Transcutaneous PCO₂ for Exercise Gas Exchange Efficiency in Chronic Obstructive Pulmonary Disease

Gas exchange inefficiency and dynamic hyperinflation contributes to exercise limitation in chronic obstructive pulmonary disease (COPD). It is also characterized by an elevated fraction of physiological dead space (V_D/V_T). Noninvasive methods for accurate V_D/V_T assessment during exercise in patients are lacking. The current study sought to compare transcutaneous PCO₂ (TcPCO₂) with the gold standard-arterial PCO₂ (PaCO₂)-and other available methods (end tidal CO₂ and the Jones equation) for estimating V_D/V_T during incremental exercise in COPD. Ten COPD patients completed a symptom limited incremental cycle exercise. TcPCO₂ was measured by a heated electrode on the ear-lobe. Radial artery blood was collected at rest, during unloaded cycling (UL) and every minute during exercise and recovery. Ventilation and gas exchange were measured breath-by-breath. Bland-Altman analysis examined agreement of PCO₂ and V_D/V_T calculated using PaCO₂, TcPCO₂, end-tidal PCO₂ (P_ETCO₂) and estimated PaCO₂ by the Jones equation (PaCO₂-Jones). Lin's Concordance Correlation Coefficient (CCC) was assessed. 114 measurements were obtained from the 10 COPD subjects. The bias between TcPCO₂ and PaCO₂ was 0.86 mmHg with upper and lower limit of agreement ranging -2.28 mmHg to 3.99 mmHg. Correlation between TcPCO₂ and PaCO₂ during rest and exercise was r²=0.907 (p < 0.001; CCC = 0.941) and V_D/V_T using TcPCO₂ vs. PaCO₂ was r²=0.958 (p < 0.0001; CCC = 0.967). Correlation between PaCO₂-Jones and P_ETCO₂ vs. PaCO₂ were r²=0.755, 0.755, (p < 0.001; CCC = 0.832, 0.718) and for V_D/V_T calculation (r²=0.793, 0.610; p < 0.0001; CCC = 0.760, 0.448), respectively. The results support the accuracy of TcPCO₂ to reflect PaCO₂ and calculate V_D/V_T during rest and exercise, but not in recovery, in COPD patients, enabling improved accuracy of noninvasive assessment of gas exchange inefficiency during incremental exercise testing.