A new method for bedside determination of effective lung volume (ELV) and functional residual capacity (FRC)

Established methods of measuring 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 a dead space of precisely known volume and recording the rising end-tidal CO₂ value of the first two breaths allows the determination of ELV and the calculation of FRC. Two sets of measurements were made on each subject 15 minutes apart.

Bland-Altman analysis of a comparison between FRC and RefFRC shows a mean bias= 0.08 l, limits of agreement (LoA) of (+1.35 to −1.19; 95%CI) l. and a percentage error of PE= 0.54. When the mean value of two observations from a subject (meanFRC) is compared to RefFRC we get a mean bias = −0.04 l (LoA) of (+0.76 to – 0.83; 95%CI) l and PE=0.23

The FRC data obtained has good absolute accuracy. A mean of two observations has sufficient precision to meet the criteria for exchangeability with reference. The simplicity of the equipment and the procedure could make this method attractive both in the pre-operative and the post-operative setting as well as in out of hospital applications.