Basics of Waveform Capnography
Waveform capnography assesses ventilation by monitoring exhaled carbon dioxide
Can use measurement and morphology during different phases of respiratory cycle to uncover pathophysiology
OVERVIEW
• Capnography measures ventilation through exhaled C02 (PECO2)
• Abnormal morphology can provide important data regarding pulmonary pathophysiology
CLINICAL APPLICATIONS
• Confirmation of endotracheal intubation
• Monitoring airway integrity
• Monitoring cardiac output
• Monitoring spontaneous respiration
• Assessing for CO2 retention
• Assessing ROSC during CPR by observing a sudden increase in waveform amplitude
ETC02 WAVEFORM
• α-Angle
- Between II and Ill
- >900 = bronchospasm or V/Q mismatch
• ß-AngIe
- Between Ill and IV/0
- >90° = rebreathing or mechanical obstruction
• PHASE I
- Inspiratory baseline
- PECO2 = zero
• PHASE II
- Beginning of expiration
- Transition as CO2 rises when anatomical dead space, then alveolar gas, is exhaled
• PHASE Ill
- Alveolar plateau
- ETCO2: Peak CO2 at end of phase Ill, Correlates with PaCO2
• PHASE IV/0
- Start of inspiration
- PECO2 rapidly falls to zero
WAVEFORM PATTERNS:
• BRONCHOSPASM AND REBREATHING/AIR TRAPPING
- Increase or loss of a-angle (aka "shark fin")
- Dead space has not finished emptying before next inspiration
- Increasing level of baseline PECO2 due to air trapping
• EMPHYSEMA
- Arterial CO2 represented by early peak, not end-tidal, due to hypercompliance and poor gas exchange surface
- Pattern can also be seen with pneumothorax with air leak
• SUDDEN LOSS OF WAVEFORM
- Critical event needing emergency intervention
- ET tube disconnected, dislodged, kinked, or obstructed
• MECHANICAL AIRWAY OBSTRUCTION
- Fixed mechanical obstruction affects both inspiration (phase IV/0) & expiration (phase II)
- α-angle and ß-angle both >900
• CARDIOGENIC OSCILLATIONS
- Pulsation transmitted from the heart to the lung parenchyma produces small volume changes that manifest as oscillations
- Sign of cardiomegaly
• DOWNTRENDING ETCO2 - Decreasing waveform size can indicate:
- Shock/low cardiac output state
- Pulmonary embolism
- Hyperventilation
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