End Tidal CO₂ monitoring 
 
End-tidal CO₂ (EtCO₂) is the partial pressure (or concentration) of carbon dioxide measured at the very end of exhalation. It reflects how well CO₂ is being produced, transported, and eliminated by the lungs. 
 
Key points 

What it represents: 
The CO₂ coming from alveoli at the end of expiration—closest non-invasive estimate of arterial CO₂. 

Normal range: 
35–45 mmHg (≈ 4.6–6.0 kPa) 

How it’s measured: 
Capnography (waveform) or capnometry (numeric value) 
 
Clinical uses 

Confirming endotracheal tube placement 
Monitoring ventilation (e.g., during anaesthesia, ICU care, procedural sedation) 
Assessing perfusion and metabolism 
– Low EtCO₂ can suggest shock, low cardiac output, or hyperventilation 
– High EtCO₂ can suggest hypoventilation or increased CO₂ production 
CPR quality: 
Higher EtCO₂ correlates with better chest compressions; sudden rise may indicate ROSC. 
 
Common causes of abnormal values 

Low EtCO₂: Hyperventilation, pulmonary embolism, shock, cardiac arrest 
High EtCO₂: Hypoventilation, airway obstruction, rebreathing, fever, sepsis 
 
EtCO₂ in the ICU 
 
Normal values 
EtCO₂: 35–45 mmHg (4.6 to 6.0 KPa) 
PaCO₂ – EtCO₂ gradient: normally 2–5 mmHg 
↑ gradient suggests V/Q mismatch (ARDS, PE, low cardiac output) 
 
Why EtCO₂ matters in ICU 
Continuous, non-invasive ventilation monitoring 
Early detection of hypoventilation, apnoea, circuit problems 
Indirect marker of pulmonary perfusion & cardiac output 
Useful during intubation, sedation, weaning, CPR 
 
 
 
 
 
Capnography Waveform (Time Capnogram) 
 
Phases of the waveform:  


 
Phase I (early expiration): Dead space gas 
Exhalation of gas from conducting airways, representing the exhalation of anatomical dead space gas, which should be CO2 free. 

Phase II (B–C): Expiratory upstroke 
Rapid rise in CO₂ 
Mixing of alveolar gas with dead space gas 

Phase III (C–D): Alveolar plateau 
Reflects exhalation of alveolar gas (CO₂) 
End of phase III (D) = EtCO₂ 
Slope ↑ → V/Q mismatch (COPD, asthma) 

Phase 0: Inspiration 
Rapid downstroke to zero 
CO2 level falls to zero as fresh gas is inhaled 
 
Abnormal Waveforms & ICU Causes 

1. Hypoventilation 
Tall waveform, ↑ EtCO₂ 
Causes: over-sedation, low RR, neuromuscular weakness 

2. Hyperventilation 
Short waveform, ↓ EtCO₂ 
Causes: excessive ventilator rate, pain, anxiety 

3. Bronchospasm / COPD (Shark-fin) 
Slanted upstroke + rising plateau 
Causes: asthma, COPD, mucus plugging 

4. Rebreathing 
Baseline does not return to zero 
Causes: exhausted CO₂ absorber, low fresh gas flow, faulty valves 

5. Sudden loss of EtCO₂ 
Flat line or near zero 
Causes: 
Oesophageal intubation 
Circuit disconnect 
Apnoea 
Cardiac arrest 

6. Pulmonary embolism / low cardiac output 
Abrupt ↓ EtCO₂, normal waveform shape 
↑ PaCO₂–EtCO₂ gradient 
 
EtCO₂ vs PaCO₂ in ICU 


 Key Points:
 
Rising EtCO₂ → think hypoventilation or increased CO₂ production 
Falling EtCO₂ → think PE, hypotension, disconnect 
Use EtCO₂ trends, not isolated numbers 
Always correlate with ABG, ventilator settings, and clinical status of the patient.