Abstract
Rebreathing in a closed system can be used to estimate mixed venous \(P_{{\rm CO}_{2}}\;(P\bar{v}_{{\text{CO}}_2})\) and cardiac output, but these estimates are affected by \(\dot{V}_{\rm A}/\dot{Q}\) heterogeneity. The purpose of this study was to validate a mathematical model of CO2 exchange during CO2 rebreathing in 29 patients with chronic obstructive pulmonary disease (COPD), with baseline arterial \(P_{{\rm CO}_{2}}\;(\hbox{Pa}_{{\rm CO}_{2}})\) ranging from 28 to 60 mmHg. Rebreathing increased end-tidal \(P_{{\rm CO}_{2}}\;(\hbox{PET}_{{\rm CO}_{2}})\) by 20 mmHg over 2.2 min. This model employed baseline values for inspired (bag) \(P_{{\rm CO}_{2}},\) estimated \(P\bar{v}_{{\text{CO}}_2},\) distribution of ventilation and blood flow in one high \(\dot{V}_{\rm A}/\dot{Q}\) and one low \(\dot{V}_{\rm A}/\dot{Q}\) compartment, the ventilation increase and conservation of mass equations to simulate time courses of \(\hbox{PI}_{{\rm CO}_{2}},\hbox{PET}_{{\rm CO}_{2}},\; P\bar{v}_{{\text{CO}}_{2}}\) and \(\hbox{Pa}_{{\rm CO}_{2}}.\) Measured \(\hbox{PI}_{{\rm CO}_{2}}\) and \(\hbox{PET}_{{\rm CO}_{2}}\) during rebreathing differed by an average (SEM) of 1.4 (0.4) mmHg from simulated values. By end of rebreathing, predicted \(P\bar{v}_{{\text{CO}}_2}\) was lower than measured and predicted \(\hbox{Pa}_{{\rm CO}_{2}},\) indicating gas to blood CO2 flux. Estimates of the ventilatory response to CO2, quantified as the slope (S) of the ventilation increase versus \(\hbox{PET}_{{\rm CO}_{2}},\) were inversely related to gas-to-blood \(P_{{\rm CO}_{2}}\) disequilibria due to \(\dot{V}_{\rm A}/\dot{Q}\) heterogeneity and buffer capacity (BC), but not airflow limitation. S may be corrected for these artifacts to restore S as a more valid noninvasive index of central CO2 responsiveness. We conclude that a rebreathing model incorporating baseline \(\dot{V}_{\rm A}/\dot{Q}\) heterogeneity and BC can simulate gas and blood \(P_{{\rm CO}_{2}}\) in patients with COPD, where \(\dot{V}_{\rm A}/\dot{Q}\) variations are large and variable.
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Acknowledgments
We thank the subjects who took part in this study for their time and cooperation. We also appreciate the facilities and funding made available for this study from the Lovelace Medical Foundation and New Mexico Resonance. A research grant from the Cardiology department, University of New Mexico School of Medicine to Milton Icenogle in the Cardiology section of the VA Medical Center in Albuquerque also provided support for this project.
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Laboratory of origin: These experiments were performed at the Lovelace Medical Foundation and at New Mexico Resonance in Albuquerque NM.
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Loeppky, J.A., Icenogle, M.V., Caprihan, A. et al. CO2 rebreathing model in COPD: blood-to-gas equilibration. Eur J Appl Physiol 98, 450–460 (2006). https://doi.org/10.1007/s00421-006-0288-4
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DOI: https://doi.org/10.1007/s00421-006-0288-4