Novel approach to estimate respiratory quotient from the alveolar gas equation: a proof-of-concept study in pigs.

BACKGROUND: The respiratory quotient (RQ) reflects metabolic substrate utilization and is traditionally measured using indirect calorimetry techniques, e.g. Douglas bag, which are technically demanding and not always feasible in clinical practice. The alveolar gas equation (AGE) provides a theoretical relationship between oxygen tension, carbon dioxide tension and RQ and may be rearranged to estimate RQ using standard respiratory measurements. This study aimed to validate a novel method for estimating RQ by reversing the alveolar gas equation. METHOD: In this prospective experimental study, twelve anesthetized, mechanically ventilated pigs were studied under steady-state conditions. RQ was determined using two methods: (1) reference measurement via Douglas bag collection using Haldane transformation (RQref), and (2) calculation from the rearranged alveolar gas equation using inspired and expired gas measurements (RQcalc). Agreement between methods was assessed using paired t-tests, Bland-Altman analysis with bootstrap confidence intervals, and linear regression. RESULTS: Mean RQref was 0.877 (95% CI 0.845-0.908), while mean RQcalc was 0.994 (95% CI 0.961-1.026), demonstrating a significant difference (p < 0.001). Bland-Altman analysis showed a bias of 0.117 (95% CI 0.103-0.131) with limits of agreement from 0.066 to 0.167 and a mean percentage error of 5%. A strong correlation was observed between methods (r = 0.895, p < 0.001), with RQcalc explaining 80% of the variance in RQref (R² = 0.801). CONCLUSION: Reversal of the alveolar gas equation enables estimation of RQ with strong correlation to reference measurements but introduces a consistent positive bias. While not directly interchangeable with Douglas bag measurements, this simplified approach may provide a practical, continuous approximation of RQ in settings where conventional metabolic monitoring is not feasible. Additional validation studies are needed to make this method ready for clinical use.