Herr: You think you know ETCO2?!?

Clinical Pearls (assisted by Dr. Lino Rafael O. Trinidad)

#1 concept: Will be the first warning of trouble in a struggling patient!

Capnography → LEAVE IT ON!!  If esophageal, will go BACK to purple

• Is the immediate picture of ventilation (SpO2 delayed)
  • Hypoventilation occurs 1-2 minutes before hypoxemia
• Ways of Measuring (Is the LAW for conscious sedation)
  • POC addition to ETT (standard intubation equipment)
  • Sidestream- can add dead space; can underestimate CO2
  • Mainstream- T-piece adaptor at the airway opening; does not add dead space
  • Mass Spectrometry- CO2 eliminated / O2 consumed

CO2 Waveform

• Evaluate for:
  • Height
  • Frequency
  • Rhythm
  • Baseline
  • Shape
• Systematic Approach
  1. Is CO2 present (waveform)
  2. Is the baseline zero
  3. Evaluate the:
     • Expiratory upstroke: steep, sloping, prolonged
     • Expiratory (alveolar) plateau: flat prolonged, notched, or sloping
     • Inspiratory downstroke: steep sloping or prolonged
  4. Just read the number
  5. Check ABG (Try to match the PaCO2, if its not within 5 a problem)

• Alpha angle: amount of V/Q mismatch
• Beta angle: if >90 degrees → rebreathing occurring

Evaluation of waveform abnormalities: 

• Waveform regular shape
  • Plateau below normal = CO2 deficiency
    • Hyperventilation
    • Decreased pulm perfusion
    • Hypothermia
    • Decreased metabolism
  • Plateau above normal = CO2 excess
    • Hypoventiltion
    • Absorption of CO2 during laparoscopy
    • Fever
    • Dietary excess
    • Sudden increases: sodium bicarb use, release of tourniquet
  • Watch for Trends:

Special uses: 

1. Assessing Intubation and Ventilation
   • 100% specific
     • Facemask can force some pharyngeal air into stomach
   • Esophageal intubation may be positive for CO2 (usually less than 10, decreases over breaths)
   • Watch out for plug, kink, apnea, severe bronchospasm
2. ETCO2 and CO
   • CO largely affect ETCO2 in dynamic situations, not when stable
   • ETCO2 < 10 after 20 mins of CPR associated with a very high mortality
     • Must make sure adequate compressions before judging ETCO2
3. Dead Space i.e. Altered CO2 clearance
   • Hypercapnea with adequate ventilation implies dead space
   • Bohr-Enghoff Equation; Dead space = (PaCO2 – PECO2) / PaCO2
     1. Normal dead space is 2ml/kg
   • Can be also calculated via Volume Capnogram
4. Ventilator Adjustment
   • Increase PEEP if gradient too high
     • Useful for weaning vent once gradient is established
   • Measure ETCO2 every time you get ABG
5. ARDS (?Dead Space Disease)
   • TV distributed to poorly or non-perfused parts:
     1. Microemboli
     2. Endothelial damage
   • Non-survivors have higher dead space, can be prognostic
   • Serial measurement of deadspace for progression
6. PE
   • Hypoxia, clinical scenario
   • 13-20% drop in dead space = CTA immediately 
     • Good NPV (100%); PPV (66%)
7. Ventilator Weaning
   • Obtain a gradient every AM (Volumetric would be more helpful)
   • ETCO2 + RSBI is indicative of the proper score for extubation
8. VA ECMO Weaning
   • ↓ECMO flow by 40% → ETCO2
     • ETCO2 will changed steeply with a flexion point (good symbol of coming off VA ECMO)