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

Abstract

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.

Keywords:

• Arterial blood gas
• transcutaneous carbon dioxide partial pressure
• exercise testing
• dead space
• end-tidal PCO₂

Acknowledgements

We are grateful to Xiuqing Guo and Jingyi Tan, for their help in mathematical and statistical analysis during the data preparation.

Conflicts of interest

None of the authors report any conflict of interest

Author’s contributions

William W. Stringer contributed to the study conception, data collection, data analysis, abstract and manuscript preparation, and is the guarantor of the manuscript. Janos Porszasz, Richard Casaburi, Harry B. Rossiter contributed to study conception. Robert Cao contributed to subject recruitment. Janos Porszasz, Robert Cao, Robert Calmelat, Susan Corey contributed to data collection. Min Cao, Arin Orogian, Fang Lin, Janos Porszasz contributed to data analysis. Arin Orogian, Janos Porszasz contributed to abstract preparation. Min Cao, Janos Porszasz, Harry B. Rossiter, Richard Casaburi contributed to manuscript preparation. All authors approved the final submitted version.

Role of the sponsors

The sponsor had no role in the design of the study, the collection and analysis of the data, or the preparation of the manuscript.

Additional information

Funding

This study was supported by a grant from the Pulmonary Education and Research Foundation.