Respiratory Foundations, Anatomy, Ventilation, and Positive-Pressure Concepts

Understanding Airway Anatomy and Physiology

Airway Structure and Function

Maintaining Acid-Base Stability

For the body to function normally, its internal physical and chemical conditions must stay within certain limits. The body uses several biological processes to detect and correct changes that move these conditions outside the normal range. This ability to maintain a stable internal environment is called homeostasis.

One important part of homeostasis is keeping the body’s acid-base levels balanced.

Acids, which have a pH below 7, are naturally produced during many normal processes in the body. Because these acids are continuously formed, the body must constantly regulate them to keep the pH of the extracellular fluid (ECF) within the normal range of 7.35 to 7.45, which is slightly alkaline.

The body regulates and maintains this balance through three primary systems:

  1. Chemical Buffer Systems
  2. Respiratory Regulation
  3. Kidney Regulation

The normal pH range of the human body is 7.35 to 7.45.

Helpful Notes:
Hydrogen ions (H+) have an inverse relationship with pH. When the amount of H+ in the extracellular fluid (ECF) increases, the pH decreases, causing the ECF to become more acidic.
Carbon dioxide (CO2) has a similar inverse relationship with pH. When CO2 levels increase, pH decreases and the body becomes more acidic.

How the Body Maintains Acid-Base Balance

Chemical Buffer System

The buffer system serves as the body’s first and fastest defense against an increase in acidity.

Respiratory Regulation

The respiratory system acts as the second defense against increasing acidity.

Kidney Regulation

The renal system provides the body’s final major defense against excessive acidity.

When Acid-Base Regulation Fails

When the body’s regulating systems are disrupted or are unable to compensate for excessive acid production, different forms of acidosis may develop.

Acidosis Caused by Respiratory Dysfunction

Respiratory acidosis occurs when the respiratory system cannot eliminate enough CO2, causing CO2 to accumulate and the body’s pH to decrease.

Metabolic Acidosis

Metabolic acidosis develops when the amount of bicarbonate in the body becomes too low, causing the pH to fall into an acidic range. It most often occurs when the body produces excessive amounts of metabolic acids. It can also result when bicarbonate is lost from the body or when normal bicarbonate production is impaired.

Important Reminder:
An acidotic condition affects the body’s extracellular fluid (ECF) as a whole, rather than occurring only in one organ system.

The terms respiratory acidosis and metabolic acidosis identify the primary cause of the imbalance. They do not describe where the acidic condition is located in the body.

When the Body Becomes Too Alkaline

Because normal metabolic processes continuously produce acids, much of the body’s acid-base regulation is focused on preventing excessive acidity. However, certain conditions can cause the body to become excessively alkaline, which can also disrupt normal homeostasis.

Respiratory Alkalosis

Respiratory alkalosis occurs when the lungs eliminate too much carbon dioxide (CO2). Excessive CO2 removal causes the body’s pH to rise.

Metabolic Alkalosis

Metabolic alkalosis occurs when there is an excessive amount of bicarbonate circulating in the body or when too much acid is lost from the extracellular fluid.

Clinical Reminder:
Although alkalotic disorders may occur less frequently and may sometimes appear less severe, they should still be taken seriously.
Both acidosis and alkalosis can result from significant underlying medical problems that may not be immediately obvious in the prehospital setting. Their signs and symptoms can also overlap, which may make it difficult for EMS providers to determine the exact acid-base disturbance in the field.

Basic Life Support Management

Key Ventilation Reminder:
In conditions such as diabetic ketoacidosis (DKA), rapid and deep breathing may be the body’s attempt to compensate for metabolic acidosis.

Reducing or interrupting this compensatory hyperventilation without providing adequate ventilation can cause additional carbon dioxide to accumulate, further lowering the patient’s pH and potentially worsening the patient’s condition.

Clinical Case

Dispatch Details

A BLS crew is dispatched for a Code 1 sick person.

Scene Assessment

The scene is determined to be safe.

Primary Complaint

The patient reports that she feels unwell. She complains of a headache and abdominal discomfort and states that she has been unable to keep down food or water since the symptoms began earlier that morning.

Initial Patient Evaluation

Vital Signs

Based on these findings, you request ALS assistance.

Further History and Assessment

Transfer of Care

An ALS ambulance arrives at the scene, and you provide the paramedic with a complete report of your assessment findings.

The ALS crew assumes care of the patient. You assist with packaging and preparing her for transport to the nearest appropriate hospital.

Case Outcome

This case was based on an actual patient experiencing diabetic ketoacidosis (DKA).

Although she had been managing her type 1 diabetes for several years, she had not consistently monitored her condition over the weekend. After arriving at the hospital, her blood glucose was found to be well above 600 mg/dL.

Many field glucose meters cannot provide an exact measurement once glucose exceeds their maximum range. Instead, they display a reading such as HI or HIGH.

The patient’s condition remained relatively stable during transport, but she required transfer to a pediatric intensive care unit (PICU) for continued treatment and close monitoring.

According to her medical record, her extracellular fluid pH was approximately 6.8, which is far below the normal range of 7.35 to 7.45.

This represented severe metabolic acidosis, a potentially life-threatening condition if it is not promptly corrected. Her deep, rapid respirations were part of the body’s attempt to compensate for the severe metabolic disturbance. Her young age may also have contributed to her ability to compensate for the condition at that time.

Airway Anatomy and Function: Upper and Lower Respiratory Passages

This study guide reviews the anatomy and function of the upper and lower airways. Understanding these structures and how they work can help EMS providers recognize, assess, and manage medical emergencies involving the respiratory system.

Functions of the Upper Airway

Major Structures of the Upper Airway

Key Reminder: The larynx marks the transition between the upper airway and lower airway.

Important Airway Structures and Terms

How Air Travels Through the Respiratory System

How Gas Exchange Occurs

Gas exchange takes place through a process called diffusion, in which molecules move from an area of higher concentration to an area of lower concentration.

For example:

Lower Airway Anatomy and Respiratory Function

Major Structures of the Lower Respiratory Tract

Important Respiratory Physiology Terms

Key Concept:
Ventilation describes the physical movement of air into and out of the lungs.
Respiration refers to the exchange of oxygen and carbon dioxide between the alveoli and the bloodstream.

Although these processes work closely together, they describe two different functions of the respiratory system.

How the Respiratory System Supports the Body

How Breathing Works: Ventilation, Gas Exchange, and Airway Anatomy

Overview

Every breath involves an exchange of gases between the body and the surrounding environment. During inhalation, oxygen enters the lungs and moves into the bloodstream. During exhalation, carbon dioxide moves from the blood into the lungs so it can be removed from the body.

This study guide explains the anatomy and physiology of breathing, including how ventilation and gas exchange occur. Understanding these processes is important because any interruption in normal breathing can quickly lead to serious problems for a patient.

Ventilation and Respiration

The Mechanics of Ventilation

Gas Exchange During Respiration
Structure of the Upper and Lower Respiratory Airways

Upper Respiratory

Lower Respiratory Airway

Identifying Breathing Problems and Respiratory Complications

Abnormal Respiratory Rates

Additional Indicators of Respiratory Difficulty

Bronchospasm

Pulmonary Edema

Clinical Case: Acute Asthma Exacerbation

Dispatch Details

Scene Assessment

Patient Presentation

Patient Management

Transport and Expected Reassessment Findings

Airway Assessment and Management

Noninvasive Positive Pressure Ventilation: CPAP and BiPAP

Continuous Positive Airway Pressure (CPAP)

What Is CPAP?

Continuous positive airway pressure (CPAP) is a form of noninvasive positive pressure ventilation (NIPPV) used to support patients who are experiencing respiratory distress but are still breathing on their own.

CPAP can improve both oxygenation and ventilation by maintaining positive pressure within the airways throughout the entire respiratory cycle.

When applied early and appropriately, CPAP may help prevent further respiratory deterioration and reduce the need for endotracheal intubation and mechanical ventilation.

CPAP commonly begins at approximately 5 cm H₂O and may be gradually increased according to the patient’s condition, response, and local treatment protocol.

How CPAP Works

CPAP delivers a constant level of positive airway pressure during both inspiration and expiration.

This continuous pressure helps:

CPAP also increases intrathoracic pressure.

This increase can reduce venous return to the heart, which decreases preload.

Reducing preload can be especially beneficial in patients experiencing cardiogenic pulmonary edema, such as pulmonary edema associated with congestive heart failure (CHF).

Common Indications for CPAP

CPAP may be considered when the patient:

Age requirements and specific treatment thresholds may vary depending on the EMS system and local protocol.

Contraindications to CPAP

CPAP may be inappropriate or unsafe in patients with:

Patients receiving CPAP should be closely monitored because positive intrathoracic pressure can contribute to a decrease in blood pressure.

Bi-Level Positive Airway Pressure (BiPAP)

What Is BiPAP?

Bi-level positive airway pressure (BiPAP) is another form of noninvasive positive pressure ventilation.

Unlike CPAP, which delivers one continuous pressure, BiPAP provides two different pressure levels:

IPAP delivers a higher level of pressure during inhalation, while EPAP provides a lower level of pressure during exhalation.

A commonly used starting setting may be approximately:

These settings may then be adjusted based on the patient’s respiratory status, response to therapy, and local protocol.

How BiPAP Works

During inspiration, IPAP assists the patient in drawing air into the lungs.

This support:

During expiration, EPAP maintains positive pressure in the airways but at a lower level than IPAP.

EPAP helps:

The difference between IPAP and EPAP is often called pressure support.

A larger difference between these pressures generally provides greater ventilatory assistance and can increase the patient’s tidal volume.

Because BiPAP can improve tidal volume and minute ventilation, it may also help remove more carbon dioxide (CO₂).

This increased ability to support ventilation and reduce CO₂ retention is one of the major differences between BiPAP and CPAP.

Common Indications for BiPAP

The general requirements for BiPAP are similar to those for CPAP.

The patient should generally:

BiPAP is particularly useful when a patient requires assistance with both oxygenation and ventilation, including situations in which inadequate ventilation may be causing CO₂ retention.

Specific indications, contraindications, and initial pressure settings should always follow local EMS or medical-control protocols.

Additional Indications for BiPAP

BiPAP may be considered when the patient:

Specific age limits, oxygen saturation thresholds, and respiratory-rate criteria may vary depending on the EMS system and local medical protocols.

Contraindications to BiPAP

The contraindications to BiPAP are generally similar to those for CPAP.

BiPAP may be inappropriate in patients with:

Special airway situations, including patients with a tracheostomy, should be managed according to the patient’s airway anatomy, available equipment, and local treatment protocols.

Clinical Benefits of CPAP and BiPAP

Both CPAP and BiPAP can improve oxygenation and reduce the work of breathing in patients who are progressing toward respiratory failure.

These forms of noninvasive positive pressure ventilation (NIPPV) may be beneficial in respiratory distress caused by conditions such as:

CPAP primarily improves oxygenation and alveolar recruitment by maintaining continuous positive airway pressure.

BiPAP provides additional ventilatory assistance by using separate inspiratory and expiratory pressures, which can improve tidal volume and carbon dioxide removal.

When used early and appropriately, noninvasive ventilation may:

Patients receiving CPAP or BiPAP should be reassessed frequently to determine whether their respiratory status is improving or whether more advanced airway management is required.

Respiratory Anatomy, Ventilation, and Common Pathophysiology

Anatomy and Physiology Review

The respiratory system is divided into the upper airway and lower airway.

Upper Airway

The upper airway includes:

These structures help conduct air from the environment toward the lower respiratory tract. They also assist with filtering, warming, and humidifying inspired air.

Lower Airway

The lower airway includes:

The trachea and branching airways transport air deeper into the lungs, while the alveoli serve as the primary location for gas exchange.

Respiratory Physiology

The primary function of the respiratory system is gas exchange.

During inspiration, oxygen-containing air enters the lungs and reaches the alveoli. Oxygen then diffuses across the alveolar-capillary membrane and enters the bloodstream.

At the same time, carbon dioxide (CO₂) moves from the blood into the alveoli.

During expiration, carbon dioxide is removed from the lungs and released into the environment.

Effective gas exchange depends on adequate:

Ventilation

Ventilation is the mechanical movement of air into and out of the lungs.

There are two primary phases of ventilation:

Normal spontaneous breathing functions as a negative-pressure system.

Inspiration

During inspiration, the diaphragm contracts and moves downward.

This increases the size of the thoracic cavity and creates negative pressure inside the chest.

Because pressure within the lungs becomes lower than atmospheric pressure, air is drawn into the lungs.

The decrease in intrathoracic pressure also lowers right atrial pressure and helps increase venous return to the heart.

Expiration

During normal expiration, the diaphragm relaxes and the lungs recoil.

Pressure within the thoracic cavity rises toward normal, causing air to move out of the lungs.

At rest, expiration is generally a passive process.

Tidal Volume

Tidal volume is the amount of air moved into or out of the lungs during a single normal respiratory cycle.

When providing mechanical or assisted ventilation, an appropriate tidal volume is commonly based on the patient’s ideal body weight rather than actual body weight.

A commonly used target is approximately:

6–8 mL/kg of ideal body weight

For an average adult, this may be around 500 mL, although the appropriate amount depends on patient size, condition, and treatment strategy.

Positive End-Expiratory Pressure

Positive end-expiratory pressure (PEEP) is positive pressure maintained within the lungs at the end of expiration.

Its primary purpose is to improve oxygenation.

PEEP helps:

As PEEP increases, greater positive pressure is maintained within the lungs. However, excessive PEEP can negatively affect blood pressure and venous return, so patients must be monitored carefully.

Respiratory Pathophysiology

Obstructive Lung Disease

Two important examples of obstructive respiratory disease are:

These conditions interfere with the patient’s ability to move air effectively through narrowed airways.

Asthma

Asthma commonly involves:

Bronchospasm causes the smooth muscle surrounding the bronchioles to tighten, narrowing the airway.

Inflammation and mucus further restrict airflow.

As a result, air may become trapped distal to the obstruction, particularly during expiration.

COPD

COPD produces chronic airflow limitation caused by a combination of airway inflammation, mucus production, structural airway changes, and, in some patients, destruction of the alveolar walls and loss of elastic recoil.

Because expiration becomes difficult, patients may develop air trapping.

Some patients with obstructive lung disease use pursed-lip breathing.

Exhaling through partially closed lips creates a small amount of positive pressure within the airways. This can help keep smaller airways open longer during expiration and reduce air trapping.

Risk Factors for COPD

Common risk factors include:

Millions of people in the United States live with asthma and COPD, including individuals whose COPD has not yet been formally diagnosed.

Both diseases can cause significant illness, repeated respiratory emergencies, and progressive impairment of lung function.

Congestive Heart Failure and Pulmonary Edema

A patient with advanced congestive heart failure (CHF) may develop significant shortness of breath, especially when the left side of the heart is unable to pump effectively.

Left-Sided Heart Failure

When the left ventricle cannot adequately pump blood forward, pressure begins to increase behind it.

This increased pressure affects:

  1. The left ventricle.
  2. The left atrium.
  3. The pulmonary veins.
  4. The pulmonary capillaries.

As pulmonary capillary pressure increases, fluid can be forced out of the vascular space and into the lung tissue.

If the condition progresses, fluid enters the alveoli and produces cardiogenic pulmonary edema.

Fluid-filled alveoli significantly interfere with oxygen diffusion and can lead to severe respiratory distress and hypoxia.

Common Causes and Risk Factors

Conditions associated with heart failure include:

Left-sided heart failure is also one of the most common causes of right-sided heart failure.

When left-sided pressures remain elevated, pressure can eventually increase within the pulmonary circulation and place additional strain on the right side of the heart.

Non-Cardiogenic Pulmonary Edema and ARDS

Acute respiratory distress syndrome (ARDS) is a severe and potentially life-threatening form of respiratory failure characterized by widespread inflammation and impaired gas exchange.

Unlike cardiogenic pulmonary edema, the fluid accumulation associated with ARDS is not primarily caused by left-sided heart failure.

Instead, inflammation damages the alveolar-capillary membrane and increases its permeability.

How ARDS Develops

Damage to the pulmonary capillary and alveolar membranes allows fluid and proteins to leak from the bloodstream into the interstitial spaces of the lungs.

As the condition progresses:

This process can dramatically reduce oxygenation even when supplemental oxygen is being administered.

Causes of Non-Cardiogenic Pulmonary Edema and ARDS

ARDS and non-cardiogenic pulmonary edema may occur after several serious illnesses or injuries, including:

Because these conditions damage the lungs at the alveolar and capillary level, patients can rapidly progress to severe respiratory distress and respiratory failure.

Recognizing and Managing Common Respiratory Emergencies

Recognition of Asthma

Patients experiencing an asthma exacerbation may present with varying degrees of respiratory distress. Common findings include:

As asthma becomes more severe, wheezing may actually become quieter if very little air is moving through the lungs. A patient with worsening respiratory distress and markedly diminished breath sounds should therefore be treated as potentially critical.

Recognition of Chronic Obstructive Pulmonary Disease

Chronic obstructive pulmonary disease (COPD) primarily includes chronic bronchitis and emphysema.

Patients may present with signs and symptoms that vary depending on the severity of their disease and whether emphysema or chronic bronchitis is more prominent.

Common findings may include:

Findings Commonly Associated With Emphysema

Patients with emphysema may demonstrate:

The traditional term “pink puffer” has historically been used to describe this presentation, although this terminology is considered outdated and should not replace an actual clinical assessment.

Findings Commonly Associated With Chronic Bronchitis

Patients with chronic bronchitis may demonstrate:

The historical term “blue bloater” has sometimes been used to describe this presentation, but it is outdated and should not be used as a substitute for specific clinical findings.

Finger clubbing may be seen with certain chronic pulmonary diseases, although it is not a defining feature of uncomplicated COPD and should prompt consideration of other underlying conditions.

Recognition of Cardiogenic Pulmonary Edema

Cardiogenic pulmonary edema commonly develops as a complication of congestive heart failure, particularly left-sided heart failure.

The patient may present with:

Signs of systemic fluid overload or right-sided heart failure may also be present, including:

JVD, ascites, and significant peripheral edema are particularly associated with right-sided heart failure or advanced congestive heart failure.

Recognition of Non-Cardiogenic Pulmonary Edema and ARDS

Non-cardiogenic pulmonary edema and acute respiratory distress syndrome (ARDS) may produce severe impairment in oxygen exchange.

The patient’s overall presentation often depends heavily on the underlying cause.

For example, a patient who develops ARDS because of sepsis may also display signs and symptoms related to the underlying infection.

Possible findings include:

As ARDS progresses, hypoxemia may become severe and increasingly difficult to correct with supplemental oxygen alone.

Treatment and Management of Asthma and COPD

Immediate Priorities

Begin with appropriate scene and provider safety.

Initial Management

Assess the patient’s:

Provide supplemental oxygen when the patient is hypoxemic or demonstrates significant respiratory distress. Oxygen therapy should be adjusted according to the patient’s clinical condition, oxygen saturation, and local EMS protocol.

Do not withhold oxygen from a critically hypoxemic COPD patient solely because of concern for carbon dioxide retention.

Ongoing Treatment Goals

For asthma or COPD exacerbations:

If the patient’s condition continues to deteriorate:

Transport Considerations

Treatment and Management of Pulmonary Edema and ARDS

Immediate Priorities

Maintain scene and provider safety.

Initial Management

Assess:

Provide supplemental oxygen when the patient is hypoxemic or in significant respiratory distress.

Positioning the conscious patient upright may help reduce respiratory discomfort when clinically appropriate and tolerated.

Ongoing Treatment Goals

For cardiogenic or non-cardiogenic pulmonary edema:

Additional treatment for cardiogenic pulmonary edema may be required according to local EMS protocols and the patient’s blood pressure and overall presentation.

Transport Considerations

Scenario: Acute Respiratory Distress With COPD and Heart Failure

Dispatch Information

It is approximately 2200 hours on a clear night, and the outside temperature is around 100°F.

Medic 10 is dispatched to a local residence for a patient experiencing difficulty breathing. Dispatch reports that the patient feels short of breath and has run out of his inhaler.

An ALS intercept is approximately 10 minutes away, while the nearest hospital is about 30 minutes away.

Scene Assessment

Upon arrival, you enter a residence that has a noticeable odor of cigarette smoke.

An albuterol inhaler is visible on the counter.

You find a 68-year-old male who appears obese and is sitting upright in a tripod position. His breathing is visibly labored, and he is wearing a nasal cannula connected to supplemental oxygen.

Patient Assessment

The patient is only able to speak in 2–3-word sentences because of his shortness of breath.

He reports that his breathing difficulty has been worsening for approximately 24 hours.

He also states that he had difficulty sleeping the previous night because of his breathing.

Over the past several weeks, he has noticed progressive weight gain.

His medical history includes:

The patient reports smoking approximately two packs of cigarettes per day.

He attempted to use his prescribed inhaler but discovered that it was empty.

Initial Vital Signs

Lung auscultation reveals:

The patient is clearly experiencing significant respiratory distress and hypoxemia.

Treatment and Reassessment

Because the patient has a history of COPD and is wheezing, you administer a nebulized albuterol treatment.

Following the treatment, you reassess the patient and notice no significant improvement in either his respiratory rate or oxygen saturation.

Because his respiratory distress and hypoxemia continue, you initiate CPAP at 5 cm H₂O.

Following CPAP application, the patient’s condition begins to improve.

Repeat findings include:

The patient is transferred onto the stretcher, and you begin emergent transport to the nearest appropriate hospital.

Transport and Continued Management

During transport, the patient’s work of breathing begins to increase again.

His oxygen saturation decreases to 88%.

Because the patient remains conscious, is breathing spontaneously, and is tolerating CPAP, you increase the CPAP pressure from 5 cm H₂O to 7.5 cm H₂O.

After increasing the pressure, the patient’s respiratory status improves again.

You observe:

The patient remains on noninvasive positive-pressure ventilation for the remainder of transport.

Upon arrival at the hospital, care is transferred to the emergency department, where the patient is transitioned from CPAP to BiPAP for continued respiratory support.

Key Clinical Findings

This patient’s presentation could initially suggest either a COPD exacerbation or an acute exacerbation of congestive heart failure.

His wheezing may be related to COPD, but wheezing can also occur in severe cardiogenic pulmonary edema, sometimes referred to as cardiac asthma.

Several findings strongly suggest that acute heart failure with pulmonary edema is contributing significantly to his respiratory distress:

His history of COPD and active cigarette use may mean that both obstructive lung disease and heart failure are contributing to his symptoms.

Important Treatment Principles

For a patient experiencing cardiogenic pulmonary edema, CPAP or BiPAP can be highly effective when the patient meets indications and has no contraindications.

Positive airway pressure can:

Supplemental oxygen alone may not fully correct severe hypoxemia when fluid within or around the alveoli is interfering with gas exchange.

For patients experiencing COPD or asthma exacerbations, bronchodilator therapy and supplemental oxygen may be appropriate first-line interventions.

If respiratory distress continues despite initial therapy and the patient begins progressing toward respiratory failure, CPAP or BiPAP may be considered, depending on the patient’s condition and local protocol.

Heart Failure Reminder

The location of fluid accumulation can help distinguish the effects of right-sided and left-sided heart failure.

Recognizing these patterns can help identify the likely cause of a patient’s respiratory distress and guide appropriate prehospital treatment.

Positive End-Expiratory Pressure in Assisted Ventilation

Introduction

This study guide reviews positive end-expiratory pressure (PEEP) and the anatomy and physiology needed to understand when PEEP may be helpful or potentially harmful.

PEEP is a method of maintaining positive pressure within the lungs at the end of expiration during positive-pressure ventilation.

In the prehospital setting, a PEEP valve can be attached to a bag-valve mask (BVM) or other ventilation device to prevent airway pressure from returning completely to zero between breaths.

PEEP shares some physiological principles with continuous positive airway pressure (CPAP). However, CPAP is generally used in patients who are still breathing spontaneously, while PEEP is commonly added when positive-pressure ventilations are being provided.

The primary purpose of PEEP is to improve oxygenation and alveolar recruitment, although increasing intrathoracic pressure can also produce important cardiovascular effects.

Review of Normal Pulmonary Ventilation

Normal breathing consists of two basic phases:

Inspiration

During spontaneous inspiration, the diaphragm contracts and moves downward while the intercostal muscles assist in expanding the chest.

This increases the volume of the thoracic cavity and decreases intrathoracic pressure.

Because pressure inside the lungs becomes lower than atmospheric pressure, air moves into the lungs.

Inspired air travels through the airways until it reaches the alveoli, where gas exchange occurs.

As the lungs expand during inspiration, the alveoli also expand, increasing the surface area available for the exchange of oxygen and carbon dioxide.

Expiration

Pulmonary Surfactant and Alveolar Stability

The inner surface of the alveoli contains a substance called pulmonary surfactant.

Surfactant is composed primarily of lipids and proteins.

Its major role is to reduce surface tension inside the alveoli.

By lowering surface tension, surfactant helps:

During expiration, alveoli naturally become smaller as the lungs recoil.

Surfactant helps prevent these alveoli from collapsing completely before the next breath.

Certain pulmonary conditions, including acute respiratory distress syndrome (ARDS) and severe pneumonia, can damage the alveolar-capillary membrane and interfere with normal surfactant function.

When alveoli collapse, the condition is known as atelectasis.

Atelectasis reduces the amount of functioning lung tissue available for gas exchange and can contribute to hypoxemia.

Purpose of PEEP

The main purpose of PEEP is to maintain positive pressure in the airways at the end of expiration.

This pressure helps keep alveoli open between ventilations.

PEEP can:

A PEEP valve contains a pressure-regulating mechanism that maintains a selected amount of positive pressure during expiration.

PEEP levels are measured in centimeters of water pressure (cm H₂O).

A common initial setting may be approximately:

5 cm H₂O

Depending on the patient’s response, equipment, and local protocol, the pressure may be increased in steps.

Some systems permit PEEP levels up to approximately 15–20 cm H₂O, although higher pressures require careful monitoring because of their potential effects on circulation and lung tissue.

Recognizing Patients Who May Benefit From PEEP

Alveolar collapse can occur for many different reasons.

Some causes involve an obstruction within the airway, while others involve disease or pressure affecting the lung tissue itself.

Obstructive Causes

Examples of airway obstruction include:

When an airway is physically blocked, pressure alone may not correct the underlying obstruction.

The cause of the blockage must be addressed whenever possible.

Non-Obstructive Causes of Alveolar Collapse

Alveoli may also collapse because of conditions such as:

In these situations, positive end-expiratory pressure may help maintain or recruit alveoli when appropriate.

Some conditions associated with lung collapse, such as pneumothorax, require special caution because positive-pressure ventilation can worsen an untreated pneumothorax.

PEEP in the Prehospital Setting

PEEP is generally considered for higher-acuity patients who are unable to maintain adequate ventilation or oxygenation on their own and are receiving positive-pressure ventilation.

A patient who is still breathing spontaneously but requires positive airway pressure may instead be a candidate for CPAP or BiPAP, depending on the clinical situation and local protocols.

A common indication for adding PEEP to BVM ventilation is:

Persistent hypoxemia despite adequate positive-pressure ventilation with high-concentration oxygen.

For example, a patient may be receiving effective BVM ventilations with supplemental oxygen but continue to have an inadequate SpO₂.

If alveolar collapse or poor alveolar recruitment is contributing to the hypoxemia, adding PEEP may improve oxygenation.

Clinical Goals of PEEP

PEEP should primarily be considered when the patient requires improved oxygenation and has sufficient circulation to tolerate the increase in intrathoracic pressure.

Treatment goals include:

The patient’s response should be reassessed after applying or increasing PEEP.

Monitor:

Cardiovascular Effects of PEEP

Although PEEP can improve oxygenation, it also increases intrathoracic pressure.

Increased pressure inside the chest can decrease the amount of venous blood returning to the heart.

This may reduce:

For this reason, PEEP must be used cautiously in patients who are already experiencing poor perfusion.

When PEEP May Be Contraindicated or Require Caution

Protocols vary, but PEEP may be contraindicated or require careful consideration in patients with:

Some EMS protocols also limit or modify the use of PEEP during cardiac arrest.

Because PEEP raises intrathoracic pressure, excessive levels can reduce venous return to the heart and potentially decrease cardiac output or blood flow during states of severe circulatory compromise.

The decision to use PEEP should therefore balance its potential benefit to oxygenation against its possible effects on hemodynamics.

Key Concept

PEEP is primarily an oxygenation tool.

It works by maintaining pressure inside the lungs after expiration, helping alveoli remain open and available for gas exchange.

When properly applied, PEEP can significantly improve oxygenation in patients with alveolar collapse or poor lung recruitment.

However, because increased intrathoracic pressure can reduce venous return and blood pressure, the patient’s cardiovascular status must be monitored closely throughout treatment.