The level of PEEP should be chosen according to the clinical situation and its effects on gas exchange ( Table.!.). Blood gas measurements show a significant improvement in arterial oxygenation when functional gas exchange units are opened and participate in gas exchange. However, alveolar overdistension of functional
alveolar units is more difficult to recognize. Indeed, overdistension typically produces an increase in dead-space ventilation by an excessively high VtQ ratio but rarely causes any fall in Pao2. The dead-space fraction of tidal ventilation ( VD/VT) also depends on tidal volume and cardiac output.
Table 1 Clinical guidelines for setting PEEP
Alveolar recruitment and overdistension can be assessed more precisely by recording a static P-V curve of the respiratory system (Suter efa/ 1975). In general this allows identification of the lower and upper inflection points ( Pflex). The lower inflection point corresponding to alveolar recruitment can easily be identified in early phases of acute lung injury and in lung edema. This inflection point is generally not found in later phases of acute respiratory distress syndrome, in pneumonia, or during acute exacerbations of chronic pulmonary dysfunction. PEEP should be set above the lower inflection point to improve pulmonary gas exchange and mechanics, and also to avoid potential lung damage by stretch injury exacerbated by opening and closing of lung units during tidal ventilation. The upper inflection area of the P-V curve corresponds to the limit above which additional volume or pressure application results in overdistension and possible (high-pressure) lung damage. Recording a static P-V curve requires a heavily sedated or paralyzed patient, but it also allows determination of the compliance of the total respiratory system.
The concept of 'optimal compliance'(SutereL§/ 1975) is similar to choosing PEEP above the lower inflection point; tidal ventilation in this range ensures recruitment of collapsed areas and avoids overdistension. Assessments of respiratory mechanics must be repeated regularly as they change due to differences in tidal volume, the amount and location of interstitial edema, changes in tissue elasticity and consolidation, and parenchymal remodeling.
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