Positive Pressure Ventilation
NIV relies on the creation of positive pressure. Understanding the physiologic pathways involved is crucial to clinical decision-making. Positive pressure ventilation (PPV) applies a consistently positive airway pressure that results in increased laminar flow.
(See Figure 1.) This leads to airway stenting and elimination of dead space through recruitment of atelectatic alveoli, resulting in increased functional residual capacity and an increase in tidal volumes due to improved lung filling, directly increasing minute ventilation.5 However, the beneficial effects of PPV are not due simply to pulmonary recruitment. In patients with pulmonary edema due to decompensated heart failure, the impact of PPV may be due more to its hemodynamic effects. PPV increases intrathoracic pressure, and this increase, relative to extrathoracic compartments, can both decrease venous return and increase left heart output. In other words, PPV can decrease both preload and afterload.
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