Is a carbon dioxide (CO2) level of 18 mmHg considered within the normal range?

CO2 Level of 18 mmHg is Below Normal Range and Indicates Respiratory Alkalosis

A carbon dioxide (CO2) level of 18 mmHg is significantly below the normal range and indicates respiratory alkalosis, requiring prompt clinical evaluation for underlying causes. 1

Normal CO2 Range and Interpretation

According to the British Thoracic Society (BTS) guidelines, the normal reference range for arterial carbon dioxide tension (PaCO2) is 4.6-6.1 kPa (34-46 mmHg) for healthy adults 1. A value of 18 mmHg is substantially below this range and represents significant hypocapnia.

Clinical Significance of Low CO2

• Respiratory Alkalosis: A CO2 of 18 mmHg indicates primary respiratory alkalosis (hypocapnia), which occurs when alveolar ventilation exceeds that required to eliminate the carbon dioxide produced by tissues 2
• pH Changes: This low CO2 level would be associated with an increase in blood pH (alkalemia) unless there is metabolic compensation
• Compensatory Mechanisms: The body initially responds with cellular uptake of bicarbonate and buffering by intracellular phosphates and proteins 2

Potential Causes to Investigate

A CO2 level this low should prompt immediate investigation for:

1. Hyperventilation states:

   • Anxiety/panic disorder
   • Pain
   • Fever
   • Hypoxemia
   • Central nervous system disorders

2. Pulmonary conditions:

   • Interstitial lung disease (ILD) - often shows considerable hyperventilation even at rest 1
   • Pulmonary vascular disease (PVD)
   • Early sepsis/pneumonia
   • Pulmonary embolism

3. Iatrogenic causes:

   • Mechanical ventilation with excessive minute ventilation
   • Inappropriate ventilator settings in critical care

Clinical Implications

Severe hypocapnia (CO2 of 18 mmHg) can lead to:

• Cerebral vasoconstriction and reduced cerebral blood flow
• Neurological symptoms (dizziness, paresthesias, tetany)
• Cardiac arrhythmias
• Decreased oxygen delivery to tissues due to left shift of the oxyhemoglobin dissociation curve

Management Approach

1. Identify and treat the underlying cause

2. Monitor acid-base status with arterial blood gases to assess for:

   • Degree of alkalemia
   • Presence of compensatory mechanisms
   • Coexisting acid-base disorders

3. Assess for tissue hypoperfusion:

   • Be cautious when interpreting venous-arterial CO2 difference (ΔCO2) during respiratory alkalosis, as it may be misleadingly elevated 3

4. Avoid rapid correction of chronic hypocapnia, as this could lead to metabolic derangements

Special Considerations

• Chronic vs. Acute: If chronic respiratory alkalosis is present, renal compensation will occur with decreased bicarbonate reabsorption, potentially normalizing pH despite the low CO2 2
• Mixed Disorders: A CO2 of 18 mmHg may be part of a mixed acid-base disorder, particularly if occurring with metabolic acidosis (as a compensatory mechanism)

Conclusion

A CO2 level of 18 mmHg is definitively abnormal and indicates significant respiratory alkalosis that requires clinical evaluation to determine the underlying cause and appropriate management.