Capnography

Introduction

• Capnography is the continuous measurement and graphical display of carbon dioxide (CO₂) in exhaled gas over time.¹ It provides a noninvasive estimate of key physiologic processes, including ventilation, pulmonary perfusion, and metabolic activity.²

Why CO₂ Monitoring Matters

• CO₂ is a byproduct of aerobic metabolism, and impaired elimination leads to accumulation, resulting in acidemia and cellular dysfunction.³
• Effective CO₂ elimination requires coordination of:
  • Cellular CO₂ production
  • Transport via the circulation to the lungs
  • Alveolar ventilation for exhalation
• Because these processes are interdependent, changes in exhaled CO₂ can indicate abnormalities in ventilation, perfusion, or metabolism.¹

How Capnography Is Used in Clinical Practice

• Capnography became widely adopted in anesthesia practice in the late 20th century and is now standard monitoring during general anesthesia, with expanding use in emergency and critical care settings.² Clinically, it is used to:
  • Confirm airway device placement and effective ventilation.³
  • Detect hypoventilation, apnea, and airway obstruction.³
  • Assess the adequacy of mechanical ventilation.²
  • Provide indirect information about cardiac output and pulmonary blood flow¹
  • Monitor for return of spontaneous circulation during cardiac arrest.²

Key Terms

• Capnometry: numeric measurement of inhaled or exhaled CO₂.¹
• Capnography: numeric measurement plus waveform display.¹

What Capnography Measures

• Capnography continuously measures the concentration or partial pressure of CO₂ in exhaled gas and displays this information both numerically and graphically as a waveform.¹
  • The numeric value, ETCO₂, represents the maximum CO₂ concentration measured at the end of expiration for each breath.¹
  • The capnogram waveform depicts how CO₂ concentration changes throughout the entire respiratory cycle, providing breath-by-breath information beyond a single numeric value.^1,4
• Because CO₂ is produced by cellular metabolism, transported to the lungs by the circulation, and eliminated through alveolar ventilation, the amount of CO₂ detected in exhaled gas reflects the integration of these key physiologic processes.²
• While ETCO₂ provides a useful quantitative measure, the waveform often conveys additional diagnostic information by revealing patterns associated with airway obstruction, hypoventilation, apnea, rebreathing, or equipment malfunction.⁴ For this reason, interpretation of capnography relies on both the numeric value and waveform morphology.

Methods of CO₂ Detection

• Several methods are used to detect CO₂ in exhaled gas. In clinical anesthesia and critical care practice, infrared (IR) spectroscopy is the primary technology used for continuous, quantitative capnography.^1,2
• A second, simpler method is qualitative colorimetric CO₂ detection, which is used in select clinical settings, particularly for rapid airway confirmation.²

Quantitative Capnography

• Quantitative capnography uses IR spectroscopy to measure the concentration or partial pressure of CO₂ in exhaled gas on a breath-by-breath basis. This method provides both:
  • A numeric ETCO₂ value, and
  • A continuous waveform that reflects changes in CO₂ throughout the respiratory cycle.¹
• Because it allows continuous monitoring and waveform analysis, IR capnography is the standard method used during general anesthesia, mechanical ventilation, and cardiopulmonary resuscitation (CPR).²

Qualitative CO₂ Detection

• Qualitative CO₂ detectors use pH-sensitive indicator paper that changes color in the presence of exhaled CO₂.² These devices do not provide a numeric value or waveform, but instead offer a rapid visual indication of CO₂ detection.

Typical color scheme:²

• Qualitative detectors are most commonly used for:
  • Rapid confirmation of tracheal intubation, particularly in emergency or prehospital settings.
  • Situations where continuous capnography is unavailable.²
• Because they provide only intermittent, non-quantitative information, colorimetric detectors are considered adjuncts rather than substitutes for continuous capnography.

IR Spectroscopy

• IR spectroscopy is the primary technology used in quantitative capnography to measure CO₂ in exhaled gas.^1,2,5
• CO₂ molecules absorb IR light at a characteristic wavelength of approximately 4.26 μm.¹ When IR light passes through a gas sample containing CO₂, a portion of the light is absorbed. The amount of absorbed light is proportional to the CO₂ concentration within the sample.¹
• Capnography systems compare the IR signal transmitted through the sampled gas with a reference signal obtained from gas that does not contain CO₂. Using this difference, the monitor calculates the CO₂ concentration or partial pressure of the exhaled gas.² These measurements occur rapidly enough to provide continuous, breath-by-breath monitoring.²

Why CO₂ Can Be Measured But O₂ Cannot

• CO2 exhibits strong and predictable IR absorption peaks, enabling its reliable detection by IR spectroscopy.¹ In contrast, oxygen and nitrogen do not significantly absorb IR light and therefore cannot be measured using this technique.⁵

Capnography System Configurations

• Capnography systems are classified according to the location of exhaled gas sampling and analysis. The two primary configurations used in clinical practice are mainstream and sidestream capnography.^1,4

Mainstream Capnography

• In mainstream capnography, the IR sensor is placed directly in the airway, typically between the endotracheal tube (ETT) and the ventilator circuit.¹

Advantages

• No transport delay: Exhaled gas is analyzed at the airway interface, eliminating transport delays.¹
• Fast rise time: The sensor responds rapidly upon exposure to exhaled gas, enabling accurate detection of rapid changes, such as apnea or airway obstruction.⁴
• High waveform fidelity: Minimal dilution or mixing improves waveform accuracy.¹
• Delay time refers to the lag caused by gas transport to the analyzer, whereas rise time describes how quickly the sensor responds once the gas reaches it.¹

Limitations

• Added dead space: The sensor occupies volume between the patient and ventilator, increasing apparatus dead space.¹
• Greater impact in small patients: In infants and children, a larger fraction of each breath may remain within dead space, contributing to CO₂ retention or hypoventilation.¹
• Heating requirements: Sensors are typically heated to approximately 40 °C to prevent condensation, which can cause facial or airway burns.¹
• Physical bulk: The sensor’s weight may increase torque on the ETT, particularly in pediatric patients, or make it more cumbersome to use than other methods.¹
• Mainstream systems are therefore best suited for mechanically ventilated adult patients.

Sidestream Capnography

Sidestream capnography continuously aspirates a small sample of airway gas through a narrow tube to a remote IR analyzer.¹

Sampling Interfaces

• ETT adapter in intubated patients.¹
• Nasal or nasal–oral cannula in non-intubated or sedated patients.²

Advantages

• Lightweight at the airway: Minimal added weight or dead space.¹
• Versatility: Can be used in both intubated and non-intubated patients.²
• Flexible setup: Less interference with airway equipment.²

Limitations

• Transport delay: Gas must travel through sampling tubing before analysis, introducing a delay time.¹
• Slower rise time: Both gas transport and sensor response contribute to lag compared with mainstream systems.¹
• Tubing obstruction: Condensation or secretions can partially or completely obstruct the sampling line.⁵
• Ambient air entrainment: Leaks, loose cannula, or open-mouth breathing can dilute the gas sample, resulting in falsely low ETCO₂ values.²

Sampling Flow Rate and Measurement Error^2,5

Sidestream systems vary in sampling flow rate, which can affect accuracy.

Low-Flow Sampling

• Draws a smaller sample volume
• Improves accuracy in patients with low tidal volumes
• Reduces aspiration of inspiratory gas into the sampling line

High-Flow Sampling

• Draws a larger sample volume
• Improves response time and accuracy in patients with larger tidal volumes
• May significantly reduce effective tidal volume in infants and neonates
• If the sidestream sampling flow is high relative to the exhaled flow, an excessive fraction of exhaled gas may be removed from the circuit, and inspiratory or ambient air may be entrained into the sampling line. These effects dilute the sampled gas and can produce artifactually low ETCO₂ values, particularly in pediatric patients and during low-tidal volume ventilation.

Technical Factors Affecting Accuracy

• Several technical and physiologic factors can affect the accuracy of capnography measurements. Understanding these limitations is essential for the appropriate interpretation of ETCO₂ values and waveform morphology.^1,4,5

Dead Space

Capnography measurements are influenced by both anatomic and apparatus dead space. Increased dead space dilutes alveolar gas with CO₂-poor gas, resulting in lower measured ETCO₂ values.¹

• Apparatus dead space is increased by airway adapters, connectors, and mainstream sensors.
• The impact is greatest in patients with small tidal volumes, particularly infants and neonates, in whom dead space constitutes a larger fraction of each breath.¹

Sampling-Related Factors

Sidestream capnography accuracy depends on appropriate sampling flow relative to the patient’s tidal volume.^2,5

• High sampling flow relative to exhaled flow can remove an excessive fraction of exhaled gas and entrain inspiratory or ambient air, diluting the sampled gas and producing artifactually low ETCO₂ values.
• Low sampling flow may improve accuracy in patients with small tidal volumes but can slow response time in patients with larger tidal volumes.

Gas Dilution and Leaks

Dilution of the sampled gas can occur when room air enters the sampling system, leading to falsely low ETCO₂ readings.²

Common causes include:

• Loose or malpositioned nasal cannula
• Open-mouth breathing during nasal sampling
• Leaks in sampling tubing or airway apparatus

Condensation and Obstruction

Moisture and secretions can accumulate within sampling tubing, particularly in sidestream systems, partially or completely obstructing gas flow.⁵

• Partial obstruction may cause waveform distortion or delayed response.
• Complete obstruction can result in loss of the capnogram or falsely low ETCO₂ values.

Heating elements and water traps are used to mitigate these effects, but do not eliminate them entirely.

Time-Based vs. Volume-Based Capnography

• Time-based capnography plots CO₂ concentration or partial pressure against time for each respiratory cycle.¹ This is the standard display used in anesthesia, emergency medicine, and critical care.
  • The waveform reflects the sequential emptying of dead space and alveolar gas during exhalation.
  • The ETCO₂ value is measured at the end of expiration and represents the highest CO₂ concentration in the exhaled breath.¹
  • Time-based capnography allows continuous assessment of ventilation, airway patency, and pulmonary perfusion, and forms the basis for waveform interpretation in routine clinical practice.⁴
• Volume capnography plots CO₂ concentration against exhaled tidal volume rather than time.¹
  • Early exhaled gas reflects anatomic and apparatus dead space and contains little or no CO₂.
  • As exhalation continues, alveolar gas with a higher CO₂ concentration predominates.
  • The shape of the curve and the volume exhaled before a rise in CO₂ provide information about the proportion of tidal volume that does not participate in gas exchange.
• Volume capnography can be used to estimate:
  • Physiologic dead-space fraction (Dead space volume / Tidal volume)
  • Efficiency of gas exchange and CO₂ elimination
  • Changes in pulmonary blood flow, as alterations in perfusion affect the volume of CO₂ delivered to the lungs.¹
• Although volume capnography is not routinely used in standard anesthesia monitoring, it is valuable for advanced physiologic assessment, critical care, and research.¹

Waveform Analysis

Overview

• Capnography provides both a numeric ETCO₂ value and a graphical waveform that displays how CO₂ concentration changes throughout the respiratory cycle. While ETCO₂ represents the maximum expired CO₂ for each breath, the waveform provides additional physiologic information by revealing patterns of ventilation that cannot be appreciated from a single numeric value alone.^1,4
• Because waveform changes often occur before alterations in oxygen saturation or arterial blood gases, capnography allows early detection of ventilatory, airway, circulatory, and equipment-related abnormalities.⁴

Normal Time-Based Capnogram

• The standard time-based capnogram plots CO₂ concentration (or partial pressure) against time for each respiratory cycle.¹ A normal waveform consists of four phases:
  • Phase I: Dead Space Exhalation: This phase represents exhalation of gas from the conducting airways (anatomic dead space), where no gas exchange occurs. As a result, CO₂ concentration is near zero during this portion of the breath.¹
  • Phase II: Transitional Phase: Phase II is characterized by a rapid rise in CO₂ concentration as alveolar gas begins to mix with dead-space gas. The steepness of this upslope reflects the synchrony of alveolar emptying.⁴
  • Phase III: Alveolar Plateau: Phase III represents predominantly alveolar gas. In healthy lungs, this segment is relatively flat because well-ventilated alveoli have similar CO₂ concentrations. The end of Phase III corresponds to the ETCO₂.^1,4
  • Phase 0: Inspiratory Downstroke: With the onset of inspiration, CO₂-free fresh gas enters the airway, and the waveform rapidly returns toward zero. Failure of the waveform to return to baseline suggests rebreathing.⁴

ETCO₂ and Its Physiologic Meaning

• ETCO₂ is measured at the end of Phase III and represents the highest CO₂ concentration in the exhaled breath. In healthy individuals, ETCO₂ is typically 2–5 mmHg lower than arterial CO₂ (PaCO₂).¹
• This normal gradient exists because:
  • Alveolar gas has a slightly lower CO₂ tension than pulmonary capillary blood.¹
  • A small proportion of alveoli are ventilated but relatively under perfused, creating physiologic alveolar dead space.⁴
• When ventilation and perfusion are well matched, ETCO₂ closely approximates PaCO₂.

Abnormal Waveform Patterns

• Changes in waveform morphology often provide earlier or more specific clues to pathology than changes in ETCO₂ alone.⁴
  • Obstructive Lung Disease (“Shark Fin” Pattern): In conditions such as asthma or chronic obstructive pulmonary disease, the Phase II upslope and Phase III plateau become slanted rather than steep and flat. This pattern reflects heterogeneous alveolar emptying, with prolonged mixing of low- and high-CO₂ gas during exhalation.^1,4
  • Hypoventilation: Inadequate ventilation relative to CO₂ production leads to accumulation of CO₂ in the alveoli, resulting in a rising ETCO₂ and an upward shift of the waveform.⁴
  • Sudden Loss of Waveform: An abrupt disappearance of the capnogram or a rapid decrease in ETCO₂ toward zero should be treated as an emergency. Potential causes include circuit disconnection, apnea, esophageal intubation, severe hypotension, cardiac arrest, or massive pulmonary embolism.^1,5
  • Rebreathing: Failure of the waveform to return to zero during inspiration indicates inhalation of CO₂-containing gas. Common causes include expiratory valve malfunction, exhausted CO₂ absorbent, or inspiratory valve failure.⁵

Capnography Outside the Operating Room

Although capnography is standard of care during general anesthesia in the operating room, its adoption has historically been less consistent in other high-risk settings such as the intensive care unit (ICU) and emergency department (ED), where airway-related adverse events are more common.^6,7

The UK National Audit Project ⁴ (NAP4) highlighted that airway complications occurred more frequently in the ICU and ED than in the operating room and were associated with higher rates of permanent harm and death. Lack of capnography was identified as a contributing factor in a substantial proportion of ICU airway-related fatalities.⁶

In response to these findings, international guidelines were revised in 2011 to recommend continuous capnography in:⁶

• The ICU and ED
• Any clinical location where patients receive advanced life support
• Capnography use outside the operating room has increased over time but remains variable. Barriers to universal adoption include equipment cost, limited availability, training requirements, and clinician perception that capnography is unnecessary in certain settings.⁶

In non-operating room environments, capnography is particularly valuable for:⁶

• Patients with tracheal tubes or tracheostomies
• Procedural sedation and noninvasive ventilation
• Early detection of airway obstruction, hypoventilation, and circuit disconnection

Global Health and Equity Considerations

• Substantial disparities exist in access to capnography between high-income countries and low- and middle-income countries (LMICs). Limited availability of anesthesia monitoring equipment contributes to significantly higher perioperative and postoperative mortality in LMICs, particularly among vulnerable populations such as children.⁷
• Esophageal intubation and unrecognized airway compromise occur more frequently outside the operating room and are especially dangerous in resource-limited settings. Capnography remains the most reliable method for confirming tracheal intubation and preventing these adverse events, yet its use in LMICs remains far from universal due to cost, lack of training, and unfamiliarity.⁷
• In response to these challenges, global initiatives have focused on expanding access to affordable, context-appropriate capnography. The Smile Train–Lifebox Capnography Project was developed to address cost and training barriers through collaboration with anesthesia providers working in low-resource environments.⁸
• Key components of this initiative include:
  • Development of low-cost capnography–pulse oximetry devices⁸
  • Emphasis on durability and usability in low-resource settings⁸
  • Structured education and training programs for anesthesia providers⁸
• These efforts reflect a growing consensus that capnography should be considered an essential monitor worldwide rather than a technology limited to high-resource settings.^7,8

Clinical Applications

• Capnography provides continuous, noninvasive information about ventilation, pulmonary perfusion, and airway integrity. Its value lies in the rapid detection of physiologic changes and equipment-related problems through both numeric ETCO₂ values and waveform morphology.^1,4

Airway Management and Anesthesia

• Capnography is standard of care during general anesthesia and a primary method for confirming effective ventilation.^1,5

Clinical uses include:¹

• Confirmation of tracheal intubation by the presence of a sustained CO₂ waveform.
• Early detection of:
  • Circuit disconnection
  • Apnea
  • Airway obstruction
  • Acute hemodynamic collapse

Limitations:

• • The presence of a waveform does not ensure optimal tube position
  • Mainstem or supraglottic placement may still generate CO₂⁵

Procedural Sedation and Nonintubated Patients

• Capnography detects hypoventilation and apnea earlier than pulse oximetry, as oxygen saturation may remain normal despite inadequate ventilation.^2,3

Common applications:

• Procedural sedation
• Postanesthesia care
  Emergency and critical care monitoring

Perfusion and Hemodynamic Assessment

• ETCO₂ reflects pulmonary blood flow in addition to ventilation^1,5

Clinical correlations⁴

• ↓ ETCO₂: reduced cardiac output, shock, pulmonary embolism
• ↑ ETCO₂: improved perfusion, release of vascular clamps or tourniquets, increased metabolic activity.

Cardiac Arrest and Resuscitation

• During CPR, ETCO₂ serves as a surrogate marker of pulmonary blood flow generated by chest compressions.¹

Clinical interpretation:

• Persistently low ETCO₂ indicates inadequate perfusion
• Sudden, sustained rise in ETCO₂ indicates return of spontaneous circulation (ROSC)^1,2

Capnography is recommended for monitoring CPR quality and early detection of ROSC.

Core Clinical Takeaway

• Capnography provides rapid, actionable information across anesthetic, critical care, and resuscitative settings. Accurate interpretation requires integration of ETCO₂ values, waveform morphology, and clinical context, as similar capnographic changes may result from abnormalities in ventilation, perfusion, metabolism, or equipment function.^1,4