Capnodynamic determination of end-expiratory lung volume in a porcine model of hypoxic pulmonary vasoconstriction.

Purpose: The capnodynamic method, End Expiratory Lung Volume CO₂ (EELV-CO₂), utilizes exhaled carbon dioxide analysis to estimate End-Expiratory Lung Volume (EELV) and has been validated in both normal lungs and lung injury models. Its performance under systemic hypoxia and variations in CO₂ elimination is not examined. This study aims to validate EELV-CO₂ against inert gas wash in/wash out (EELV- SF6, sulfur hexafluoride) in a porcine model of stable hemodynamic conditions followed by hypoxic pulmonary vasoconstriction and inhaled nitric oxide (iNO).

Methods: Ten mechanically ventilated piglets were exposed to a hypoxic gas mixture and selective pulmonary vasoconstriction. Inhalation of nitric oxide was used to reverse the pulmonary vasoconstriction. Paired recordings of EELV-CO₂ and EELV-SF6, were conducted to assess their agreement of absolute values.

Results: EELV-CO₂ showed a bias of + 5 ml kg^- 1 compared to EELV-SF6, upper limit of agreement of 11 ml kg^- 1 (95%CI: 9-13 ml kg^- 1), lower limit of agreement - 1 ml kg^- 1 (95%CI: -3- 0 ml kg^- 1), mean percentage error 34%. Agreement between EELV-CO₂ and EELV-SF6 was largely constant but was affected by progressing hypoxia and reached maximum limit of agreement after iNO exposure. Re-introduction of normoxemia then stabilized bias and limits of agreement to baseline levels.

Conclusion: EELV-CO₂ generates absolute values in parallel with EELV -SF6. Stressing EELV-CO₂ with hypoxic pulmonary vasoconstriction and iNO, transiently impairs the agreement which stabilizes once normoxemia is reestablished.