Addressing Assumptions for the Use of Non-invasive Cardiac Output Measurement Techniques During Exercise in COPD

The multifactorial functional limitation of COPD increasingly demonstrates the need for an integrated circulatory assessment. In this study cardiac output (Qc) derived from non-inert (CO2-RB), inert (N2O-RB) gas rebreathing approaches and bioimpedance were compared to examine the limitations of currently available non-invasive techniques for exercise Qc determination in patients with chronic lung disease. Thirteen COPD patients (GOLD II-III) completed three constant cycling bouts at 20, 35, and 50% of peak work on two occasions to assess Qc with bioimpedance as well as using CO2-RB and N2O-RB for all exercise tests. Results showed significantly lower Qc using the N2O-RB or end-tidal CO2-derived Qc compared to the PaCO2-derived CO2-RB or the bioimpedance at rest and for all exercise intensities. End-tidal CO2-derived values are however not statistically different from those obtained using inert-gas rebreathing. This study show that in COPD patients, CO2-rebreathing Qc values obtained using PaCO2 contents which account for any gas exchange impairment or inadequate gas mixing are similar to those obtained using thoracic bioimpedance. Alternately, the lower values for N2O rebreathing derived Qc indicates the inability of this technique to account for gas exchange impairment in the computation of Qc. These findings indicate that the choice of a gas rebreathing technique to measure Qc in patients must be dictated by the ability to include in the derived computations a correction for either gas exchange inadequacies and/or a vascular shunt.