A new method for bedside determination of effective lung volume (ELV) and functional residual capacity (FRC)
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Established methods of measuring FRC involve sophisticated equipment and elaborate procedures. Here we present and evaluate a new method based on CO 2 rebreathing and end-tidal CO 2 monitoring using simple equipment and procedure applicable at bedside.
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 the Cosmed Quark CPET unit using the single-breath helium dilution technique. Rebreathing through a dead space of precisely known volume and recording the resulting rise in the end-tidal CO 2 value for the first two breaths allows the determination of ELV. FRC is then calculated from ELV. Two sets of measurements were made on each subject 15 minutes apart.
Bland-Altman analysis of a comparison between FRC and RefFRC shows a bias= 0.08 l and SD=0.67 l. Comparing the first set of data to the second, (alfa =0.05) paired t-test shows the two sets to be independent. When the mean value of two observations is calculated (meanFRC) and compared to RefFRC we obtain bias= −0.04 l with SD=0.38 l which results in a percentage error PE=0.23
The FRC data obtained has good absolute accuracy. Due to the short measuring time, a mean of two observations can be formed that has sufficient precision to meet criteria for exchangeability with reference. The simplicity of equipment and procedure, could make this method attractive both in the pre-operative and the post-operative setting.
