What are the implications of an end-tidal CO2 (etCO2) level less than 35 mmHg?

Why ETCO2 Less Than 35 mmHg is Problematic

An ETCO2 below 35 mmHg indicates hyperventilation, which causes cerebral vasoconstriction, reduced cerebral blood flow, and potential secondary brain injury—particularly dangerous in trauma patients and those post-cardiac arrest where maintaining adequate cerebral perfusion is critical for survival and neurologic outcomes. 1

Physiologic Consequences of Low ETCO2

Cerebral Blood Flow Impairment

• Hypocapnia (low CO2) causes cerebral vasoconstriction, reducing oxygen delivery to already-injured brain tissue 1
• The 2024 International Consensus on CPR explicitly recommends targeting normocapnia (PaCO2 35-45 mmHg) in adults with return of spontaneous circulation after cardiac arrest to avoid this harm 1
• This recommendation carries moderate-certainty evidence that deviating from normal CO2 ranges—either above or below—provides no benefit and may cause harm 1

Clinical Context: Traumatic Brain Injury

• Inadvertent hyperventilation is extremely common and dangerous in severe traumatic brain injury, with 79% of patients experiencing ETCO2 <30 mmHg despite monitoring and target parameters 2
• Even with paramedics instructed to target ETCO2 of 30-35 mmHg, mean minimum ETCO2 values reached 23.5 mmHg, well below safe thresholds 2
• ETCO2 values ≤30 mmHg in trauma patients are associated with increased risk of severe injury, need for ICU admission, emergency operative intervention, and acute blood loss anemia 3, 4

Specific Clinical Scenarios Where Low ETCO2 Causes Harm

Post-Cardiac Arrest Care

• The 2024 guidelines provide a weak recommendation with moderate-certainty evidence to target normocapnia (35-45 mmHg) specifically because RCTs failed to show benefit from targeting CO2 values outside this range 1
• Both hypocapnia and hypercapnia showed either harm or no benefit in observational studies and RCTs 1

Trauma and Hemorrhage

• ETCO2 ≤27 mmHg has 72.7% sensitivity and 72.2% specificity for predicting need for transfusion, operative intervention, or death in the first 24 hours 4
• Mean prehospital ETCO2 of 25.7 mmHg in patients requiring transfusion/surgery/death versus 30.6 mmHg in those who did not (p=0.049) 4

Critical Monitoring Thresholds

When ETCO2 <35 mmHg Signals Danger

• ETCO2 values <30 mmHg indicate significant hyperventilation requiring immediate ventilation adjustment 5, 6
• An absolute change from baseline ETCO2 >10 mmHg in either direction warrants immediate clinical reassessment 5, 7, 6
• In trauma patients, ETCO2 <30 mmHg occurred in 79% of cases and ETCO2 <25 mmHg in 59% of cases, with mean durations exceeding 6 minutes 2

Important Caveats and Pitfalls

ETCO2-PaCO2 Correlation Issues

• ETCO2 may not accurately reflect PaCO2 in critically ill patients, particularly those with cardiorespiratory dysfunction or altered ventilation-perfusion matching 1, 8, 9
• In emergency patients with cardiorespiratory failure, there is often no correlation between ETCO2 and arterial PaCO2 due to ventilation-perfusion mismatch 9
• Trauma patients ventilated to "normal" ETCO2 range (35-40 mmHg) were actually underventilated (PaCO2 >40 mmHg) 80% of the time 8

When to Rely on Arterial Blood Gas Instead

• In trauma patients and those with severe cardiorespiratory dysfunction, arterial blood gas analysis is required for accurate ventilation assessment, as ETCO2 correlation is poor (R²=0.277) 8, 9
• Correlation is worst in patients with evidence of poor tissue perfusion (elevated lactate, shock index, base deficit) 8
• Only patients without primary cardiorespiratory damage show acceptable ETCO2-PaCO2 correlation 9

Practical Management Algorithm

For Post-Cardiac Arrest Patients

1. Target normocapnia: PaCO2 35-45 mmHg (ETCO2 approximately 30-40 mmHg if good correlation exists) 1
2. Confirm with arterial blood gas, as ETCO2 may not accurately reflect PaCO2 in this population 1
3. Avoid both hypocapnia and hypercapnia unless specific compensatory needs exist (severe lung injury, metabolic acidosis) 1

For Trauma Patients

1. Do not rely solely on ETCO2 for ventilation guidance—obtain arterial blood gas 8, 9
2. If ETCO2 <30 mmHg, immediately reduce ventilation rate and recheck with arterial blood gas 2, 3
3. ETCO2 ≤27 mmHg should trigger hemorrhage protocol activation (transfusion preparation, surgical consultation) 4

For Procedural Sedation

• ETCO2 >50 mmHg indicates hypoventilation requiring intervention 5, 7, 6
• ETCO2 <30 mmHg indicates excessive ventilation requiring rate reduction 5, 6
• Normal range: 35-40 mmHg 6