Capnography Summary

Normal Values

  • ETCO₂: 35-40 mmHg (4.3-5.5 kPa)
  • Slightly lower than arterial PaCO₂
  • Reflects cardiac output and ventilation

Key Indications

  • CPR quality monitoring
  • Sedation and procedural sedation
  • Endotracheal tube confirmation
  • Ongoing ventilation monitoring

During CPR

  • Confirms airway placement
  • Monitors ventilation rate
  • Assesses compression quality
  • Identifies ROSC
  • Provides prognostic information

Key Patterns

  • Low ETCO₂: Shock, PE, poor CPR
  • Sudden increase: ROSC
  • Absent: No circulation, esophageal intubation
  • Shark-fin: Bronchospasm, obstruction

Waveform Capnography Guide

Comprehensive reference for end-tidal CO₂ monitoring and interpretation

Introduction

The concentration of CO₂ in expired air varies throughout expiration, being maximal at the end, and it is this value, the end-tidal CO₂ (ETCO₂) that is most useful. ETCO₂ reflects cardiac output and pulmonary blood flow, as well as the ventilation minute volume.

Clinical Utility

Waveform capnography enables continuous real time ETCO₂ to be monitored. It works most reliably in patients who have a tracheal tube but can also be used with a supraglottic airway device or bag mask.

Normal Values

A normal ETCO₂ is considered between 35-40 mmHg (4.3-5.5 kPa), however results may be influenced by various physiological factors. Note that ETCO₂ is typically slightly lower than arterial PaCO₂.

Indications for Waveform Capnography

CPR

Monitoring quality of chest compressions, confirming airway placement, and detecting return of spontaneous circulation (ROSC).

Sedation and Procedural Sedation

Monitoring ventilation adequacy during procedures requiring sedation.

Endotracheal Intubation

Confirmation of proper endotracheal tube placement in the trachea.

Ongoing Monitoring of Ventilation

Continuous assessment of ventilation status in critically ill patients.

The Capnography Waveform

Normal Capnogram

A
B
C
D
E
A-B: Baseline
B-C: Expiratory upstroke
C-D: Expiratory plateau
D: End-tidal CO₂
D-E: Inspiration
A-B: Baseline - This is the baseline and indicates the end of inspiration. In effect the concentration of CO₂ is being measured in air (or whatever gas is being delivered) which is virtually 0.
B-C: Expiratory upstroke - This represents the start of expiration, with a rapid rise in the concentration of expired CO₂. The expired gas initially contains no CO₂ as it has come from the anatomical dead space.
C-D: Expiratory plateau - Eventually the alveolar plateau is reached. This represents gas from alveoli taking part in gas exchange. The slight gradual increase during this phase is due to the fact that not all alveoli empty at the same rate.
D: End-tidal CO₂ concentration - At the end of expiration the concentration of CO₂ is maximal, this is the end-tidal CO₂. In healthy patients, this is normally 4.8 kPa (4.3 - 5.5 kPa), slightly lower than if the arterial PaCO₂ was measured at the same point in time.
D-E: Inspiration - As inspiration starts, air containing no CO₂ is mixed with a small amount of residual expired gas in the breathing circuit. This is rapidly diluted until gas containing no CO₂ is being inspired.

Waveform Characteristics

The shape of the waveform that is displayed will depend upon the time scale and the patient's condition. Changes in the waveform can provide valuable diagnostic information about the patient's respiratory and circulatory status.

The Role of Waveform Capnography During CPR

CPR Context

During CPR, end-tidal CO₂ values are low, reflecting low cardiac output generated by chest compressions. Monitoring ETCO₂ provides valuable feedback on the effectiveness of resuscitation efforts.

Airway Placement Confirmation

The presence of an end-tidal CO₂ waveform indicates the tracheal tube is in the airway; correct tube placement also relies on observation and auscultation to ensure both lungs are ventilated.

Ventilation Rate Monitoring

Monitoring ventilation rate during CPR helps avoid hyperventilation, which can decrease coronary perfusion pressure and survival rates.

Chest Compression Quality

Monitoring the quality of chest compressions during CPR - a greater depth of chest compression will increase the ETCO₂ value.

ROSC Identification

An increase in end-tidal CO₂ during CPR may be the first indicator of ROSC and prevent unnecessary administration of adrenaline in a patient with ROSC; this is as a result of the improved circulation transporting accumulated CO₂ from the tissues to the lungs.

Prognostication

Low end-tidal CO₂ values during CPR have been associated with lower ROSC rates and increased mortality, and high values with better ROSC and survival; failure to achieve an end-tidal CO₂ value > 1.33 kPa after 20 minutes of CPR is associated with a poor outcome in observational studies.

Interpreting Waveform Capnography

Normal Capnograph

A normal capnograph is present when the patient is spontaneously breathing or adequately ventilated, has normal cardiac output, and has normal metabolic function.

Waveform Patterns and Their Causes

Low ETCO₂ Levels

  • Shock
  • Pulmonary embolism
  • Ineffective CPR being performed during cardiac arrest

Sudden Significant Increase in ETCO₂

  • Return of spontaneous circulation (ROSC)

Absent ETCO₂ Levels and Waveform

  • No metabolic activity
  • No CPR in cardiac arrest
  • Exsanguination/profound shock
  • Endotracheal tube in the esophagus

Increasing ETCO₂ Values Away From Normal

  • Respiratory depression/failure
  • Inadequate respiratory rate and/or tidal volume
  • Increased CO₂ production through increased metabolic rate or temperature or reperfusion of ischemic tissue

Decreasing ETCO₂ Values Away From Normal

  • Hyperventilation
  • Diminished CO₂ production through decreased metabolic rate
  • Falling cardiac output

Increasing ETCO₂ Values Toward Normal

  • Restoration of normal respiratory rate and/or tidal volume
  • Cardiac output improved
  • Improved integrity of airway seal

Decreasing ETCO₂ Values Toward Normal

  • Restoration of normal metabolism/CO₂ production
  • Normalized respiratory rate and/or tidal volume

'Shark-fin' Appearance

  • Obstruction in the expiratory limb of the breathing circuit
  • Presence of a foreign body in the upper airway
  • Partially kinked or occluded artificial airway
  • Bronchospasm (e.g., asthma, COPD)

Elevation of Baseline CO₂

  • Insufficient expiratory time
  • Faulty expiratory valve on ventilator
  • Inadequate inspiratory flow
  • Rebreathing of CO₂