Simulation of maximum respiratory venous PCO2 in vitro

Area of Science:

• Physiology
• Biophysics
• Respiratory Medicine

Background:

• Maximum respiratory venous PCO2 (PvmrCO2) represents theoretical full oxygen extraction, simulating maximum aerobic PCO2 in tissues.
• The Dill nomogram provides PvmrCO2 values but is limited to specific conditions ([Hb] = 15 g/dL, [BE] = 0).
• Understanding PvmrCO2 variability is crucial for identifying critical tissue PCO2 levels.

Purpose of the Study:

• To determine the variability of PvmrCO2 with changes in hemoglobin concentration ([Hb]), arterial base excess ([BE]), and arterial PCO2 (PaCO2).
• To develop a predictive equation for in vitro PvmrCO2 across a range of physiological conditions.

Main Methods:

• Simulated venous CO2 titrations using published in vitro formulas for [BE] and whole blood [total CO2].
• Analyzed PvmrCO2 across a respiratory quotient (RQ) range of 0.7 to 1.0.
• Varied parameters including [Hb], [BE], and PaCO2 to assess their impact on PvmrCO2.

Main Results:

• Simulations yielded PvCO2 values consistent with the Dill nomogram within the RQ range of 0.7 to 1.0.
• Predicted that PvmrCO2 decreases proportionally with [Hb] and increases non-linearly with decreasing arterial [BE].
• Demonstrated that the PvmrCO2-PaCO2 difference increases linearly with increasing PaCO2, with significant variations based on [BE] and [Hb].

Conclusions:

• The relationship between venous PCO2 (PvCO2) and oxygen saturation (SO2) is not linear when arterial [Hb] and/or [BE] vary.
• A novel equation is provided to predict in vitro PvmrCO2 as a function of arterial [BE], [Hb], RQ, and PaCO2.
• The accuracy of the predictive equation in vivo warrants further investigation.