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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 PCO 2 (TcPCO 2) with the gold standard-arterial PCO 2 (PaCO 2)-and other available methods (end tidal CO 2 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. TcPCO 2 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-bybreath. Bland-Altman analysis examined agreement of PCO 2 and V D /V T calculated using PaCO 2 , TcPCO 2 , end-tidal PCO 2 (P ET CO 2) and estimated PaCO 2 by the Jones equation (PaCO 2-Jones). Lin's Concordance Correlation Coefficient (CCC) was assessed. 114 measurements were obtained from the 10 COPD subjects. The bias between TcPCO 2 and PaCO 2 was 0.86 mmHg with upper and lower limit of agreement ranging À2.28 mmHg to 3.99 mmHg. Correlation between TcPCO 2 and PaCO 2 during rest and exercise was r 2 ¼0.907 (p < 0.001; CCC ¼ 0.941) and V D /V T using TcPCO 2 vs. PaCO 2 was r 2 ¼0.958 (p < 0.0001; CCC ¼ 0.967). Correlation between PaCO 2-Jones and P ET CO 2 vs. PaCO 2 were r 2 ¼0.755, 0.755, (p < 0.001; CCC ¼ 0.832, 0.718) and for V D /V T calculation (r 2 ¼0.793, 0.610; p < 0.0001; CCC ¼ 0.760, 0.448), respectively. The results support the accuracy of TcPCO 2 to reflect PaCO 2 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.

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