Lung mechanics and gas exchange during pressure-control ventilation in dogs: Augmentation of CO₂ elimination by an intratracheal catheter

Increased awareness of pressure-related injury to the alveolar-capillary interface has renewed interest in modes of ventilation that limit alveolar distention such as pressure-controlled ventilation (PCV). We examined respiratory system mechanics and gas exchange during PCV in six dogs. Our data conformed to the predictions of our single-compartment mathematical model of respiratory dynamics during PCV (J Appl Physiol 1989; 67:1081-92). For a fixed pressure (Pset) and inspiratory time fraction (TI/Ttot) (15 cm H₂O and 0.3, respectively), minute ventilation (V̇E) reached a well-defined plateau as frequency (f) increased from 10 to 50 breaths/min and tidal volume (VT) fell progressively. Concomitantly, the physiologic dead-space fraction (VD/VT) increased from 0.50 ± 0.04 to 0.85 ± 0.04, and arterial P(CO₂) (Pa(CO₂)) rose from 39 ± 4 to 76 ± 12 mm Hg. At a fixed combination of frequency, applied pressure, and TI/Ttot (40 breaths/min, 15 cm H₂O, and 0.3), V̇E did not change when we introduced fresh gas continuously from an intratracheal catheter. However, Pa(CO₂) and VD/VT fell progressively as catheter flow increased from zero to 14 L/min (60 ± 12 to 40 ± 12 mm Hg and 0.83 ± 0.03 to 0.25 ± 0.14 mm Hg, respectively). We conclude that during PCV at a fixed Pset and TI/Ttot increasing frequency caused VT to fall and V̇E to reach a plateau. Declining VT was associated with a rise in Pa(CO₂) because of a subsequent fall in alveolar ventilation. Insufflating fresh gas by an intratracheal catheter increased alveolar ventilation and improved CO₂ elimination by washing out the anatomic dead space. These results suggest that such techniques may be a useful clinical adjunct to conventional mechanical ventilation.