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.