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

PurposeThe 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).MethodsTen 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.ResultsEELV-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.ConclusionEELV-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.