What is a normal carbon dioxide (CO2) level for a patient with asthma?

Normal Carbon Dioxide Levels in Asthma Patients

In patients with asthma, a normal arterial carbon dioxide tension (PaCO2) is 4.6-6.1 kPa (34-46 mmHg), the same as in healthy individuals, but any increase in PaCO2 into the normal range during an acute asthma exacerbation is concerning and indicates severe airway obstruction. 1

Understanding CO2 Levels in Asthma

• The normal reference range for arterial carbon dioxide tension (PaCO2) is 4.6-6.1 kPa (34-46 mmHg) for both healthy adults and stable asthma patients 1
• During mild to moderate asthma exacerbations, PaCO2 is typically low (hypocapnia) due to increased ventilation and respiratory alkalemia 2
• A normal or elevated PaCO2 in a breathless asthmatic patient is a marker of a very severe, life-threatening attack 3

Clinical Significance of CO2 Levels in Asthma

• In early stages of an asthma exacerbation, ventilation is generally increased with excessive elimination of carbon dioxide, resulting in hypocapnia 2
• As an asthma attack worsens, when FEV1 falls below 20-25% of predicted and work of breathing becomes excessive, PaCO2 begins to increase 2
• A rising PaCO2, even into the normal range (4.6-6.1 kPa), in a patient experiencing an asthma attack indicates respiratory muscle fatigue and patient exhaustion 2

Monitoring CO2 Levels in Asthma

• Arterial blood gas measurements should always be obtained in patients with acute severe asthma who are admitted to hospital 3
• End-tidal CO2 (EtCO2) measurements can be useful in the prehospital setting, with extreme values (either very low or very high) associated with poor outcomes 4
• Both hypocapnia (<4.6 kPa) and hypercapnia (>6.1 kPa) can occur in asthma patients depending on the severity of the exacerbation 1, 2

Warning Signs and Management Implications

• A normal or rising PaCO2 in a breathless asthmatic patient requires immediate intervention 3, 2
• Patients with extreme EtCO2 values (either <28 mmHg or >50 mmHg) have higher rates of intubation, ICU admission, and mortality 4
• Some asthma patients may have inherently lower respiratory center sensitivity to CO2, potentially predisposing them to CO2 retention during severe attacks 5

Treatment Considerations

• High-flow oxygen (40-60%) should be administered during severe asthma attacks, as CO2 retention is not aggravated by oxygen therapy in asthma 3
• Patients with normal or elevated PaCO2 during an asthma attack may require transfer to intensive care with preparation for possible intubation 3
• Arterial blood gas measurements should be repeated within 2 hours of starting treatment if initial PaCO2 was normal or raised 3

Key Pitfalls to Avoid

• Do not assume that a "normal" PaCO2 reading in an asthmatic having an exacerbation is reassuring - it may actually indicate severe respiratory compromise 3, 2
• The severity of an asthma attack is often underestimated by patients, relatives, and doctors due to failure to make objective measurements 3
• Central cyanosis may be an unusual finding in acute severe asthma, so its absence should not be interpreted as indicating a less severe attack 6