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Constant‐flow ventilation in canine experimental pulmonary emphysema

The efficacy of constant‐flow ventilation (CFV) was investigated in eight mongrel dogs before (control‐phase) and after development of papain‐induced panlobular emphysema (PLE‐phase). For CFV, heated, humidified and oxygen‐enriched air was continuously delivered via two catheters positioned within each mainstem bronchus at flow rates (V) of 0.33, 0.5 and 0.66 1/s. Data obtained during intermittent positive pressure ventilation (IPPV) served as reference. In the control‐phase, Pao2 was lower (P≤0.05) and alveolo‐arterial O2 difference (P(a‐a)O2 was higher (P≤0.01) during CFV at all flow rates when compared with IPPV. This may be due to inhomogeneities of intrapulmonary gas distribution and increased ventilation‐perfusion (Va/Q·) mismatching. Paco2 and V· showed a hyperbolic relationship; constant normocapnia (5.3 kPa) was achieved at 0.48 ± 0.21 1/s (≤5.3). Development of PLE resulted in an increase of functional residual capacity (FRC), residual volume (RV) and static compliance (Cstat) (P≤0.05). Pao2 had decreased and P(a‐a)O2 had increased (P≤0.05), indicating moderate pulmonary dysfunction. Oxygenation during CFV was not significantly different in the PLE‐phase when compared with the control‐phase. Paco2 and V· showed a hyperbolic relationship and V·5.3 was even lower than in the control‐group (0.42 ± 0.13 1/s). In dogs with emphysematous lungs CFV maintains sufficient gas exchange. This may be due to preferential ventilation of basal lung units, thereby counterbalancing the effects of impaired lung morphometry and increased airtrapping. Conventional mechanical ventilation is more effective in terms of oxygenation and CO2‐elimination.