Capnography
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capnography waveform end tidal carbon dioxide clinical monitoring guideline
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https://www.ncbi.nlm.nih.gov/books/NBK482169
Capnography
Capnography is continuous measurement and graphical display of carbon dioxide (CO₂) in exhaled gas over time. The key displayed value is end-tidal CO₂ (ETCO₂ or PetCO₂), the CO₂ concentration at the end of exhalation.
It provides real-time information about:
- Ventilation: whether CO₂ is being eliminated
- Airway patency: obstruction, apnea, disconnection
- Pulmonary perfusion: especially during CPR or shock
- Metabolism: increased CO₂ production can raise ETCO₂
It does not directly measure oxygenation. Use pulse oximetry alongside it.
Normal values
- Normal ETCO₂: approximately 35-45 mmHg
- Normally, ETCO₂ is slightly lower than arterial PaCO₂, commonly by about 2-5 mmHg in healthy lungs.
- A widening PaCO₂-ETCO₂ gap can occur with increased dead space, such as pulmonary embolism, severe hypotension, or significant lung disease.
Normal capnogram waveform
A normal waveform has four phases:
-
Phase I, inspiratory baseline
Exhaled dead-space gas contains essentially no CO₂, so the tracing is near zero. -
Phase II, expiratory upstroke
Dead-space gas mixes with alveolar gas, producing a rapid rise in CO₂. -
Phase III, alveolar plateau
Mostly alveolar gas is exhaled. ETCO₂ is measured at the end of this phase. -
Phase 0, inspiratory downstroke
Inhalation of fresh gas causes CO₂ to return rapidly to baseline.
Common waveform patterns
| Pattern | Likely interpretation |
|---|---|
| ETCO₂ progressively rises | Hypoventilation, respiratory depression, rebreathing |
| ETCO₂ falls | Hyperventilation, falling cardiac output, reduced perfusion, leak or disconnection |
| Flat line / sudden loss of waveform | Apnea, esophageal intubation, tube/circuit disconnection, complete obstruction, cardiac arrest |
| “Shark-fin” upsloping plateau | Bronchospasm or expiratory airflow obstruction, such as asthma or COPD |
| Baseline fails to return to zero | Rebreathing CO₂, exhausted CO₂ absorber, faulty expiratory valve, inadequate fresh gas flow |
| Small, irregular waves | Partial obstruction, shallow breathing, leak, sampling problem |
Major clinical uses
1. Confirmation of endotracheal tube placement
Persistent waveform capnography after intubation is the most reliable bedside confirmation that the tube is in the trachea when there is pulmonary blood flow. A sudden loss of a previously present waveform requires immediate assessment for extubation, disconnection, obstruction, or circulatory collapse.
2. Monitoring ventilation during sedation and anesthesia
Capnography detects hypoventilation and apnea earlier than pulse oximetry, particularly when supplemental oxygen is used. Oxygen saturation may remain normal for several minutes despite absent ventilation, whereas the capnogram changes immediately. Roberts and Hedges’ Clinical Procedures in Emergency, p. 711.
3. CPR
During effective chest compressions, ETCO₂ reflects pulmonary blood flow and indirectly cardiac output.
- Persistently low ETCO₂ can suggest poor-quality compressions, inadequate perfusion, or prolonged arrest.
- An abrupt sustained rise in ETCO₂ may indicate return of spontaneous circulation (ROSC).
Interpret the number in the full clinical context, rather than using it alone to decide prognosis.
4. Mechanical ventilation
ETCO₂ supports adjustment and monitoring of ventilation. In general:
- High ETCO₂: lower minute ventilation, increased CO₂ production, or rebreathing
- Low ETCO₂: excessive minute ventilation, reduced cardiac output, increased dead space, or leak
5. Airway disease
The characteristic shark-fin capnogram indicates prolonged, obstructed expiration, often seen in bronchospasm.
Measurement methods
- Mainstream capnography: sensor is placed directly in the airway circuit. It has rapid response but can add weight and dead space.
- Sidestream capnography: a small sample is aspirated to an external analyzer. It is commonly used with nasal/oral sampling cannulas in nonintubated patients, but sampling dilution, secretions, or kinking can affect accuracy.
Key limitation
ETCO₂ is influenced by ventilation, perfusion, and metabolism. A low value does not automatically mean the patient is hyperventilating. For example, it may instead reflect low pulmonary perfusion in shock or increased dead space.
Clinical takeaway: A capnogram answers, breath by breath, “Is CO₂ reaching the airway and being exhaled?” It is an early warning monitor for ventilation failure, but it complements rather than replaces clinical assessment, pulse oximetry, and blood gas testing where needed.
Recent evidence: a 2024 systematic review/meta-analysis evaluated continuous pulse oximetry and capnography for preventing postoperative respiratory compromise (Khanna et al., PMID 38184918).
Pictures of capnography
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capnography waveform normal phases and abnormal shark fin bronchospasm waveform diagram
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capnography waveform normal abnormal shark fin image
Capnography
Capnography is continuous measurement and graphical display of carbon dioxide (CO₂) in exhaled gas over time. The key displayed value is end-tidal CO₂ (ETCO₂ or PetCO₂), the CO₂ concentration at the end of exhalation.
It provides real-time information about:
- Ventilation: whether CO₂ is being eliminated
- Airway patency: obstruction, apnea, disconnection
- Pulmonary perfusion: especially during CPR or shock
- Metabolism: increased CO₂ production can raise ETCO₂
It does not directly measure oxygenation. Use pulse oximetry alongside it.
Normal values
- Normal ETCO₂: approximately 35-45 mmHg
- Normally, ETCO₂ is slightly lower than arterial PaCO₂, commonly by about 2-5 mmHg in healthy lungs.
- A widening PaCO₂-ETCO₂ gap can occur with increased dead space, such as pulmonary embolism, severe hypotension, or significant lung disease.
Normal capnogram waveform
A normal waveform has four phases:
-
Phase I, inspiratory baseline
Exhaled dead-space gas contains essentially no CO₂, so the tracing is near zero. -
Phase II, expiratory upstroke
Dead-space gas mixes with alveolar gas, producing a rapid rise in CO₂. -
Phase III, alveolar plateau
Mostly alveolar gas is exhaled. ETCO₂ is measured at the end of this phase. -
Phase 0, inspiratory downstroke
Inhalation of fresh gas causes CO₂ to return rapidly to baseline.
Common waveform patterns
| Pattern | Likely interpretation |
|---|---|
| ETCO₂ progressively rises | Hypoventilation, respiratory depression, rebreathing |
| ETCO₂ falls | Hyperventilation, falling cardiac output, reduced perfusion, leak or disconnection |
| Flat line / sudden loss of waveform | Apnea, esophageal intubation, tube/circuit disconnection, complete obstruction, cardiac arrest |
| “Shark-fin” upsloping plateau | Bronchospasm or expiratory airflow obstruction, such as asthma or COPD |
| Baseline fails to return to zero | Rebreathing CO₂, exhausted CO₂ absorber, faulty expiratory valve, inadequate fresh gas flow |
| Small, irregular waves | Partial obstruction, shallow breathing, leak, sampling problem |
Major clinical uses
1. Confirmation of endotracheal tube placement
Persistent waveform capnography after intubation is the most reliable bedside confirmation that the tube is in the trachea when there is pulmonary blood flow. A sudden loss of a previously present waveform requires immediate assessment for extubation, disconnection, obstruction, or circulatory collapse.
2. Monitoring ventilation during sedation and anesthesia
Capnography detects hypoventilation and apnea earlier than pulse oximetry, particularly when supplemental oxygen is used. Oxygen saturation may remain normal for several minutes despite absent ventilation, whereas the capnogram changes immediately. Roberts and Hedges’ Clinical Procedures in Emergency, p. 711.
3. CPR
During effective chest compressions, ETCO₂ reflects pulmonary blood flow and indirectly cardiac output.
- Persistently low ETCO₂ can suggest poor-quality compressions, inadequate perfusion, or prolonged arrest.
- An abrupt sustained rise in ETCO₂ may indicate return of spontaneous circulation (ROSC).
Interpret the number in the full clinical context, rather than using it alone to decide prognosis.
4. Mechanical ventilation
ETCO₂ supports adjustment and monitoring of ventilation. In general:
- High ETCO₂: lower minute ventilation, increased CO₂ production, or rebreathing
- Low ETCO₂: excessive minute ventilation, reduced cardiac output, increased dead space, or leak
5. Airway disease
The characteristic shark-fin capnogram indicates prolonged, obstructed expiration, often seen in bronchospasm.
Measurement methods
- Mainstream capnography: sensor is placed directly in the airway circuit. It has rapid response but can add weight and dead space.
- Sidestream capnography: a small sample is aspirated to an external analyzer. It is commonly used with nasal/oral sampling cannulas in nonintubated patients, but sampling dilution, secretions, or kinking can affect accuracy.
Key limitation
ETCO₂ is influenced by ventilation, perfusion, and metabolism. A low value does not automatically mean the patient is hyperventilating. For example, it may instead reflect low pulmonary perfusion in shock or increased dead space.
Clinical takeaway: A capnogram answers, breath by breath, “Is CO₂ reaching the airway and being exhaled?” It is an early warning monitor for ventilation failure, but it complements rather than replaces clinical assessment, pulse oximetry, and blood gas testing where needed.
Recent evidence: a 2024 systematic review/meta-analysis evaluated continuous pulse oximetry and capnography for preventing postoperative respiratory compromise (Khanna et al., PMID 38184918).
Quick answer
I could not retrieve suitable images from the medical image library before you stopped the search.
Here are the waveform pictures to look for in any capnography chart:
- Normal waveform: square-shaped curve with a flat alveolar plateau.
- Bronchospasm/COPD/asthma: “shark-fin” waveform, with a slanted, prolonged expiratory upstroke.
- Apnea or circuit disconnection: waveform progressively diminishes then becomes a flat line at zero.
- Rebreathing CO₂: baseline does not return to zero between breaths.
- Hypoventilation: waveform remains shaped normally but ETCO₂ becomes higher and breaths may be less frequent.
- Hyperventilation: lower ETCO₂, often with closer-spaced waveforms.
A simple schematic:
Normal:
_________
/ |
_______________/ |_______________
Bronchospasm ("shark fin"):
______
___/
__/
_____________/
Apnea / disconnection:
_______________________________
Rebreathing:
_______ _______
____/ |_____/ |____
↑ baseline stays above zero
```### Normal and abnormal capnography waveforms
### “Shark-fin” waveform
- A normal trace has a steep expiratory upstroke, near-flat alveolar plateau, then a sharp inspiratory downstroke.
- A **shark-fin** pattern indicates slowed expiration and obstructed airflow, commonly with asthma or COPD.
- A tracing that does not return to zero can indicate **CO₂ rebreathing**.
- An abruptly flat trace suggests **apnea, disconnection, complete obstruction**, or loss of pulmonary blood flow.
The [Masimo waveform reference](https://www.masimo.com/siteassets/us/documents/pdf/plm-11098b-quick-reference-guide-capnography-waveforms-global.pdf) contains labelled examples of normal, obstructive, apnea, rebreathing, and rising/falling ETCO₂ patterns.
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