What does an expiratory flow waveform that does not return to baseline suggest?
It suggests incomplete exhalation and possible air trapping or auto-PEEP.

86. How can a pressure-volume loop suggest poor compliance?
A pressure-volume loop may suggest poor compliance when the curve appears flatter and more pressure is needed to deliver volume.

87. What can a widened pressure-volume loop suggest?
A widened pressure-volume loop can suggest increased airway resistance during active airflow.

88. Why are dynamic pressure-volume loops different from static pressure-volume loops?
Dynamic pressure-volume loops are different because they include the effects of airflow and airway resistance.

89. What information is needed to calculate dynamic compliance?
The therapist needs tidal volume, peak inspiratory pressure, and PEEP to calculate dynamic compliance.

90. What is the pressure difference if PIP is 35 cm H₂O and PEEP is 10 cm H₂O?
The pressure difference is 25 cm H₂O.

91. What is the dynamic compliance if VT is 500 mL, PIP is 35 cm H₂O, and PEEP is 10 cm H₂O?
The dynamic compliance is 20 mL/cm H₂O.

92. What is the dynamic compliance if VT is 600 mL, PIP is 40 cm H₂O, and PEEP is 5 cm H₂O?
The dynamic compliance is approximately 17 mL/cm H₂O.

93. What is the dynamic compliance if VT is 450 mL, PIP is 25 cm H₂O, and PEEP is 5 cm H₂O?
The dynamic compliance is 22.5 mL/cm H₂O.

94. What is corrected tidal volume?
Corrected tidal volume is the exhaled tidal volume minus compressed volume.

95. Why is compressed volume considered in dynamic compliance calculations?
Compressed volume is considered because some delivered volume may be lost in the ventilator circuit and may not reach the patient.

96. How is compressed volume calculated for dynamic compliance?
Compressed volume is calculated by multiplying the tubing compliance factor by the pressure above PEEP.

97. What should the therapist assess when dynamic compliance suddenly drops?
The therapist should assess for airway obstruction, bronchospasm, secretions, tube kinking, circuit problems, worsening lung disease, or reduced chest wall movement.

98. Why is dynamic compliance considered a clue rather than a complete diagnosis?
Dynamic compliance is a clue because it shows that pressure requirements have changed, but further assessment is needed to identify the cause.

99. What is a common exam rule for dynamic compliance?
If peak pressure changes but plateau pressure does not, think increased or decreased airway resistance.

100. What is the most important takeaway about dynamic compliance?
Dynamic compliance reflects how easily a breath is delivered during active airflow, including both airway resistance and lung or chest wall compliance.

Final Thoughts

Dynamic compliance is a practical measurement that helps respiratory therapists assess how easily a tidal volume is delivered during active airflow. Because it uses peak inspiratory pressure, it reflects both airway resistance and the elastic properties of the lungs and chest wall.

A low dynamic compliance may indicate bronchospasm, secretions, tube obstruction, circuit problems, stiff lungs, or restricted chest wall movement. Comparing dynamic compliance with static compliance helps separate airway resistance problems from compliance problems.

By trending dynamic compliance and interpreting it with ventilator pressures, graphics, and patient assessment, clinicians can identify changes earlier and respond more appropriately.