What is the target partial pressure of carbon dioxide (PaCO2) in patients with raised intracranial pressure (ICP)?

Target PaCO2 in Raised Intracranial Pressure

Maintain PaCO2 between 35-40 mmHg (4.7-5.3 kPa) in patients with raised intracranial pressure, reserving temporary hypocapnia (PaCO2 30-35 mmHg) only for acute cerebral herniation as a bridge to definitive treatment. 1

Standard Target Range

• The target PaCO2 is 35-40 mmHg during management of raised ICP, whether during emergency neurosurgery, life-threatening hemorrhage interventions, or general ICU management of severe traumatic brain injury 1, 2
• This range represents normocapnia and avoids the detrimental effects of both hypocapnia (cerebral vasoconstriction, decreased cerebral blood flow, potential ischemia) and hypercapnia (increased cerebral blood volume, elevated ICP) 1, 2
• The 2019 World Society of Emergency Surgery consensus guidelines achieved 97.5% agreement on maintaining PaCO2 at 35-40 mmHg in severe brain-injured patients 1

Temporary Hypocapnia for Herniation

Use temporary hyperventilation to PaCO2 of 30-35 mmHg only in cases of impending or active cerebral herniation, and only as a temporizing measure until definitive treatment is available. 1, 3

• Hypocapnia should be used temporarily during cerebral herniation while awaiting or during emergency neurosurgery 1
• This represents a rescue therapy, not a maintenance strategy 3, 4
• Prolonged hyperventilation beyond a few hours can cause cerebral ischemia and should be avoided 3, 4

Evidence Against Aggressive Hyperventilation

• Hypocapnia (PaCO2 < 35 mmHg) is independently associated with unfavorable outcomes and delayed cerebral ischemia in brain-injured patients 1
• Severe hypocapnia (PaCO2 26-31 mmHg) and forced hypocapnia (PaCO2 < 26 mmHg) show a U-shaped relationship with mortality, with both extremes associated with increased in-hospital death 5
• PaCO2 should never be allowed to decrease to 20 mmHg or lower, and even values of 21-25 mmHg may predispose to ischemia in some patients 6
• A 2021 multicenter study of 1100 TBI patients found that centers using profound hyperventilation more frequently did not have worse outcomes, but the mean minimum PaCO2 was still maintained at 35.2 mmHg overall 7

Permissive Mild Hypercapnia

• Some evidence suggests permissive mild hypercapnia (PaCO2 > 37.5 mmHg) may be beneficial, as it has been associated with decreased risk of unfavorable outcomes in subarachnoid hemorrhage patients 1
• However, hypercapnia (PaCO2 > 45 mmHg) increases cerebral blood flow and can elevate ICP, particularly in patients without external ventricular drainage 1
• This approach requires careful ICP monitoring and should not be used in patients with uncontrolled intracranial hypertension 1

Monitoring and Individualization

• Patients with ICP monitoring tend to have lower PaCO2 values (mean 34.5 mmHg) compared to those without monitoring (36.7 mmHg), reflecting active management of intracranial dynamics 7
• In patients with documented intracranial hypertension, daily PaCO2 nadir is typically lower (33.8 vs 35.7 mmHg) as clinicians use ventilation to control ICP 7
• Maintain cerebral perfusion pressure ≥60 mmHg when ICP monitoring is available, adjusting based on neuromonitoring data and cerebral autoregulation status 1

Stepwise Approach to ICP Management

When managing elevated ICP, follow this hierarchy before resorting to hyperventilation 1, 3:

1. First-tier interventions: Head elevation to 30 degrees, sedation/analgesia, CSF drainage (if EVD present), osmotherapy (mannitol 1 g/kg or hypertonic saline)
2. Second-tier interventions: Optimize ventilation to maintain PaCO2 35-40 mmHg, deeper sedation/paralysis, higher doses of osmotherapy
3. Third-tier interventions: Moderate hyperventilation (PaCO2 30-35 mmHg) only for refractory cases or herniation, barbiturate coma, decompressive craniectomy

Critical Pitfalls to Avoid

• Do not use prolonged aggressive hyperventilation (PaCO2 < 30 mmHg) as a maintenance strategy for ICP control, as this causes cerebral vasoconstriction and ischemia 1, 3, 4
• Do not assume normocapnia is always safe—monitor for extreme fluctuations in PaCO2, as variability itself may be harmful 8
• Do not delay osmotherapy and neurosurgical consultation while attempting to control ICP solely with hyperventilation 3
• In subarachnoid hemorrhage specifically, avoid hypocapnia as it is independently associated with delayed cerebral ischemia and poor outcomes 1
• Ensure adequate oxygenation (PaO2 60-100 mmHg) while managing PaCO2, as hypoxemia compounds brain injury 1, 2