Transcutaneous PCO2-based dead space ventilation is highly accurate to discriminate COPD patients from healthy controls at submaximal exercise

Background: Increased physiological dead space ventilation (Vd/Vt) at exercise reflects impairment of pulmonary gas exchange and is a sensitive marker of cardio-respiratory disease. At present, Vd/Vt is typically not measured during routine cardiopulmonary exercise testing (CPET) because its calculation requires PaCO₂, and thus arterial puncture. Instead, dead space ventilation is indirectly evaluated as a determinant of the ventilation (VE)/VCO₂ slope, which also depends on the PaCO₂ setpoint. We hypothesized that Vd/Vt calculations based on transcutaneous PCO₂ (PtcCO₂) measurement had better diagnostic characteristics than the VE/VCO₂ slope for the discrimination of healthy subjects from patients with COPD, a common disease with impaired pulmonary gas exchange. Methods: Nineteen healthy Controls and 24 COPD patients underwent CPET with monitoring of PtcCO₂. Areas under receiver operating characteristics curves (AUC) were calculated to assess diagnostic accuracy of CPET measurements for the discrimination of COPD and Controls. Results: The AUC for PtcCO₂-based Vd/V at VT1 (0.977) was significantly higher than for the VE/VCO₂ slope (0.660), SpO₂ at peak exercise (0.913), decrease in inspiratory capacity (0.719), and ventilatory reserve (0.708). At a threshold of 0.24, the sensitivity and specificity of PtcCO₂-based Vd/Vt were 100% and 84%, respectively. Conclusions: PtcCO₂-based Vd/Vt was the most accurate measurement to discriminate healthy controls from subjects with chronic lung disease associated with altered pulmonary gas exchange, ie COPD. Non-invasive monitoring of PtcCO₂ may be highly useful for routine CPET.