What is the difference between low and high end-tidal CO2 (etco2) levels?

Understanding End-Tidal CO2 (ETCO2) Levels

Low ETCO2 means your body isn't making enough carbon dioxide or can't get it out properly, while high ETCO2 means too much carbon dioxide is building up in your body - both can indicate serious health problems that need immediate attention.

What is ETCO2?

End-tidal CO2 (ETCO2) is the partial pressure of carbon dioxide measured at the end of exhalation. It represents the concentration of CO2 that is exhaled from your lungs 1. This measurement is most accurately obtained using waveform capnography, which shows the actual CO2 waveform during breathing 1.

Low ETCO2 (Below 35 mmHg)

Low ETCO2 values (especially below 10 mmHg during CPR) indicate:

• Poor blood flow: During cardiac arrest, low ETCO2 reflects inadequate cardiac output from chest compressions 1
• Poor CPR quality: Values below 10 mmHg during resuscitation suggest ineffective chest compressions 1
• Poor prognosis: Persistently low values (below 10 mmHg) during CPR are associated with unlikely survival 1, 2
• Metabolic problems: Can indicate metabolic acidosis 3
• Hyperventilation: Breathing too fast can lower CO2 levels 1
• Technical issues: Can be caused by equipment problems like airway leaks or ETT kinking 1

High ETCO2 (Above 45 mmHg)

High ETCO2 values indicate:

• Return of spontaneous circulation (ROSC): A sudden increase in ETCO2 (>10 mmHg) during CPR often signals that the heart has started beating again 2
• Hypoventilation: Not breathing enough causes CO2 buildup
• Increased metabolic rate: More CO2 production from fever, seizures, or other high-energy states
• CO2 retention: Seen in conditions like COPD where patients retain carbon dioxide

Clinical Applications

1. During CPR:

   • ETCO2 <10 mmHg: Attempt to improve CPR quality 1
   • Sudden ETCO2 increase >10 mmHg: Likely indicates ROSC 2

2. Ventilation monitoring:

   • Helps adjust ventilation parameters in intubated patients
   • However, in emergency patients with cardiorespiratory dysfunction, ETCO2 may not accurately reflect arterial CO2 4

3. Metabolic assessment:

   • ETCO2 ≤25 mmHg: Higher likelihood of metabolic acidosis 3
   • ETCO2 ≥36 mmHg: Can help rule out severe metabolic disturbances 3

4. Trauma assessment:

   • ETCO2 ≤27 mmHg in trauma patients may predict need for blood transfusion or surgical intervention 5

Important Caveats

• ETCO2 is most reliable in intubated patients; measurements may be less accurate with bag-mask ventilation 1
• The relationship between ETCO2 and arterial CO2 varies in patients with cardiorespiratory failure due to ventilation-perfusion mismatch 4
• ETCO2 monitoring is widely available but underutilized in pediatric emergency departments, especially for non-intubated patients 6

Remember that ETCO2 monitoring provides valuable real-time information about a patient's ventilation, perfusion, and metabolism, making it an essential tool in emergency and critical care settings.