No detailed clinical information is available for guidance regarding the maximum peak and mean alveolar pressures that can be applied safely for extended periods without inducing alveolar damage or retarding lung healing. Clearly, the answer differs between disease stages and among individual patients. The stiffness of the chest wall influences the airway pressure that can be tolerated without overdistension. Moreover, alveolar volumes and stresses undoubtedly vary from site to site within the damaged lung. If avoiding ventilator-induced lung damage is the goal, the pressure applied to the endotracheal tube should allow for the distensibility and vulnerability of all lung units. Although failure to preserve a certain minimum end-expiratory transalveolar pressure intensifies pre-existing alveolar damage in the laboratory setting, this phenomenon has not yet been clearly demonstrated in humans. Moreover, once recruitment has been completed, additional PEEP is probably ineffectual or damaging. Consequently, expert opinion differs on whether applying the least PEEP that accomplishes adequate gas exchange or, alternatively, guaranteeing some minimum value of end-expiratory alveolar pressure is the best course to follow within the first few days of onset of illness. Periodic application of sustained high inflating pressures to recruit unstable lung units (recruiting maneuvers) continues to be advocated by some knowledgeable investigators, particularly when small tidal volumes (< 4-5 ml/kg) are used or high-frequency ventilation is employed. PEEP should be withdrawn later in the disease process, particularly if no inflection region can be identified on the static pressure-volume curve of the respiratory system. Because tidal compliance depends on tidal volume, the appropriate tidal volume to select undoubtedly varies with the level of PEEP and vice versa.
There is no firm consensus regarding the contribution of vascular pressures, position changes, infections, inspired oxygen concentration, and other clinical variables on the incidence or intensity of ventilator-induced lung edema. Furthermore, detailed information is lacking regarding ventilation pressures and patterns of inflation which are safe to apply for extended periods. At fractional inspired oxygen (Fio 2) levels below 0.Z, limiting airway pressure to 'safe' levels generally takes precedence over limiting Fio2. In the absence of definitive data obtained in a clinical context, some practitioners increase end-expiratory lung volume in an attempt to minimize Fio2, whereas others prefer to use higher Fio2 rather than increase peak, mean, and end-expiratory airway pressures. Whether methods for achieving a similar mean airway pressure (such as PEEP and inverse ratio ventilation) differ with respect to risks and benefits has not been adequately clarified.
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