Static Compliance (Cstat) vs. Dynamic Compliance (Cdyn)

Static compliance and dynamic compliance are two important measurements used to evaluate lung mechanics during mechanical ventilation. Both describe the relationship between volume and pressure, but they are measured under different conditions and provide different clinical information.

Static compliance focuses on the elastic properties of the lungs and chest wall when airflow is paused, while dynamic compliance reflects both elastic resistance and airway resistance during active airflow.

Understanding the difference helps respiratory therapists interpret ventilator pressures, identify the cause of worsening mechanics, and respond appropriately at the bedside.

 

 

What is Lung Compliance?

Lung compliance describes how easily a structure expands when pressure is applied. In respiratory care, it usually refers to how easily the lungs and thorax accept volume during ventilation.

The basic relationship is:

Compliance = Change in volume / Change in pressure

A highly compliant respiratory system accepts a larger volume with a smaller pressure change. A poorly compliant respiratory system is stiff and requires more pressure to deliver the same volume.

For example, if a patient receives a tidal volume of 500 mL with a small pressure change, compliance is relatively high. If the same tidal volume requires a much larger pressure change, compliance is lower. This means the lungs, chest wall, or both are harder to inflate.

Compliance is especially important during mechanical ventilation because the ventilator delivers breaths using pressure, volume, or both. The pressure needed to deliver a tidal volume gives the respiratory therapist information about the patient’s lung condition, airway resistance, artificial airway, circuit, and chest wall mechanics.

However, compliance is not just one measurement. Two major forms are commonly discussed in mechanical ventilation:

• Static compliance
• Dynamic compliance

These two values are related, but they are not interchangeable. Static compliance is measured when airflow is absent. Dynamic compliance is measured while airflow is occurring. That difference changes what each value represents.

What is Static Compliance?

Static compliance is the measurement of how easily the lungs and chest wall expand when airflow is absent. It reflects the elastic properties of the respiratory system under no-flow conditions.

In mechanical ventilation, static compliance is measured using plateau pressure. Plateau pressure is obtained during an inspiratory pause after the tidal volume has been delivered. During this pause, airflow stops briefly, allowing pressure to equilibrate between the airway opening and the alveoli.

Because no gas is moving during the pause, the pressure needed to overcome airway resistance is removed from the measurement. This is why static compliance is considered a better reflection of lung and chest wall stiffness.

The formula is:

Static compliance = Tidal volume / Plateau pressure – PEEP

Or:

Cst = VT / (Pplat − PEEP)

In this formula:

• Cst means static compliance
• VT means tidal volume
• Pplat means plateau pressure
• PEEP means positive end-expiratory pressure

Static compliance is usually expressed in mL/cmH₂O.

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

What Static Compliance Tells You

Static compliance tells you how stiff or expandable the lungs and chest wall are. Since it is measured when airflow is absent, it focuses mainly on elastic resistance rather than airway resistance.

A low static compliance means the lungs or chest wall are harder to inflate. This may occur when lung tissue becomes stiff, alveoli collapse, fluid fills the lungs, pleural pressure restricts expansion, or the chest wall cannot move normally.

Common causes of decreased static compliance include:

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

These conditions increase the pressure required to deliver a given tidal volume. In a volume-controlled breath, this often appears as an increase in plateau pressure. In a pressure-controlled breath, it may appear as a decrease in delivered tidal volume.

Note: Static compliance is important because high plateau pressure can indicate increased stress on the alveoli and lung tissue. In lung-protective ventilation, clinicians often monitor plateau pressure and static compliance to reduce the risk of ventilator-induced lung injury.

What is Dynamic Compliance?

Dynamic compliance is the measurement of how easily the lungs and thorax expand during active airflow. It reflects the pressure-volume relationship while gas is moving through the airways.

In mechanical ventilation, dynamic compliance is calculated using peak inspiratory pressure. Peak inspiratory pressure is the highest pressure reached during inspiration. Because it is measured while gas is flowing, it includes both the pressure needed to overcome airway resistance and the pressure needed to expand the lungs and chest wall.

The formula is:

Dynamic compliance = Tidal volume / Peak inspiratory pressure – PEEP

Or:

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

Dynamic compliance is also expressed in mL/cmH₂O.

Because dynamic compliance uses peak inspiratory pressure, it reflects the total pressure cost of delivering a breath during active ventilation.

What Dynamic Compliance Tells You

Dynamic compliance gives information about how difficult it is to deliver a breath during actual airflow. Unlike static compliance, it includes both elastic resistance and airway resistance.

A low dynamic compliance may indicate:

• Increased airway resistance
• Decreased lung compliance
• Decreased chest wall compliance
• Artificial airway obstruction
• Circuit resistance
• A combination of these problems

Common causes of decreased dynamic compliance related to 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

Dynamic compliance can also decrease when static compliance decreases. For example, ARDS, pneumonia, pulmonary edema, atelectasis, and fibrosis can all make the lungs harder to inflate. Since peak pressure rises when the lungs are stiff, dynamic compliance falls as well.

Note: This is why dynamic compliance is less specific than static compliance. It is useful for detecting that something has changed, but it does not always identify the cause by itself.

The Key Difference Between Static and Dynamic Compliance

The key difference is the condition under which the measurement is taken.

• Static compliance is measured when airflow is absent.
• Dynamic compliance is measured during active airflow.

This difference determines which pressure is used.

• Static compliance uses plateau pressure.
• Dynamic compliance uses peak inspiratory pressure.

Plateau pressure is measured during an inspiratory pause when airflow has stopped. Since there is no airflow, airway resistance has less influence. Therefore, static compliance mainly reflects the elastic properties of the lungs and chest wall.

Peak inspiratory pressure is measured while gas is flowing. It includes pressure needed to move gas through the airways, artificial airway, and circuit, as well as pressure needed to expand the lungs and chest wall. Therefore, dynamic compliance reflects both airway resistance and elastic resistance.

Note: This is the most important distinction for students and clinicians to remember.

Comparing the Formulas

The formulas look similar, but the pressure used in the denominator is different.

Static compliance:

• Cst = VT / (Pplat − PEEP)

Dynamic compliance:

• Cdyn = VT / (PIP − PEEP)

Both use tidal volume in the numerator. Both subtract PEEP from the measured pressure. Both are expressed in mL/cmH₂O. The difference is that static compliance uses plateau pressure, while dynamic compliance uses peak inspiratory pressure.

Note: This difference makes static compliance more specific for lung and chest wall stiffness, while dynamic compliance reflects the combined effects of airway resistance and respiratory system compliance.

Why Dynamic Compliance is Usually Lower

Dynamic compliance is usually lower than static compliance because peak inspiratory pressure is usually higher than plateau pressure. Peak pressure includes resistive pressure and elastic pressure. Plateau pressure mainly reflects elastic pressure.

For example, suppose a patient has the following values:

• Tidal volume: 500 mL
• PIP: 30 cmH₂O
• Pplat: 20 cmH₂O
• PEEP: 5 cmH₂O

Dynamic compliance:

Cdyn = 500 / (30 − 5)

Cdyn = 500 / 25

Cdyn = 20 mL/cmH₂O

Static compliance:

Cst = 500 / (20 − 5)

Cst = 500 / 15

Cst = 33 mL/cmH₂O

Note: The dynamic compliance is lower because the denominator is larger. The denominator is larger because peak pressure includes airway resistance. This does not automatically mean static compliance is normal. It simply means dynamic compliance includes more pressure components than static compliance.

Normal Values

Normal values can vary depending on the patient, age, body size, ventilator settings, artificial airway, and measurement method.

Dynamic compliance is often listed as approximately 30 to 40 mL/cmH₂O in mechanically ventilated adults. Static compliance is often listed as approximately 40 to 60 mL/cmH₂O in mechanically ventilated adults.

Some references list normal adult lung-thorax static compliance closer to 100 mL/cmH₂O when discussing broader pulmonary mechanics. In infants, values are much lower because the lungs are smaller. A normal 3-kg infant may have a static compliance around 5 mL/cmH₂O.

These numbers are helpful, but trends are often more useful than isolated values. A single measurement gives a snapshot. Serial measurements show whether the patient’s condition is improving, worsening, or remaining stable.

For example, a dynamic compliance of 25 mL/cmH₂O may be low, but if it improved from 15 mL/cmH₂O after suctioning, that trend suggests improved airway resistance. A static compliance of 35 mL/cmH₂O may still be below normal, but if it improved from 20 mL/cmH₂O, that may suggest better lung recruitment or improving lung disease.

Peak Pressure vs. Plateau Pressure

To understand static and dynamic compliance, it is essential to understand peak pressure and plateau pressure.

Peak inspiratory pressure is the highest pressure reached during inspiration. It is measured while gas is moving into the lungs. It includes the pressure needed to overcome airway resistance, artificial airway resistance, ventilator circuit resistance, and elastic resistance from the lungs and chest wall.

Plateau pressure is measured after the tidal volume has been delivered and airflow is briefly paused. Since no gas is moving, airway resistance is not a major part of the measurement. Plateau pressure reflects the pressure needed to keep the lungs inflated at the delivered volume.

This distinction is clinically important. If peak pressure increases but plateau pressure stays the same, airway resistance has likely increased. If plateau pressure increases, static compliance has likely decreased.

Note: If both peak and plateau pressures increase, the patient may have decreased compliance, increased resistance, or both. The difference between peak and plateau pressure helps clarify the pattern.

The Peak-to-Plateau Difference

The difference between peak pressure and plateau pressure is an important clue. Peak pressure reflects both resistance and compliance. Plateau pressure mainly reflects compliance.

Therefore, the difference between peak pressure and plateau pressure reflects the resistive pressure component.

If the peak-to-plateau difference increases, airway resistance has likely increased. If both peak and plateau pressures rise but the difference remains the same, the main problem is decreased compliance.

For example:

Previous values:

• PIP: 30 cmH₂O
• Pplat: 20 cmH₂O
• Difference: 10 cmH₂O

New values:

• PIP: 40 cmH₂O
• Pplat: 30 cmH₂O
• Difference: 10 cmH₂O

Both pressures increased, but the difference stayed the same. This suggests decreased static compliance. The lungs or chest wall have become stiffer, but airway resistance has not significantly changed.

Now consider another pattern:

Previous values:

• PIP: 30 cmH₂O
• Pplat: 20 cmH₂O
• Difference: 10 cmH₂O

New values:

• PIP: 45 cmH₂O
• Pplat: 30 cmH₂O
• Difference: 15 cmH₂O

Note: Both pressures increased, and the difference increased. This suggests decreased compliance plus increased airway resistance.

Decreased Dynamic Compliance with Stable Static Compliance

This pattern occurs when peak pressure rises, but plateau pressure remains unchanged. Because dynamic compliance uses peak pressure, dynamic compliance decreases. Because static compliance uses plateau pressure, static compliance remains stable.

This pattern usually indicates increased airway resistance.

Possible causes include:

• Bronchospasm
• Secretions
• Mucus plugging
• Airway edema
• Water in the circuit
• Kinked ventilator tubing
• Kinked endotracheal tube
• Patient biting the tube
• Small artificial airway
• Partial airway obstruction

For example, a patient’s PIP rises from 28 to 40 cmH₂O, but plateau pressure remains 20 cmH₂O. The lungs and chest wall have not become stiffer. Instead, gas is having more difficulty moving through the airways or circuit.

Note: The respiratory therapist should assess breath sounds, check the tube and circuit, suction if indicated, evaluate for bronchospasm, drain water from the tubing, and ensure the patient is not biting the tube.

Increased Dynamic Compliance with Stable Static Compliance

This pattern occurs when peak pressure decreases, but plateau pressure remains unchanged. Because dynamic compliance uses peak pressure, dynamic compliance improves. Because static compliance uses plateau pressure, static compliance remains the same.

This pattern suggests that airway resistance has improved.

Possible explanations include:

• Secretions were removed
• Mucus plugging improved
• Bronchospasm improved
• A kinked tube was corrected
• Water was removed from the circuit
• The patient stopped biting the tube

For example, after suctioning, a patient’s peak pressure decreases from 40 to 30 cmH₂O, while plateau pressure remains 20 cmH₂O. This suggests that resistance improved, but the elastic properties of the lungs and chest wall did not significantly change.

Note: This is a common and useful bedside pattern. It helps confirm that the intervention addressed an airway resistance problem.

Decreased Static and Dynamic Compliance with No Change in Peak-to-Plateau Difference

This pattern occurs when both peak pressure and plateau pressure increase, but the difference between them remains the same.

Because plateau pressure increases, static compliance decreases. Because peak pressure also increases, dynamic compliance decreases. However, the unchanged peak-to-plateau difference suggests airway resistance has not significantly changed.

This is mainly a compliance problem.

Possible causes include:

• ARDS
• Pneumonia
• Pulmonary edema
• Atelectasis
• Pulmonary fibrosis
• Pleural effusion
• Pneumothorax
• Chest wall restriction
• Abdominal distention

For example:

Before:

• PIP: 30 cmH₂O
• Pplat: 20 cmH₂O
• Difference: 10 cmH₂O

After:

• PIP: 40 cmH₂O
• Pplat: 30 cmH₂O
• Difference: 10 cmH₂O

The peak pressure increased, but only because the plateau pressure increased. The main issue is that the respiratory system has become harder to inflate.

Note: The clinician should assess for worsening lung disease, atelectasis, pulmonary edema, pneumothorax, pleural effusion, or chest wall restriction.

Decreased Static and Dynamic Compliance with Increased Airway Resistance

This pattern occurs when both peak and plateau pressures increase, and the peak-to-plateau difference also increases. This means dynamic compliance has decreased, static compliance has decreased, and airway resistance has increased.

The patient has both a compliance problem and a resistance problem.

For example:

Before:

• PIP: 30 cmH₂O
• Pplat: 20 cmH₂O
• Difference: 10 cmH₂O

After:

• PIP: 45 cmH₂O
• Pplat: 30 cmH₂O
• Difference: 15 cmH₂O

The plateau pressure increased, so compliance worsened. The difference between PIP and plateau pressure also increased, so airway resistance worsened too.

Possible causes include:

• ARDS with secretions
• Pneumonia with mucus plugging
• Pulmonary edema with bronchospasm
• Atelectasis with retained secretions
• Chest wall restriction with tube obstruction

Note: In this case, the respiratory therapist must assess both the lungs and the airway. The patient may need suctioning or bronchodilator therapy, but they may also need evaluation for worsening lung disease or a ventilator strategy change.

Increased Static and Dynamic Compliance with No Change in Peak-to-Plateau Difference

This pattern occurs when both peak and plateau pressures decrease, but the difference between them remains unchanged.

Because plateau pressure decreases, static compliance improves. Because peak pressure also decreases, dynamic compliance improves. The unchanged difference suggests airway resistance has not significantly changed.

This is mainly an improvement in lung or chest wall compliance.

For example:

Before:

• PIP: 40 cmH₂O
• Pplat: 30 cmH₂O
• Difference: 10 cmH₂O

After:

• PIP: 30 cmH₂O
• Pplat: 20 cmH₂O
• Difference: 10 cmH₂O

Note: The lungs are easier to inflate, but airway resistance is about the same.

Possible reasons include:

• Improved atelectasis
• Improved pulmonary edema
• Better lung recruitment
• Improving pneumonia
• Improved chest wall mechanics
• Resolution of pneumothorax or pleural restriction

Note: This pattern may suggest that treatment is improving the elastic properties of the respiratory system.

Increased Static and Dynamic Compliance with Improved Airway Resistance

This pattern occurs when both peak and plateau pressures decrease, and the peak-to-plateau difference also decreases. This indicates improvement in both lung or chest wall compliance and airway resistance.

For example:

Before:

• PIP: 45 cmH₂O
• Pplat: 30 cmH₂O
• Difference: 15 cmH₂O

After:

• PIP: 30 cmH₂O
• Pplat: 22 cmH₂O
• Difference: 8 cmH₂O

The plateau pressure decreased, so static compliance improved. The peak pressure decreased, so dynamic compliance improved. The difference between peak and plateau pressure decreased, so airway resistance also improved.

Note: This may occur after multiple improvements, such as suctioning secretions, bronchodilator therapy, improved lung recruitment, treatment of pulmonary edema, or improvement in pneumonia.

Static Compliance and Lung Stiffness

Static compliance is most useful when evaluating lung stiffness. When the lungs become stiff, plateau pressure rises and static compliance falls.

ARDS is a common example. In ARDS, inflammation, alveolar flooding, surfactant dysfunction, and reduced aerated lung volume make the respiratory system harder to inflate. The same tidal volume requires more pressure, and plateau pressure may increase.

Pneumonia can also decrease static compliance. Consolidated lung tissue is less expandable. As more lung tissue becomes involved, the ventilator may need more pressure to deliver the same volume.

Pulmonary edema decreases compliance because fluid in the interstitial spaces and alveoli makes the lungs heavier and less expandable.

Atelectasis reduces compliance because collapsed alveoli are not participating in ventilation. The remaining open lung units receive more of the tidal volume, which can increase pressure and reduce measured compliance.

Pulmonary fibrosis causes chronic stiffness due to scarring. Patients with fibrotic lungs may have low compliance and require higher pressures for small volume changes.

Static compliance is also affected by the chest wall and abdomen. A patient with abdominal distention, chest wall restriction, or circumferential burns may have low static compliance even if the lung tissue itself is not the only problem.

Dynamic Compliance and Airway Resistance

Dynamic compliance is especially useful for recognizing changes in airway resistance. When airway resistance increases, peak pressure rises. If plateau pressure stays the same, static compliance has not changed. This pattern points toward the airways, artificial airway, or ventilator circuit.

Bronchospasm is a common cause. Narrowed airways increase resistance to airflow, requiring more pressure to deliver the breath. Dynamic compliance falls.

Secretions and mucus plugs can also reduce dynamic compliance. They narrow or obstruct the airway lumen. Suctioning may lower peak pressure and improve dynamic compliance.

The artificial airway is another frequent source of resistance. A small endotracheal tube, kinked tube, partial obstruction, cuff issue, or biting can raise peak pressure.

Ventilator circuit issues can create similar findings. Water in the circuit, kinked tubing, or increased circuit resistance may increase peak pressure and reduce dynamic compliance.

Note: Because dynamic compliance is affected by these problems, it is often useful for rapid bedside troubleshooting.

Static vs. Dynamic Compliance in Ventilator Modes

Compliance interpretation depends partly on the ventilator mode.

In volume-control ventilation, the ventilator delivers a set tidal volume. If airway resistance increases, peak pressure rises. If static compliance decreases, plateau pressure rises. Since volume is fixed, changes in pressure are often easy to detect.

In pressure-control ventilation, the ventilator delivers a set pressure. If airway resistance increases or compliance decreases, delivered tidal volume may fall. Instead of seeing a pressure increase, the clinician may notice a drop in exhaled tidal volume or minute ventilation.

The principles remain the same, but the signs may look different.

In volume-control ventilation:

• Increased resistance usually raises peak pressure
• Decreased compliance usually raises plateau pressure

In pressure-control ventilation:

• Increased resistance may reduce delivered tidal volume
• Decreased compliance may reduce delivered tidal volume
• Further assessment is needed to determine the cause

Note: Regardless of mode, the clinician should interpret compliance with pressure waveforms, flow waveforms, volume delivery, patient effort, and clinical findings.

Corrected Tidal Volume and Circuit Compliance

Both static and dynamic compliance calculations may be affected by compressed volume in the ventilator circuit. During positive-pressure ventilation, some of the set volume may not reach the patient because gas is compressed in the circuit and the tubing expands under pressure. This is called compressed volume.

For accurate calculations, compressed volume may need to be subtracted from the exhaled tidal volume.

Static compliance with corrected volume:

• Cst = (Exhaled VT − compressed volume) / (Pplat − PEEP)

Dynamic compliance with corrected volume:

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

The compressed volume is calculated by multiplying the tubing compliance factor by the relevant pressure above PEEP.

• For static compliance, use plateau pressure minus PEEP.
• For dynamic compliance, use peak pressure minus PEEP.

For example, if the tubing compliance factor is 4 mL/cmH₂O and the pressure above PEEP is 20 cmH₂O:

Compressed volume = 4 × 20

Compressed volume = 80 mL

If the exhaled tidal volume is 600 mL, the corrected tidal volume is:

600 − 80 = 520 mL

Note: Some modern ventilators compensate for circuit compliance automatically, but respiratory therapists should understand the concept for manual calculations and exam preparation.

Example Calculation: Static Compliance

A patient has the following values:

• Tidal volume: 500 mL
• Plateau pressure: 25 cmH₂O
• PEEP: 5 cmH₂O

First, calculate the pressure above PEEP:

25 − 5 = 20 cmH₂O

Then calculate static compliance:

500 / 20 = 25 mL/cmH₂O

This value is reduced and suggests that the lungs or chest wall are stiff.

Now consider the same patient with a higher plateau pressure:

• Tidal volume: 500 mL
• Plateau pressure: 35 cmH₂O
• PEEP: 5 cmH₂O

Pressure above PEEP:

35 − 5 = 30 cmH₂O

Static compliance:

500 / 30 = 16.7 mL/cmH₂O

Note: This shows a more severe reduction in static compliance. More pressure is required to deliver the same tidal volume.

Example Calculation: Dynamic Compliance

A patient has the following values:

• Tidal volume: 500 mL
• Peak inspiratory pressure: 30 cmH₂O
• PEEP: 10 cmH₂O

First, calculate the pressure above PEEP:

30 − 10 = 20 cmH₂O

Then calculate dynamic compliance:

500 / 20 = 25 mL/cmH₂O

This is below the normal dynamic compliance range often used for ventilated adults.

Now consider a higher peak pressure:

• Tidal volume: 500 mL
• Peak inspiratory pressure: 40 cmH₂O
• PEEP: 10 cmH₂O

Pressure above PEEP:

40 − 10 = 30 cmH₂O

Dynamic compliance:

500 / 30 = 16.7 mL/cmH₂O

Note: Dynamic compliance has decreased because more pressure is needed to deliver the same tidal volume during active airflow.

The next step is to determine whether the pressure increase is caused by airway resistance, decreased compliance, or both. Plateau pressure is needed for that interpretation.

Example: Airway Resistance Problem

A ventilated patient has the following earlier values:

• VT: 500 mL
• PIP: 30 cmH₂O
• Pplat: 20 cmH₂O
• PEEP: 5 cmH₂O

Dynamic compliance:

500 / (30 − 5) = 20 mL/cmH₂O

Static compliance:

500 / (20 − 5) = 33 mL/cmH₂O

Later, the values are:

• VT: 500 mL
• PIP: 45 cmH₂O
• Pplat: 20 cmH₂O
• PEEP: 5 cmH₂O

Dynamic compliance:

500 / (45 − 5) = 12.5 mL/cmH₂O

Static compliance:

500 / (20 − 5) = 33 mL/cmH₂O

Note: Dynamic compliance decreased, but static compliance remained the same. This points toward increased airway resistance. Possible causes include bronchospasm, secretions, mucus plugging, water in the circuit, kinked tubing, or patient biting the tube.

Example: Compliance Problem

A ventilated patient has the following earlier values:

• VT: 500 mL
• PIP: 30 cmH₂O
• Pplat: 20 cmH₂O
• PEEP: 5 cmH₂O

Dynamic compliance:

500 / (30 − 5) = 20 mL/cmH₂O

Static compliance:

500 / (20 − 5) = 33 mL/cmH₂O

Later, the values are:

• VT: 500 mL
• PIP: 40 cmH₂O
• Pplat: 30 cmH₂O
• PEEP: 5 cmH₂O

Dynamic compliance:

500 / (40 − 5) = 14.3 mL/cmH₂O

Static compliance:

500 / (30 − 5) = 20 mL/cmH₂O

Note: Both dynamic and static compliance decreased. The peak-to-plateau difference remained 10 cmH₂O. This points toward decreased lung or chest wall compliance without a major change in airway resistance. Possible causes include ARDS, pulmonary edema, pneumonia, atelectasis, pneumothorax, pleural effusion, or chest wall restriction.

Example: Combined Resistance and Compliance Problem

A ventilated patient has the following earlier values:

• VT: 500 mL
• PIP: 30 cmH₂O
• Pplat: 20 cmH₂O
• PEEP: 5 cmH₂O
• Peak-to-plateau difference: 10 cmH₂O

Later, the values are:

• VT: 500 mL
• PIP: 50 cmH₂O
• Pplat: 30 cmH₂O
• PEEP: 5 cmH₂O
• Peak-to-plateau difference: 20 cmH₂O

Dynamic compliance:

500 / (50 − 5) = 11.1 mL/cmH₂O

Static compliance:

500 / (30 − 5) = 20 mL/cmH₂O

Note: Both values decreased. Plateau pressure increased, so compliance worsened. The peak-to-plateau difference also increased, so airway resistance worsened. This may occur when a patient has pneumonia and retained secretions, ARDS and bronchospasm, or atelectasis with mucus plugging.

Relationship to Driving Pressure

Driving pressure is the difference between plateau pressure and PEEP.

Driving pressure = Pplat − PEEP

This is the denominator in the static compliance formula. Static compliance can be viewed as tidal volume divided by driving pressure.

Cst = VT / driving pressure

Driving pressure is useful because it shows how much pressure above PEEP is required to deliver the tidal volume.

If tidal volume stays the same and driving pressure increases, static compliance decreases.

For example:

• VT: 500 mL
• Driving pressure: 10 cmH₂O
• Static compliance: 50 mL/cmH₂O

If driving pressure increases to 20 cmH₂O:

Static compliance: 25 mL/cmH₂O

The tidal volume did not change, but the pressure required to deliver it doubled. This means the respiratory system has become stiffer.

Note: Dynamic compliance also uses a pressure difference above PEEP, but it uses peak pressure. This includes the resistance component, so it is not the same as driving pressure.

Relationship to Time Constants

Static and dynamic compliance are also connected to time constants.

A time constant is calculated as:

Time constant = Resistance × Compliance

A time constant describes how quickly a lung unit fills or empties after a pressure change. Lung units with short time constants fill and empty quickly. Lung units with long time constants fill and empty slowly.

In obstructive disease, airway resistance is high. This lengthens the time constant and slows exhalation. If the ventilator 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 increase the pressure the patient must overcome to trigger a breath. It can also increase intrathoracic pressure and reduce venous return.

Compliance also affects the time constant. A high-compliance lung unit can take longer to fill and empty, while a low-compliance unit may fill and empty quickly but accept less volume.

Note: In real lungs, different compartments may have different resistance and compliance values. This means ventilation may be uneven. Some units fill quickly, while others fill slowly. Dynamic compliance can be affected by these timing issues because it is measured during active airflow.

Multiple Compartments and Pendelluft

The respiratory system is not a single uniform compartment. Different lung regions may have different airway resistance, compliance, and time constants.

In a simple single-compartment model, static and dynamic measurements may seem easier to compare. In real patients, however, some lung units fill quickly, others fill slowly, and gas distribution may be uneven.

Pendelluft refers to the movement of gas between lung compartments even when there is no airflow at the airway opening. This can occur because different lung regions have different mechanical properties.

This matters because 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 units may not receive enough volume. If exhalation is too short, slower-emptying units may retain air.

Note: Static compliance may better isolate elastic properties, but even static measurements are affected by the complexity of real lung behavior.

Ventilator Graphics and Compliance

Ventilator graphics can help interpret both static and dynamic compliance.

• Pressure waveforms show peak pressure and may show plateau pressure during an inspiratory pause. A rising peak pressure suggests increased resistance, decreased compliance, or both. A rising plateau pressure suggests decreased static compliance.
• 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 especially common in obstructive lung disease.
• Pressure-volume loops show the relationship between pressure and volume. The slope of the pressure-volume curve reflects compliance.
• A steep slope suggests higher compliance because a small pressure change produces a larger volume change.
• A flatter slope suggests lower compliance because more pressure is needed to achieve the same volume.
• Dynamic pressure-volume loops are affected by airflow and resistance. Static or quasi-static pressure-volume loops reduce the effect of airflow and are more useful for assessing elastic properties.

Note: Ventilator graphics should not be interpreted alone. They should be combined with patient assessment, breath sounds, oxygenation, ventilation, hemodynamics, and imaging.

Clinical Response to Decreased Dynamic Compliance

When dynamic compliance suddenly decreases, the therapist should first consider whether airway resistance has increased.

The assessment may include:

• Checking the patient for distress or asynchrony
• Listening for wheezing or diminished breath sounds
• Passing a suction catheter if secretions are suspected
• Checking for mucus plugging
• Looking for a kinked endotracheal tube
• Making sure the patient is not biting the tube
• Checking for water in the circuit
• Checking ventilator tubing for obstruction or kinking
• Evaluating recent changes in flow, tidal volume, or ventilator settings

If peak pressure rises but plateau pressure is stable, airway resistance is the likely problem. The response may include suctioning, bronchodilator therapy, correcting tube obstruction, draining water from the circuit, or improving patient-ventilator synchrony.

Note: Dynamic compliance is useful because it often changes quickly when resistance changes.

Clinical Response to Decreased Static Compliance

When static compliance decreases, the therapist should consider conditions that make the lungs or chest wall harder to inflate.

The assessment may include:

• Checking plateau pressure
• Evaluating oxygenation
• Reviewing chest imaging
• Assessing breath sounds
• Checking for asymmetrical chest movement
• Considering pneumothorax or pleural effusion
• Assessing for atelectasis
• Evaluating for pulmonary edema
• Considering worsening pneumonia or ARDS
• Assessing abdominal distention or chest wall restriction

If both peak and plateau pressures rise, decreased compliance is likely. The response may involve evaluating the cause, adjusting ventilator settings, considering PEEP changes, reducing tidal volume if plateau pressure is high, improving lung recruitment when appropriate, or treating the underlying disease.

Note: Static compliance is especially important for lung-protective ventilation because it relates closely to plateau pressure and driving pressure.

Common Mistakes

One common mistake is using peak pressure to calculate static compliance. Static compliance uses plateau pressure, not peak pressure. Another mistake is using plateau pressure to calculate dynamic compliance. Dynamic compliance uses peak inspiratory pressure.

A third mistake is forgetting to subtract PEEP. Both formulas use the pressure above baseline, so PEEP must be subtracted.

Another mistake is assuming that a high peak pressure always means low static compliance. Peak pressure can rise from airway resistance alone. Plateau pressure must be checked to determine whether static compliance has changed.

It is also a mistake to assume that low dynamic compliance always means bronchospasm or secretions. Dynamic compliance can fall because of stiff lungs, chest wall restriction, or both.

Another common error is ignoring the trend. Compliance values are most useful when compared over time under similar conditions. Finally, compliance should not be interpreted without the patient. Ventilator numbers are important, but they must be matched with clinical assessment.

Static vs. Dynamic Compliance in Exam Questions

For respiratory therapy exams, static and dynamic compliance often appear in calculation and interpretation questions.

Important points to remember include:

• Static compliance uses plateau pressure.
• Dynamic compliance uses peak inspiratory pressure.
• Both formulas subtract PEEP.
• Static compliance reflects elastic resistance.
• Dynamic compliance reflects elastic resistance plus airway resistance.
• Dynamic compliance is usually lower than static compliance.
• A change in peak pressure alone suggests airway resistance.
• A change in plateau pressure suggests compliance.
• If both peak and plateau pressures change, compare the peak-to-plateau difference.
• If the difference increases, airway resistance has increased.
• If the difference stays the same, airway resistance has not significantly changed.
• If both pressures decrease and the difference stays the same, compliance has improved.
• If both pressures decrease and the difference decreases, both compliance and resistance have improved.

Note: These rules help students answer ventilator troubleshooting questions more efficiently.

Quick Comparison

• Static compliance is measured during no-flow conditions. It uses plateau pressure and mainly reflects lung and chest wall elasticity.
• Dynamic compliance is measured during active airflow. It uses peak inspiratory pressure and reflects both airway resistance and elastic resistance.
• Static compliance is more specific for lung stiffness.
• Dynamic compliance is more sensitive to airway resistance.
• Static compliance requires an inspiratory pause maneuver.
• Dynamic compliance can often be monitored breath by breath.
• Low static compliance suggests stiff lungs or restricted chest wall movement.
• Low dynamic compliance may suggest airway resistance, stiff lungs, chest wall restriction, artificial airway obstruction, or circuit problems.
• Both values are clinically useful, but they answer different questions.
• Static compliance asks: How stiff are the lungs and chest wall?
• Dynamic compliance asks: How difficult is it to deliver a breath during active airflow?

Final Thoughts

Static compliance and dynamic compliance are related measurements, but they should not be interpreted the same way. Static compliance uses plateau pressure during no-flow conditions, making it more useful for evaluating lung and chest wall stiffness. Dynamic compliance uses peak inspiratory pressure during active airflow, so it includes both airway resistance and elastic resistance.

Comparing the two helps respiratory therapists determine whether rising ventilator pressures are caused by secretions, bronchospasm, tube obstruction, stiff lungs, chest wall restriction, or a combination of problems. When trended over time, these measurements support safer ventilator assessment and more accurate bedside decision-making.