Positive End-Expiratory Pressure (PEEP): A Complete Guide

Positive end-expiratory pressure, or PEEP, is a fundamental component of mechanical ventilation that helps maintain positive pressure in the lungs at the end of expiration.

By preventing airway pressure from returning completely to atmospheric pressure, PEEP can stabilize alveoli, increase functional residual capacity, reduce intrapulmonary shunting, and improve oxygenation. However, PEEP also affects intrathoracic pressure, venous return, cardiac output, pulmonary vascular resistance, and lung mechanics.

Effective use requires balancing alveolar recruitment and oxygenation against the risks of overdistention, hemodynamic compromise, barotrauma, and other complications.

What is PEEP?

Positive end-expiratory pressure is the pressure that remains in the airways and alveoli at the end of expiration during positive-pressure mechanical ventilation. Instead of allowing airway pressure to return to atmospheric pressure after each breath, the ventilator maintains a predetermined level of positive pressure.

This positive baseline pressure becomes the starting point for the next mechanical breath. PEEP is not considered a separate ventilator mode. Instead, it is added to modes such as assist/control ventilation, synchronized intermittent mandatory ventilation, or pressure support ventilation.

A similar physiologic principle is used with continuous positive airway pressure, or CPAP. The major distinction is that CPAP is generally applied to a spontaneously breathing patient, while PEEP is commonly used during mechanical ventilation. Both strategies maintain positive pressure at the end of expiration to support alveolar stability and oxygenation.

Why is PEEP Used?

The primary purpose of PEEP is to improve and maintain oxygenation by preventing alveolar collapse.

At the end of a normal expiration, the lungs retain a certain amount of gas known as functional residual capacity. This remaining lung volume helps keep alveoli open and provides a reservoir of oxygen between breaths.

When functional residual capacity decreases, some alveoli may partially or completely collapse. This reduces the number of lung units available for gas exchange and can contribute to hypoxemia. PEEP raises the pressure within the lungs at the end of expiration, helping keep unstable alveoli open.

The major physiologic effects include:

• Increasing functional residual capacity
• Preventing end-expiratory alveolar collapse
• Recruiting previously collapsed or poorly aerated alveoli
• Increasing the amount of lung available for gas exchange
• Improving ventilation-perfusion relationships
• Reducing intrapulmonary shunting
• Improving arterial oxygenation
• Potentially reducing the need for high concentrations of inspired oxygen

Note: The goal is not simply to apply more pressure. The goal is to use enough PEEP to maintain adequate lung volume and oxygenation without causing excessive alveolar distention or cardiovascular compromise.

How PEEP Improves Oxygenation

PEEP improves oxygenation primarily by maintaining alveolar recruitment. When alveoli collapse, pulmonary blood flow may continue through those regions even though little or no ventilation is occurring.

Blood therefore passes through the lungs without being adequately oxygenated. This process is known as intrapulmonary shunting.

Increasing the fraction of inspired oxygen may improve some forms of hypoxemia, but significant shunting may respond poorly to oxygen therapy alone. If an alveolus is completely collapsed and receives no ventilation, increasing FiO₂ cannot effectively deliver additional oxygen into that unit.

PEEP addresses the underlying mechanical problem by helping reopen or stabilize the alveolus.

Once more alveoli are open, a greater percentage of pulmonary blood flow can participate in effective gas exchange. This can improve PaO₂ and oxygen saturation while reducing the severity of the shunt.

PEEP and Functional Residual Capacity

Functional residual capacity (FRC) is the amount of gas remaining in the lungs after a normal exhalation. FRC is important because it helps maintain alveolar stability and serves as an oxygen reservoir between breaths.

Several respiratory disorders can significantly reduce FRC. When lung volume falls, dependent or unstable alveoli become more likely to collapse. PEEP increases end-expiratory lung volume and therefore increases FRC.

The resulting increase in lung volume can:

• Restore ventilation to collapsed regions
• Improve ventilation-perfusion matching
• Increase available gas-exchange surface area
• Reduce alveolar opening and closing
• Improve oxygenation

Note: These effects make PEEP especially useful in conditions associated with widespread alveolar collapse or reduced lung volume.

Indications for PEEP

PEEP is most useful when hypoxemia is caused by alveolar collapse, reduced FRC, intrapulmonary shunting, or diffuse pulmonary disease.

Common clinical situations include:

• Acute respiratory distress syndrome
• Acute lung injury
• Generalized atelectasis
• Pulmonary edema
• Infant respiratory distress syndrome
• Severe refractory hypoxemia
• Significant intrapulmonary shunting
• Reduced functional residual capacity
• Decreased lung compliance

PEEP may also be used when a patient requires prolonged exposure to a high FiO₂. Improving oxygenation with PEEP may allow the clinician to lower the inspired oxygen concentration and reduce the risks associated with prolonged high oxygen exposure.

One commonly used clinical pattern involves persistent hypoxemia despite a relatively high FiO₂. For example, a PaO₂ below approximately 60 mm Hg despite an FiO₂ greater than 0.50 to 0.60 suggests that increasing oxygen concentration alone may not be sufficient.

In this situation, increasing PEEP may be more effective if alveolar collapse or intrapulmonary shunting is contributing to the hypoxemia.

PEEP and Acute Respiratory Distress Syndrome

PEEP plays an especially important role in the management of acute respiratory distress syndrome (ARDS).

ARDS produces a heterogeneous pattern of lung injury. Some areas may remain relatively normal, while others are collapsed, fluid-filled, poorly aerated, or extremely stiff. This decreases the amount of functional lung available for ventilation.

Without adequate end-expiratory pressure, unstable lung units may repeatedly collapse during expiration and reopen during inspiration. Repetitive opening and closing can expose alveoli to mechanical stress and contribute to ventilator-associated lung injury. PEEP helps maintain recruited alveoli in an open state and reduces repeated collapse.

In ARDS, ventilation is typically managed with a lung-protective approach. PEEP is adjusted along with FiO₂ to provide acceptable oxygenation while limiting excessive airway pressures.

Oxygenation targets may allow a PaO₂ of approximately 55 to 80 mm Hg or an SpO₂ of approximately 88% to 95% rather than attempting to achieve unnecessarily high oxygen values. A minimum PEEP level of approximately 5 cm H₂O is commonly used, with higher levels considered according to oxygenation, lung mechanics, hemodynamics, and disease severity.

Plateau pressure should also be monitored carefully. A plateau pressure of approximately 30 cm H₂O or less is commonly targeted during lung-protective ventilation. Whenever PEEP is increased, plateau pressure should be reassessed because additional end-expiratory pressure may increase total lung stress and contribute to overdistention.

Understanding Optimal PEEP

Optimal PEEP is the level that provides the greatest overall physiologic benefit with the least harm. The highest possible PEEP is not necessarily the best PEEP.

Too little PEEP may allow alveoli to collapse, reducing FRC and worsening shunt. Too much PEEP may overdistend the lungs, reduce pulmonary blood flow, impair cardiac output, and decrease respiratory-system compliance. The optimal level therefore lies between these extremes.

Potential goals of PEEP titration include:

• Maintaining recruited alveoli
• Improving oxygenation
• Reducing intrapulmonary shunting
• Improving static compliance
• Maintaining acceptable airway pressures
• Preserving cardiac output
• Maximizing systemic oxygen delivery
• Avoiding alveolar overdistention

Note: Different methods may be used to estimate the most appropriate PEEP level.

Minimum PEEP

One strategy is known as minimum PEEP. The purpose is to apply only enough end-expiratory pressure to provide adequate oxygenation while allowing the FiO₂ to be reduced to a safer level.

A common goal may be to maintain a PaO₂ above approximately 60 mm Hg or an SpO₂ above 90% while keeping FiO₂ at or below approximately 0.60.

With this approach, PEEP may frequently remain within the range of approximately 5 to 15 cm H₂O, although individual requirements vary considerably. Minimum PEEP emphasizes using the lowest level of pressure that accomplishes the desired oxygenation goal.

Best or Optimum PEEP

Another strategy focuses on identifying the PEEP level that provides the best overall balance between oxygenation, lung mechanics, and cardiovascular performance.

This approach may involve higher PEEP levels, particularly in patients with severe ARDS. The clinician evaluates several variables rather than looking only at arterial oxygenation.

Potential indicators of appropriate PEEP include:

• Increased PaO₂
• Improved static compliance
• Reduced intrapulmonary shunting
• Stable cardiac output
• Stable or improved mixed venous oxygenation
• Acceptable blood pressure
• Acceptable pulmonary vascular resistance
• Absence of overdistention

Note: If oxygenation improves but cardiac output falls substantially, the higher PEEP may not actually improve overall tissue oxygen delivery. This is why PEEP must be assessed as part of the entire cardiopulmonary system.

PEEP and Oxygen Delivery

Arterial oxygenation does not tell the complete story of tissue oxygen delivery. Oxygen delivery depends on both the oxygen content of arterial blood and cardiac output.

PEEP may improve arterial oxygen content by recruiting alveoli and increasing PaO₂. At the same time, excessive PEEP may decrease venous return and cardiac output.

If the reduction in cardiac output is large enough, total oxygen delivery to the tissues may fall despite a better PaO₂. This explains why an improved blood gas does not automatically mean that a higher PEEP setting is beneficial.

One method of PEEP titration involves increasing PEEP in small increments while monitoring parameters such as:

• Cardiac output
• Cardiac index
• Blood pressure
• Mixed venous oxygen saturation
• Mixed venous oxygen tension
• Calculated oxygen delivery

Note: If increasing PEEP begins to reduce oxygen delivery or produces significant hemodynamic instability, the optimal level has probably been exceeded. The clinician may then return to the previous PEEP setting that provided the best balance between pulmonary and cardiovascular performance.

PEEP and Static Compliance

Respiratory-system compliance provides another useful method of assessing PEEP. Static compliance describes how easily the respiratory system expands for a given pressure.

It can be calculated as:

Crs = VT ÷ (Pplat − PEEP)

where:

• Crs is static respiratory-system compliance
• VT is tidal volume
• Pplat is plateau pressure
• PEEP is the externally applied end-expiratory pressure

When tidal volume remains constant, an improvement in compliance means less driving pressure is required to deliver the same volume. As PEEP recruits previously collapsed alveoli, compliance may improve.

If PEEP becomes excessive, alveoli may become overinflated and compliance may begin to decrease. This creates a useful clinical concept: the PEEP associated with the highest static compliance may approximate the level at which recruitment is optimized without excessive overdistention.

Pressure-Volume Relationships and PEEP

Pressure-volume curves may also provide information about alveolar recruitment and overdistention. The lower portion of the pressure-volume curve may contain an inflection point representing the pressure at which collapsed or unstable lung units begin opening.

PEEP may be set high enough to maintain alveolar pressure above the level where collapse is likely to occur. The upper portion of the curve may contain another inflection region associated with overdistention.

The objective is to operate between these zones whenever possible, maintaining adequate recruitment without pushing the lungs into excessive inflation. This concept reinforces the idea that more pressure is not always better.

PEEP Following Lung Recruitment Maneuvers

A lung recruitment maneuver temporarily increases airway pressure in an attempt to reopen collapsed alveoli. Once recruitment has occurred, an appropriate PEEP level is needed to prevent those newly opened lung units from collapsing again.

One approach is a decremental PEEP study. After recruitment, the patient may be ventilated using a low tidal volume, such as approximately 4 to 6 mL/kg of predicted body weight. PEEP is then reduced gradually, often in steps of approximately 2 cm H₂O.

Static compliance is measured after each adjustment. The clinician continues decreasing PEEP until compliance reaches its best value and then begins to decline.

A recruitment maneuver may then be repeated, and PEEP may be set approximately 2 cm H₂O above the level associated with peak compliance. The purpose is to identify the lowest PEEP capable of maintaining recruited alveoli while avoiding unnecessary pressure.

Cardiovascular Effects of PEEP

PEEP has important cardiovascular effects because positive airway pressure increases intrathoracic and pleural pressure. Venous blood normally returns to the right side of the heart because of a pressure gradient between the systemic veins and the thorax.

As intrathoracic pressure rises, this gradient decreases.

The result may include:

• Reduced venous return
• Reduced right ventricular preload
• Reduced stroke volume
• Reduced pulmonary blood flow
• Reduced cardiac output
• Hypotension

Note: These effects become more significant when high levels of PEEP are used or when the patient is hypovolemic. A patient with adequate circulating volume may tolerate a PEEP increase better than a patient who is already volume depleted.

Effects on Pulmonary Vascular Resistance

PEEP can also affect the pulmonary circulation. When alveoli become excessively inflated, they may compress surrounding pulmonary capillaries. This can increase pulmonary vascular resistance and increase right ventricular afterload.

The right ventricle must then generate greater pressure to move blood through the pulmonary circulation. Excessive right ventricular pressure and volume loading can also shift the interventricular septum toward the left ventricle, potentially interfering with left ventricular filling.

Both very low and very high lung volumes may increase pulmonary vascular resistance. For this reason, an appropriate PEEP level may actually improve pulmonary vascular function by maintaining lung volume without producing excessive distention.

Hemodynamic Monitoring During PEEP Therapy

When higher PEEP levels are required, hemodynamic monitoring becomes increasingly important.

Useful measurements may include:

• Heart rate
• Blood pressure
• Cardiac output
• Cardiac index
• Mixed venous oxygen saturation
• Central venous pressure
• Pulmonary artery pressures
• Pulmonary capillary wedge pressure
• Urine output
• Peripheral perfusion

A sudden fall in arterial blood pressure following a PEEP increase suggests that the new setting may be impairing venous return and cardiac output. In such a situation, reducing PEEP toward the previous level may be appropriate while the patient is reassessed.

Positive intrathoracic pressure can also influence invasive pressure measurements. Central venous pressure and pulmonary artery wedge pressure may appear artificially elevated because pressure from the thorax is transmitted to the vascular system.

Measurements are therefore generally obtained at end-expiration. The patient should not be disconnected from PEEP merely to obtain these measurements because abrupt removal of PEEP can cause alveolar derecruitment and cardiovascular changes.

Complications of Excessive PEEP

PEEP can be highly beneficial, but excessive pressure can cause important complications.

Reduced Cardiac Output

One of the most important complications is decreased cardiac output. Increasing intrathoracic pressure can reduce venous return to the heart, particularly in patients who are hypovolemic or have relatively compliant lungs.

Signs may include:

• Hypotension
• Tachycardia
• Reduced urine output
• Cool or poorly perfused extremities
• Decreased mixed venous oxygen saturation
• Reduced cardiac output

Note: When these findings develop after increasing PEEP, excessive intrathoracic pressure should be considered.

Alveolar Overdistention

PEEP that exceeds the amount needed for recruitment can stretch already open alveoli. Overdistention may reduce compliance and compress pulmonary capillaries.

This can worsen lung mechanics and cardiovascular function even when oxygenation appears satisfactory. Monitoring plateau pressure, compliance, tidal volume, and pressure-volume relationships can help identify excessive inflation.

Barotrauma

High airway pressure can contribute to alveolar rupture.

Potential forms of barotrauma include:

• Pneumothorax
• Tension pneumothorax
• Pneumomediastinum
• Subcutaneous emphysema
• Pneumopericardium
• Pneumoperitoneum
• Pulmonary interstitial emphysema, particularly in neonates

Note: A sudden deterioration in respiratory status, falling blood pressure, increased airway pressures, or unilateral changes in breath sounds should raise concern for pneumothorax in a patient receiving positive-pressure ventilation. An untreated tension pneumothorax is an absolute contraindication to applied PEEP.

Increased Intracranial Pressure

High intrathoracic pressure may interfere with venous drainage from the brain. This can increase intracranial pressure and potentially reduce cerebral perfusion.

PEEP should therefore be used cautiously in patients with elevated intracranial pressure or significant neurologic injury. The effect varies according to lung compliance, cardiovascular function, cerebral physiology, and the amount of pressure being transmitted into the thorax.

Renal Effects

PEEP can indirectly affect renal function by decreasing cardiac output and renal blood flow. Reduced renal perfusion may lead to decreased urine output and, when severe or prolonged, contribute to renal dysfunction.

Urine output should therefore be considered part of the overall assessment of a patient receiving significant levels of positive pressure.

PEEP and Heart Failure

Although excessive PEEP can reduce venous return and cardiac output, positive intrathoracic pressure may be beneficial in selected patients with left ventricular dysfunction or cardiogenic pulmonary edema.

Positive pressure can reduce some of the loading conditions placed on the left ventricle while also helping recruit fluid-filled or collapsed alveoli. Improved lung volume and oxygenation may reduce respiratory distress and decrease the work of breathing.

This is one reason CPAP and PEEP can be useful in selected patients with acute cardiogenic pulmonary edema. The patient’s blood pressure and overall cardiovascular status must still be monitored carefully because excessive pressure may cause hypotension.

What is Auto-PEEP?

Auto-PEEP, also known as intrinsic PEEP, occult PEEP, or dynamic hyperinflation, is positive pressure that remains in the lungs because the patient has not completely exhaled before the next breath begins.

Unlike therapeutic PEEP, auto-PEEP is not intentionally set on the ventilator. It develops because gas becomes trapped within the respiratory system. Auto-PEEP is particularly common in patients with airflow obstruction, including asthma and COPD.

Causes of Auto-PEEP

Factors that can cause or worsen auto-PEEP include:

• High respiratory rate
• Excessive tidal volume
• Short expiratory time
• Long inspiratory time
• Low inspiratory flow during volume-controlled ventilation
• Bronchospasm
• Airway inflammation
• Retained secretions
• Mucus plugging
• Partially obstructed endotracheal tube
• Wet or obstructed expiratory filters
• Expiratory valve malfunction
• Severe airflow obstruction
• Patient-ventilator asynchrony

Note: All of these factors can prevent complete lung emptying before the next inspiration begins.

How to Recognize Auto-PEEP

Ventilator graphics are particularly useful for detecting auto-PEEP. One of the most important clues is expiratory flow that fails to return to zero before the next mechanical inspiration begins. This indicates that gas is still leaving the lungs when the ventilator initiates another breath.

A flow-volume loop may show similar evidence of incomplete exhalation. Auto-PEEP can also be measured using an end-expiratory hold maneuver.

During the maneuver, the expiratory valve is temporarily closed so airway and alveolar pressures can equilibrate. The total end-expiratory pressure is then compared with the externally set PEEP.

For example, if the total end-expiratory pressure is 12 cm H₂O and the ventilator PEEP is set at 5 cm H₂O:

Auto-PEEP = 12 − 5 = 7 cm H₂O

Note: The patient therefore has 7 cm H₂O of intrinsic PEEP.

How Auto-PEEP Affects Triggering

Auto-PEEP can significantly increase the work required for a spontaneously breathing patient to trigger the ventilator. Before the ventilator senses an inspiratory effort, the patient must first reduce alveolar pressure enough to overcome the intrinsic positive pressure trapped within the lungs.

The patient must then generate additional pressure or flow to satisfy the ventilator trigger setting.

For example, a patient with 6 cm H₂O of auto-PEEP and a pressure trigger requiring an additional 2 cm H₂O change may effectively need to generate approximately 8 cm H₂O of inspiratory effort before receiving ventilator assistance.

This increased workload can cause:

• Missed triggering
• Delayed triggering
• Increased work of breathing
• Respiratory muscle fatigue
• Patient-ventilator asynchrony

Note: The ventilator may show inspiratory efforts that fail to produce assisted breaths.

Treating Auto-PEEP

The primary goal when treating auto-PEEP is to improve lung emptying. Management should focus on correcting the cause rather than simply increasing external PEEP.

Potential strategies include:

• Decreasing respiratory rate
• Decreasing tidal volume when appropriate
• Increasing inspiratory flow during volume-controlled ventilation
• Decreasing inspiratory time during pressure-controlled ventilation
• Increasing expiratory time
• Treating bronchospasm
• Administering bronchodilator therapy
• Clearing secretions
• Correcting airway obstruction
• Addressing ventilator circuit problems

Note: Patients with severe obstructive disease may require prolonged expiratory times, sometimes producing I:E ratios such as 1:4 or 1:5. The goal is to allow expiratory flow to approach or return to zero before the next breath begins.

Using External PEEP in Patients With Auto-PEEP

It may initially seem inappropriate to add PEEP to a patient who already has trapped positive pressure. However, carefully selected external PEEP can reduce the inspiratory threshold load in spontaneously breathing patients with dynamic airflow obstruction.

External PEEP raises the airway pressure closer to the pressure already present within the alveoli. This reduces the amount of pressure the patient must generate before triggering the ventilator.

Applied PEEP may also function as a pneumatic stent in patients with dynamic airway collapse, helping support unstable airways during expiration. A commonly described approach is to apply external PEEP at approximately 50% to 80% of the measured auto-PEEP.

PEEP may be increased gradually in small increments, such as 1 to 2 cm H₂O, while observing whether triggering improves.

The clinician should monitor:

• Triggering effectiveness
• Total PEEP
• Airway pressure
• Tidal volume
• Expiratory flow
• Lung mechanics
• Hemodynamics

External PEEP should not be increased blindly. If it worsens hyperinflation or intrinsic pressure, the strategy should be reconsidered.

This approach is primarily useful when the patient is actively attempting to trigger ventilator breaths. During fully controlled ventilation, increasing external PEEP solely because auto-PEEP is present is generally not indicated.

PEEP During Pressure Support Ventilation

PEEP and pressure support can be used together during partial ventilatory support. Pressure support assists the patient’s inspiratory effort and helps overcome artificial airway resistance or increase tidal volume.

PEEP serves a different purpose by supporting oxygenation and maintaining end-expiratory lung volume. A patient may therefore receive pressure support to reduce the work of inspiration while simultaneously receiving PEEP to prevent alveolar collapse.

Note: As the patient’s condition improves, pressure support and PEEP may be reduced separately according to respiratory muscle function, gas exchange, lung mechanics, and oxygen requirements.

PEEP During Weaning

PEEP requirements are considered when determining readiness for liberation from mechanical ventilation.

A patient being evaluated for a spontaneous breathing trial should generally demonstrate improving disease status, hemodynamic stability, adequate inspiratory effort, and acceptable oxygenation with relatively modest ventilator support.

Common oxygenation criteria may include:

• P/F ratio of approximately 150 to 200 or greater
• FiO₂ approximately 0.40 to 0.50 or less
• PEEP approximately 5 to 8 cm H₂O

These values are not absolute for every patient but provide a general framework.

CPAP may be used during spontaneous breathing trials. A small amount of baseline pressure may be particularly helpful in patients with intrinsic PEEP because it can reduce the effort needed to trigger or sustain breathing through the ventilator circuit.

Reducing PEEP

PEEP should generally be reduced gradually as pulmonary function improves. Reductions may be made in steps of approximately 2 to 5 cm H₂O with reassessment after each adjustment.

The clinician evaluates whether oxygenation remains acceptable and whether the patient continues to tolerate the lower pressure. The sequence of reducing FiO₂ and PEEP depends partly on the patient’s oxygen requirement.

If FiO₂ is already below approximately 0.50, PEEP may be reduced first. If FiO₂ remains above approximately 0.50, FiO₂ may be reduced before lowering PEEP, provided oxygenation remains adequate.

If the patient develops a PEEP-related complication such as hypotension, decreased cardiac output, severe overdistention, or barotrauma, PEEP may need to be decreased promptly regardless of the FiO₂.

Monitoring a Patient Receiving PEEP

PEEP should never be evaluated as an isolated ventilator setting. Its effect depends on the interaction between lung mechanics, oxygenation, hemodynamics, disease process, and patient effort.

Important monitoring parameters include:

• SpO₂
• PaO₂
• FiO₂
• PaCO₂
• Arterial pH
• Peak inspiratory pressure
• Plateau pressure
• Mean airway pressure
• Static compliance
• Tidal volume
• Expiratory flow
• Total PEEP
• Blood pressure
• Heart rate
• Cardiac output when available
• Urine output
• Signs of tissue perfusion
• Patient comfort and synchrony

Note: Ventilator waveforms and loops can provide additional information about air trapping, overdistention, missed triggering, leaks, and incomplete expiration. Low PEEP alarms should also be investigated because they may indicate a circuit leak, disconnection, or failure to maintain the intended baseline pressure.

Contraindications and Precautions

PEEP is not appropriate in every situation. An untreated tension pneumothorax is an absolute contraindication because additional positive pressure can worsen intrathoracic pressure and cardiovascular collapse.

PEEP should also be used cautiously in patients with:

• Significant hypotension
• Severe hypovolemia
• Elevated intracranial pressure
• Significant right ventricular dysfunction
• Severe pulmonary overdistention
• Existing barotrauma

Note: These conditions do not always mean that PEEP can never be used, but they require careful consideration of risks, benefits, and monitoring.

Key Principles of PEEP Management

Effective PEEP management can be summarized by several important principles:

• PEEP is primarily an oxygenation strategy. It improves oxygenation by increasing end-expiratory lung volume, stabilizing alveoli, and reducing shunting.
• The appropriate PEEP level is individualized. A number that works well for one patient may produce excessive pressure or inadequate recruitment in another.
• Oxygenation should never be considered in isolation. A higher PaO₂ does not necessarily represent improvement if cardiac output and tissue oxygen delivery are simultaneously decreasing.
• Respiratory mechanics can help guide PEEP selection. Improved static compliance may indicate successful recruitment, while declining compliance after a PEEP increase may suggest overdistention.
• External PEEP and auto-PEEP are different. Auto-PEEP results from incomplete exhalation and requires correction of airflow obstruction and ventilator timing.
• Every PEEP adjustment should be followed by reassessment. The patient’s oxygenation, ventilation, hemodynamics, lung mechanics, and ventilator graphics should be evaluated together.

PEEP Practice Questions

1. What is positive end-expiratory pressure (PEEP)?
PEEP is positive pressure maintained in the airways and alveoli at the end of expiration during mechanical ventilation.

2. What is the primary purpose of PEEP?
The primary purpose of PEEP is to prevent alveolar collapse, maintain alveolar recruitment, and improve oxygenation.

3. How does PEEP affect functional residual capacity (FRC)?
PEEP increases functional residual capacity by maintaining greater lung volume at the end of expiration.

4. How does PEEP help reduce intrapulmonary shunting?
PEEP helps recruit and stabilize collapsed or poorly ventilated alveoli so that more pulmonary blood flow participates in gas exchange.

5. Why is PEEP especially important in patients with ARDS?
PEEP helps keep unstable alveoli open in ARDS, improving oxygenation and reducing repeated alveolar collapse and reopening.

6. What is a common starting level of PEEP during mechanical ventilation?
A common starting level of PEEP is approximately 5 cm H₂O.

7. What is meant by optimal PEEP?
Optimal PEEP is the level that provides adequate alveolar recruitment and oxygenation without causing excessive overdistention or cardiovascular compromise.

8. Why should oxygenation not be the only factor used to determine optimal PEEP?
An increase in PEEP may improve arterial oxygenation while simultaneously decreasing cardiac output and overall tissue oxygen delivery.

9. How can excessive PEEP reduce cardiac output?
Excessive PEEP increases intrathoracic pressure, which can reduce venous return, right ventricular preload, stroke volume, and cardiac output.

10. Why are hypovolemic patients more vulnerable to the hemodynamic effects of PEEP?
Hypovolemic patients already have reduced circulating blood volume, so additional decreases in venous return from PEEP can cause significant hypotension and reduced cardiac output.

11. How can excessive PEEP affect pulmonary vascular resistance?
Excessive PEEP can overdistend alveoli and compress pulmonary capillaries, increasing pulmonary vascular resistance and right ventricular afterload.

12. What happens to static compliance when PEEP successfully recruits collapsed alveoli?
Static compliance may improve because the lungs require less pressure to deliver the same tidal volume.

13. What may a decrease in static compliance after increasing PEEP indicate?
A decrease in static compliance after increasing PEEP may indicate alveolar overdistention.

14. How is static respiratory-system compliance calculated in a patient receiving PEEP?
Static compliance is calculated as tidal volume divided by the difference between plateau pressure and PEEP.

15. What is auto-PEEP?
Auto-PEEP is unintended positive pressure remaining in the lungs at the end of expiration because the patient has not completely exhaled before the next breath begins.

16. Which patients are especially prone to developing auto-PEEP?
Patients with obstructive airway diseases such as COPD and asthma are especially prone to developing auto-PEEP.

17. What ventilator waveform finding suggests the presence of auto-PEEP?
Auto-PEEP is suggested when expiratory flow fails to return to zero before the next inspiration begins.

18. How can auto-PEEP be measured on a mechanical ventilator?
Auto-PEEP can be measured using an end-expiratory hold maneuver that allows airway and alveolar pressures to equilibrate.

19. How is the amount of auto-PEEP calculated?
Auto-PEEP is calculated by subtracting the externally set PEEP from the total end-expiratory pressure measured during an expiratory hold.

20. How does auto-PEEP affect a spontaneously breathing patient’s ability to trigger the ventilator?
Auto-PEEP increases the inspiratory effort required to overcome trapped alveolar pressure before the ventilator can be triggered.

21. Why might external PEEP be applied to a patient who already has auto-PEEP?
Carefully applied external PEEP can reduce the inspiratory threshold load and make it easier for the patient to trigger the ventilator.

22. How much external PEEP may be used to offset auto-PEEP in a patient with dynamic airflow obstruction?
External PEEP may be applied at approximately 50% to 80% of the measured auto-PEEP while the patient is monitored closely.

23. What ventilator adjustments can help reduce auto-PEEP?
Reducing respiratory rate or tidal volume, increasing inspiratory flow, decreasing inspiratory time, and increasing expiratory time can help reduce auto-PEEP.

24. What are major complications of excessive PEEP?
Major complications include decreased venous return, reduced cardiac output, hypotension, alveolar overdistention, barotrauma, and increased intracranial pressure.

25. What is an absolute contraindication to applying PEEP?
An untreated tension pneumothorax is an absolute contraindication to applied PEEP.

26. How does PEEP affect mean airway pressure?
PEEP increases mean airway pressure because positive pressure is maintained throughout the expiratory phase.

27. Why can PEEP allow the FiO₂ to be reduced?
PEEP can improve alveolar recruitment and oxygenation, allowing adequate oxygenation to be maintained with a lower inspired oxygen concentration.

28. Why is reducing prolonged exposure to a high FiO₂ important?
Reducing prolonged exposure to a high FiO₂ helps decrease the risk of oxygen toxicity.

29. What PaO₂ value may suggest refractory hypoxemia when the patient is receiving a high FiO₂?
A PaO₂ below approximately 60 mm Hg despite a high FiO₂ may suggest refractory hypoxemia.

30. What does the 60-60 rule suggest about PEEP use?
If the PaO₂ is below 60 mm Hg while the FiO₂ is greater than 0.60, significant shunting may be present and PEEP may need to be added or increased.

31. What oxygen saturation range may be acceptable in patients with ARDS?
An SpO₂ of approximately 88% to 95% may be an acceptable target in patients with ARDS.

32. What PaO₂ range may be acceptable in patients with ARDS?
A PaO₂ of approximately 55 to 80 mm Hg may be an acceptable target in patients with ARDS.

33. Why should plateau pressure be reassessed after increasing PEEP?
Plateau pressure should be reassessed because increasing PEEP can increase total lung pressure and contribute to alveolar overdistention.

34. What plateau pressure is commonly targeted during lung-protective ventilation in ARDS?
A plateau pressure of approximately 30 cm H₂O or less is commonly targeted.

35. What is minimum PEEP?
Minimum PEEP is the lowest level of PEEP needed to maintain adequate oxygenation while allowing the FiO₂ to be reduced to a safer level.

36. What oxygenation goal is commonly used with a minimum PEEP strategy?
A common goal is to maintain a PaO₂ above approximately 60 mm Hg or an SpO₂ above 90% while using an FiO₂ of approximately 0.60 or less.

37. What is best or optimum PEEP designed to achieve?
Best or optimum PEEP is designed to maximize physiologic benefit while minimizing alveolar overdistention and cardiovascular compromise.

38. How does PEEP influence the pressure gradient for venous return?
PEEP increases intrathoracic pressure and reduces the pressure gradient that normally promotes venous return to the right side of the heart.

39. How can excessive PEEP affect right ventricular afterload?
Excessive PEEP can compress pulmonary capillaries and increase pulmonary vascular resistance, which increases right ventricular afterload.

40. How can excessive PEEP interfere with left ventricular filling?
Excessive PEEP can contribute to right ventricular enlargement and septal displacement toward the left ventricle, reducing left ventricular filling.

41. Why can both very low and very high lung volumes increase pulmonary vascular resistance?
Low lung volumes can reduce vessel support, while excessive lung inflation can compress pulmonary capillaries, so both extremes can increase resistance.

42. What hemodynamic variable should remain stable when PEEP is appropriately titrated?
Cardiac output should remain stable or acceptable when PEEP is appropriately titrated.

43. What can mixed venous oxygen saturation help assess during PEEP titration?
Mixed venous oxygen saturation can help indicate whether overall tissue oxygen delivery is being maintained.

44. Why should a patient be hemodynamically stable before an aggressive PEEP titration?
A hemodynamically unstable patient may poorly tolerate the reduction in venous return and cardiac output caused by higher PEEP levels.

45. How can PEEP affect central venous pressure measurements?
PEEP can increase intrathoracic pressure and artificially elevate measured central venous pressure.

46. How can PEEP affect pulmonary artery wedge pressure measurements?
PEEP can transmit pressure into the thorax and cause pulmonary artery wedge pressure measurements to appear higher than the true cardiac filling pressure.

47. When should invasive hemodynamic pressures generally be measured in a patient receiving PEEP?
They should generally be measured at end-expiration.

48. Why should a patient not be disconnected from PEEP just to obtain a hemodynamic pressure measurement?
Disconnecting PEEP can cause alveolar derecruitment and alter both respiratory and cardiovascular conditions.

49. What is a decremental PEEP study?
A decremental PEEP study gradually lowers PEEP after lung recruitment while monitoring respiratory mechanics to identify the level that best maintains recruitment.

50. Why may PEEP be set slightly above the level associated with peak compliance after a decremental PEEP study?
PEEP may be set slightly above that level to help prevent recruited alveoli from collapsing again.

51. How does PEEP help reduce repeated alveolar opening and closing?
PEEP maintains positive pressure at end-expiration so unstable alveoli are less likely to collapse between breaths and require reopening during the next inspiration.

52. Why can repeated alveolar collapse and reopening be harmful?
Repeated collapse and reopening can create mechanical stress on lung tissue and contribute to ventilator-associated lung injury.

53. What is the relationship between PEEP and alveolar recruitment?
PEEP helps maintain alveoli that have been recruited so they remain open at the end of expiration.

54. What is the difference between PEEP and CPAP?
PEEP is generally applied during mechanical ventilation, while CPAP maintains continuous positive airway pressure in a spontaneously breathing patient.

55. Can PEEP be used with pressure support ventilation?
Yes. PEEP can support oxygenation while pressure support assists the patient’s spontaneous inspiratory effort.

56. What is the role of pressure support when it is used together with PEEP?
Pressure support reduces inspiratory workload and helps produce an adequate tidal volume, while PEEP helps maintain end-expiratory lung volume and oxygenation.

57. Why can PEEP be beneficial in pulmonary edema?
PEEP can recruit poorly aerated alveoli, improve oxygenation, and increase end-expiratory lung volume in patients with pulmonary edema.

58. How can positive airway pressure benefit some patients with left ventricular dysfunction?
Positive intrathoracic pressure can alter cardiac loading conditions while improving lung expansion and oxygenation.

59. What effect can excessive PEEP have on cerebral venous drainage?
Excessive PEEP can impede venous drainage from the brain and contribute to an increase in intracranial pressure.

60. Why should PEEP be used cautiously in patients with elevated intracranial pressure?
Higher intrathoracic pressure may further increase intracranial pressure and potentially reduce cerebral perfusion.

61. How can PEEP affect renal function?
PEEP can reduce cardiac output and renal blood flow, which may decrease urine output and impair renal perfusion.

62. What change in urine output may suggest cardiovascular compromise from excessive PEEP?
A decrease in urine output may indicate reduced cardiac output and diminished renal perfusion.

63. What pulmonary complication can result from alveolar rupture caused by excessive airway pressure?
Alveolar rupture can cause a pneumothorax.

64. What is a tension pneumothorax in a mechanically ventilated patient?
A tension pneumothorax is progressive accumulation of pressurized air in the pleural space that can compress the lungs and impair cardiovascular function.

65. What other forms of barotrauma may occur with excessive positive pressure?
Barotrauma may include pneumomediastinum, subcutaneous emphysema, pneumopericardium, pneumoperitoneum, and pulmonary interstitial emphysema.

66. What sudden clinical change should raise concern for barotrauma in a patient receiving PEEP?
Sudden respiratory deterioration, hypotension, increased airway pressures, or an abrupt change in breath sounds should raise concern for barotrauma such as pneumothorax.

67. How can a partially obstructed endotracheal tube contribute to auto-PEEP?
A partially obstructed endotracheal tube increases expiratory resistance and can prevent complete exhalation before the next breath begins.

68. How can retained secretions contribute to auto-PEEP?
Retained secretions can obstruct airflow and slow expiration, increasing the likelihood of gas trapping.

69. How can bronchospasm contribute to auto-PEEP?
Bronchospasm narrows the airways and increases expiratory resistance, making it more difficult for the lungs to empty completely.

70. How can a high respiratory rate cause auto-PEEP?
A high respiratory rate shortens the time available for expiration, allowing the next breath to begin before the previous breath has been fully exhaled.

71. Why can increasing inspiratory flow during volume-controlled ventilation reduce auto-PEEP?
Increasing inspiratory flow shortens inspiratory time and leaves more of the respiratory cycle available for expiration.

72. Why can decreasing inspiratory time during pressure-controlled ventilation reduce auto-PEEP?
A shorter inspiratory time increases the time available for exhalation and can reduce gas trapping.

73. What I:E ratios may be required in severe obstructive lung disease to promote adequate exhalation?
Patients with severe obstructive disease may require prolonged expiratory times such as I:E ratios of approximately 1:4 or 1:5.

74. Why is external PEEP generally not added solely to treat auto-PEEP during fully controlled ventilation?
External PEEP is mainly useful for reducing the inspiratory threshold load in patients who are actively triggering the ventilator, so adding it solely because auto-PEEP exists during fully controlled ventilation is generally not indicated.

75. How can external PEEP function as a pneumatic stent in obstructive airway disease?
External PEEP can help support collapsible airways during expiration, potentially reducing dynamic airway collapse and improving expiratory flow.

76. What does total PEEP represent in a mechanically ventilated patient?
Total PEEP represents the combined effect of externally applied PEEP and any intrinsic or auto-PEEP present in the lungs.

77. How is total PEEP related to set PEEP and auto-PEEP?
Total PEEP equals the externally set PEEP plus the amount of intrinsic PEEP caused by incomplete exhalation.

78. What level of intrapulmonary shunting may support the use of therapeutic PEEP?
An intrapulmonary shunt greater than approximately 15% may support the use of therapeutic PEEP.

79. Why may PEEP be considered when a patient requires an FiO₂ greater than 0.50 for an extended period?
PEEP may improve oxygenation enough to allow the FiO₂ to be reduced, limiting prolonged exposure to high oxygen concentrations.

80. In what increments may therapeutic PEEP commonly be increased during titration?
PEEP may commonly be increased in increments of approximately 2 to 5 cm H₂O while the patient’s response is reassessed.

81. What should be done after each increase in PEEP?
The clinician should reassess oxygenation, lung mechanics, airway pressures, blood pressure, and the patient’s overall hemodynamic response.

82. What does an improvement in PaO₂ after increasing PEEP suggest?
An improvement in PaO₂ may indicate better alveolar recruitment and gas exchange, provided cardiovascular function remains stable.

83. Why might pulmonary vascular resistance decrease with appropriately selected PEEP?
Appropriate PEEP can restore lung volume without excessive overdistention, helping maintain a more favorable pulmonary vascular resistance.

84. What can a stable mixed venous oxygen level indicate during PEEP therapy?
A stable mixed venous oxygen level suggests that overall oxygen delivery and tissue oxygen extraction remain acceptable.

85. How does lung compliance influence transmission of PEEP into the pleural space?
More compliant lungs may transmit a greater portion of airway pressure into the pleural space, increasing the potential for cardiovascular effects.

86. Why may patients with very stiff lungs tolerate relatively high PEEP better from a hemodynamic standpoint?
Stiff lungs may transmit less of the applied airway pressure into the pleural space, reducing its effect on venous return.

87. What ventilator equipment problem can contribute to auto-PEEP by increasing expiratory resistance?
A wet or obstructed expiratory filter can increase expiratory resistance and contribute to incomplete lung emptying.

88. How can an expiratory valve malfunction contribute to auto-PEEP?
An expiratory valve malfunction can interfere with normal exhalation and cause pressure and gas to remain trapped in the lungs.

89. What is missed triggering in a patient with auto-PEEP?
Missed triggering occurs when the patient makes an inspiratory effort but cannot overcome intrinsic pressure enough to trigger an assisted ventilator breath.

90. Why can auto-PEEP increase the patient’s work of breathing?
The patient must first overcome the intrinsic positive pressure in the lungs before generating enough additional effort to trigger the ventilator.

91. What does the lower inflection point on a pressure-volume curve represent?
The lower inflection point represents the pressure range where previously collapsed or unstable lung units begin to open.

92. How can the lower inflection point help guide PEEP selection?
PEEP may be set high enough to help keep alveolar pressure above the level at which recruited lung units are likely to collapse.

93. What does the upper inflection region of a pressure-volume curve suggest?
The upper inflection region suggests that further increases in pressure may produce excessive alveolar distention.

94. Why should PEEP be kept below levels associated with excessive overdistention on a pressure-volume curve?
Excessive overdistention can reduce compliance, compress pulmonary capillaries, and increase the risk of lung injury.

95. What is the purpose of PEEP after a lung recruitment maneuver?
PEEP helps keep newly recruited alveoli open after the temporary higher pressures of the recruitment maneuver are removed.

96. How may PEEP benefit a patient with postoperative atelectasis?
PEEP can help reopen and stabilize collapsed alveoli, increase lung volume, and improve postoperative oxygenation.

97. What might a low PEEP alarm indicate on a mechanical ventilator?
A low PEEP alarm may indicate a circuit leak, disconnection, or another problem preventing the ventilator from maintaining the intended baseline pressure.

98. What is the usual relationship between airway pressure and atmospheric pressure at end-expiration without applied PEEP?
Without applied PEEP, airway pressure generally returns toward atmospheric pressure at the end of expiration.

99. What PEEP level is commonly considered when evaluating readiness for a spontaneous breathing trial?
PEEP is commonly reduced to approximately 5 to 8 cm H₂O when evaluating readiness for a spontaneous breathing trial.

100. What is the overall goal when balancing FiO₂ and PEEP during mechanical ventilation?
The goal is to maintain adequate oxygenation with the lowest reasonable FiO₂ and enough PEEP to preserve alveolar recruitment without causing excessive pressure-related complications.

Final Thoughts

Positive end-expiratory pressure is an important tool for maintaining alveolar recruitment, increasing functional residual capacity, reducing intrapulmonary shunting, and improving oxygenation during mechanical ventilation.

Its benefits are greatest when lung units are unstable or prone to collapse, particularly in conditions such as ARDS, atelectasis, and pulmonary edema. However, excessive PEEP can overdistend alveoli, reduce venous return, impair cardiac output, increase pulmonary vascular resistance, contribute to barotrauma, and affect intracranial and renal function.

Safe PEEP management therefore requires individualized titration, careful monitoring, and continual reassessment of both pulmonary and cardiovascular responses.