Lung resistance and elastance are different in ex vivo sheep lungs ventilated by positive and negative pressures

Lung resistance (R_L) and elastance (E_L) can be measured during positive or negative pressure ventilation. Whether the different modes of ventilation produce different R_L and E_L is still being debated. Although negative pressure ventilation (NPV) is more physiological, positive pressure ventilation (PPV) is more commonly used for treating respiratory failure. In the present study, we measured lung volume, airway diameter, and airway volume, as well as R_L and E_L with PPV and NPV in explanted sheep lungs. We found that lung volume under a static pressure, either positive or negative, was not different. However, R_L and E_L were significantly higher in NPV at high inflation pressures. Interestingly, diameters of smaller airways (diameters <3.5 mm) and total airway volume were significantly greater at high negative inflation pressures compared with those at high positive inflation pressures. This suggests that NPV is more effective in distending the peripheral airways, likely due to the fact that negative pressure is applied through the pleural membrane and reaches the central airways via the peripheral airways, whereas positive pressure is applied in the opposite direction. More distension of lung periphery could explain why R_L is higher in NPV (vs. PPV), because the peripheral parenchyma is a major source of tissue resistance, which is a part of the R_L that increases with pressure. This explanation is consistent with the finding that during high frequency ventilation (>1 Hz, where R_L reflects airway resistance more than tissue resistance), the difference in R_L between NPV and PPV disappeared.