Comparing estimated versus actual dead space measurements in cardiopulmonary exercise testing (CPET): implications for clinical practice

<bold>Introduction:</bold> Accurate dead space/tidal volume ratio (Vd/Vt) is important in assessing lung function but requires an arterial blood gas (ABG) analysis to measure PaCO₂. Estimated Vd/Vt based on Jones et al.’s work (J Appl Physiol 1979; 47:954-60) is commonly used in CPET machines. Its reliability warrants investigation. <bold>Methods:</bold> We conducted a study involving 63 patients referred for CPET. ABGs were obtained at rest and peak exercise (<30 sec). Vd/Vt was derived from 9 formulas (Fig. 1). Agreement between Vd/Vt values derived from formulas (F) 2-9 and the gold standard F1 were assessed using Intra-Class Correlation (ICC). ICC>0.7 indicated good agreement. <bold>Results:</bold> 39.7% of participants were male (mean age 53.8±17.5 years). At rest, there was poor agreement between F1 and F2-9 (ICC=0.20; 0.45; 0.66; 0.25; 0.26; 0.20; 0.56; 0.18 respectively). At peak exercise, only F3, F4 and F6 demonstrated good agreement with F1 (ICC=0.81, p=0.048; ICC=0.96, p=0.006; ICC=0.70, p<10³ respectively). F2, F5, F7, F8 and F9 showed unacceptable agreement (ICC=0.68; 0.60; 0.30; 0.51; 0.11 respectively). <bold>Conclusion:</bold> While ventilatory equivalents for CO₂ and its slope may serve as reasonable proxies for calculating Vd/Vt, our findings underscore the continued necessity of ABG analysis. Specifically, the formula employing PaCO₂ estimated by Jones et al. fails to accurately predict actual Vd/Vt values. <fig><object-id>erj;64/suppl_68/PA2591/F1</object-id><object-id>F1</object-id><object-id>F1</object-id><graphic></graphic></fig>