Combination of constant-flow and continuous positive-pressure ventilation in canine pulmonary edema

Constant-flow ventilation (CFV) maintains alveolar ventilation without tidal excursion in dogs with normal lungs, but this ventilatory mode requires high CFV and bronchoscopic guidance for effective subcarinal placement of two inflow catheters. We designed a circuit that combines CFV with continuous positive-pressure ventilation (CPPV; CFV-CPPV), which negates the need for bronchoscopic positioning of CFV cannula, and tested this system in seven dogs having oleic acid-induced pulmonary edema. Addition of positive end-expiratory pressure (PEEP, 10cmH₂O) reduced venous admixture from 44 ± 17 to 10.4 ± 5.4% and kept arterial CO₂ tension (Pa(CO₂)) normal. With the innovative CFV-CPPV circuit at the same PEEP and respiratory rate (RR), we were able to reduce tidal volume (VT) from 437 ± 28 to 184 ± 18 ml (P < 0.001) and elastic end-inspiratory pressure (PEI) from 25.6 ± 4.6 to 17.7 ± 2.8 cmH₂O (P < 0.001) without adverse effects on cardiac output or pulmonary exchange of O₂ or CO₂; indeed, Pa(CO₂) remained at 35 ± 4 Torr even though CFV was delivered above the carina and at lower (1.6 l · kg^-1 · min^-1) flows than usually required to maintain eucapnia during CFV alone. At the same PEEP and RR, reduction of VT in the CPPV mode without CFV resulted in CO₂ retention (Pa(CO₂) 59 ± 8 Torr). We conclude that CFV-CPPV allows CFV to effectively mix alveolar and dead spaces by a small bulk flow bypassing the zone of increased resistance to gas mixing, thereby allowing reduction of the CFV rate, VT, and PEI for adequate gas exchange.