Clinical validation of a capnodynamic method for measuring end-expiratory lung volume in critically ill patients

Abstract

Rationale: End-expiratory lung volume (EELV) is reduced in mechanically ventilated patients, especially in pathologic conditions. The resulting heterogeneous distribution of ventilation increases the risk for ventilation induced lung injury. Clinical measurement of EELV however, remains difficult.

Objective: Validation of a novel continuous capnodynamic method based on expired carbon dioxide (CO₂) kinetics for measuring EELV in mechanically ventilated critically-ill patients.

Methods: Prospective study of mechanically ventilated patients scheduled for a diagnostic computed tomography exploration. Comparisons were made between absolute and corrected EELVCO₂ values, the latter accounting for the amount of CO₂ dissolved in lung tissue, with the reference EELV measured by computed tomography (EELVCT). Uncorrected and corrected EELVCO₂ was compared with total CT volume (density compartments between - 1000 and 0 Hounsfield units (HU) and functional CT volume, including density compartments of - 1000 to - 200HU eliminating regions of increased shunt. We used comparative statistics including correlations and measurement of accuracy and precision by the Bland Altman method.

Measurements and main results: Of the 46 patients included in the final analysis, 25 had a diagnosis of ARDS (24 of which COVID-19). Both EELVCT and EELVCO₂ were significantly reduced (39 and 40% respectively) when compared with theoretical values of functional residual capacity (p < 0.0001). Uncorrected EELVCO₂ tended to overestimate EELVCT with a correlation r² 0.58; Bias - 285 and limits of agreement (LoA) (+ 513 to - 1083; 95% CI) ml. Agreement improved for the corrected EELVCO₂ to a Bias of - 23 and LoA of (+ 763 to - 716; 95% CI) ml. The best agreement of the method was obtained by comparison of corrected EELVCO₂ with functional EELVCT with a r² of 0.59; Bias - 2.75 (+ 755 to - 761; 95% CI) ml. We did not observe major differences in the performance of the method between ARDS (most of them COVID related) and non-ARDS patients.

Conclusion: In this first validation in critically ill patients, the capnodynamic method provided good estimates of both total and functional EELV. Bias improved after correcting EELVCO₂ for extra-alveolar CO₂ content when compared with CT estimated volume. If confirmed in further validations EELVCO₂ may become an attractive monitoring option for continuously monitor EELV in critically ill mechanically ventilated patients.

Trial registration: clinicaltrials.gov (NCT04045262).

Keywords: Carbon dioxide kinetics; End-expiratory lung volume; Lung strain; Mechanical ventilation; Respiratory monitoring; Ventilation induced lung injury; Volumetric capnography.