These observations suggest that CO2 elimination is impaired by a flow-resistive mechanism and that the degree of airway obstruction modulates the rate at which PCO2 rises during expiration. However, despite the strong correlation between Rrs and the expired CO2 slope, the confidence intervals were wide. Therefore, the expired CO2 slope has limited clinical applicability in accurately predicting Rrs at the bedside (BJanch. ..et.. al 1994).
In the same study, a weak correlation was found between PaCO2 - PetCO2 gradients and Rrs. Because the PaCO2 - PetCO2 gradient is influenced by structural
airway abnormalities as well as by high and low VlQ regions, this may explain why some patients exhibit a large PaC02 - PetC02 gradient with normal Rrs and expired CO2 slope. Finally, low cardiac output states may increase the PaCO2 - PetCO2 gradient as pulmonary perfusion decreases while lung mechanical properties remain unchanged (B!aDch etlla/ 19.87,; Hess 1990). These observations suggest that CO2 elimination in critically ill patients is strongly modulated by lung, airway, endotracheal tube, and ventilator equipment resistances.
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