A new method for bedside determination of effective lung volume and functional residual capacity

Established methods of measuring functional residual capacity (FRC) involve sophisticated equipment and elaborate procedures. Here we present a new method based on CO₂ rebreathing that has a simple fast procedure and only requires end-tidal CO₂ monitoring. Ten healthy subjects with diverse anthropometric and respiratory parameters were studied in the sitting position. Reference FRC (RefFRC) and tidal volume (TV) were measured with a Cosmed Quark PFT/DLCO unit using the single-breath methane dilution technique in combination with spirometry. Rebreathing through an external dead space of precisely known volume and recording the rising end-tidal CO₂ value of the first two breaths allows the determination of effective lung volume (ELV) and the calculation of FRC. Two sets of measurements were made on each subject 15 min apart. Bland-Altman analysis of a comparison between FRC and RefFRC showed a mean bias of 0.04 L, with limits of agreement (LoA, 95% CI) of -1.24 to +1.32 L and a percentage error (PE) of 0.54. When the mean value of two observations from a subject (meanFRC) was compared to RefFRC we obtained a mean bias of -0.08 L, LoA (95% CI) of -0.88 to +0.72 L and PE of 0.23. The FRC data obtained demonstrate good absolute accuracy. An average of repeated measurements improves precision indicating that a criterion for exchangeability with the reference method can be met. The simplicity of the equipment and the procedure could make this method attractive in the pre-operative and the post-operative settings, as well as in out-of-hospital applications.