CO2 rebreathing model in COPD: blood-to-gas equilibration.

Autor: Loeppky, Jack A., Icenogle, Milton V., Caprihan, Arvind, Vidal Melo, Marcos F., Altobelli, Stephen, Altobelli, Stephen A
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Zdroj: European Journal of Applied Physiology; Nov2006, Vol. 98 Issue 5, p450-460, 11p, 2 Charts, 5 Graphs
Abstrakt: 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. [ABSTRACT FROM AUTHOR]
Databáze: Complementary Index
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