Practical considerations for assessing pulmonary gas exchange and ventilation during flume swimming using the MetaSwim metabolic cart

The MetaSwim (MS) metabolic cart can assess pulmonary gas exchange and ventilation in aquatic environments. The aims of this study were: 1) to determine the agreement between minute ventilation (V̇E), pulmonary oxygen uptake (V̇O₂) and carbon dioxide output (V̇CO₂) using the MS and Douglas Bag (DB) methods during flume swimming; 2) to assess the repeatability of these and other MS derived parameters. Sixteen trained swimmers completed a combined incremental and supramaximal verification cardiopulmonary swimming test to determine maximal V̇O₂, two progressive intensity swimming tests during which MS and DB measurements were made (agreement protocol), and/or three-four constant velocity submaximal swimming tests during which only the MS was used (repeatability protocol). Agreement was determined using limits of agreement (LoA), bias, random error and 95% confidence intervals with systematic bias assessed using paired samples t-tests. Within-trial and between trial repeatability were determined using the coefficient of variation (CV) and the repeatability coefficient (CR). Where data were heteroscedastic, LoA and CR were logtransformed, anti-logged and displayed as ratios. MS underestimated peak V̇O₂ and V̇CO₂ (<0.39 L.min^-1) and V̇E (9.08 L.min^-1), while submaximal values varied between 2-5% for CV and ±1.09-1.22 for ratio CR. The test re-test CV during constant velocity swimming for V̇E, tidal volume, breathing frequency, V̇O₂, V̇CO₂, and end-tidal pressures of O₂ and CO₂ was < 9% (ratio CR of ±1.09-1.34). Thus, the MS and DB cannot be used interchangeably. Whether the MS is suitable for evaluating ventilatory and pulmonary responses in swimming will depend upon the size of effect required.