PaCO2n

PaCO2n is a proposed normalized measure of arterial carbon dioxide tension, intended mainly for physiological research to facilitate comparisons across individuals with different baseline CO2 levels or to track relative changes over time. It is not an established standard in routine clinical practice, and there is no universal agreement on its calculation or interpretation.

Definition and calculation: PaCO2n is defined as the ratio of the measured PaCO2 to a reference or baseline PaCO2 for the individual, often expressed as PaCO2n = PaCO2 / PaCO2_base. The baseline PaCO2 is typically obtained under resting conditions and can vary by study, though 40 mmHg is commonly used as a reference in healthy adults. A PaCO2n greater than 1 indicates a higher CO2 level relative to baseline, while a PaCO2n less than 1 indicates a lower CO2 level relative to baseline.

Usage and interpretation: In research examining responses to interventions such as ventilation strategies or pharmacologic changes, PaCO2n can help normalize data and emphasize relative changes rather than absolute PaCO2 values. It is not used to guide clinical management; decisions in practice rely on absolute PaCO2 values and their trends over time.

Measurement considerations and limitations: The value of PaCO2n depends on the chosen baseline, so consistent baseline definition is essential. PaCO2n requires accurate arterial blood gas measurements and standardized sampling. Because it is not standardized, different studies may use different baselines or formulas, which can limit comparability. Additionally, individual factors such as CO2 production, ventilation-perfusion status, and acid-base balance can affect PaCO2 independently of baseline normalization, complicating interpretation.