Key messages

• A certain amount of PEEP (or CPAP in the spontaneously breathing patient) can be considered 'physiological' up to the time of extubation in all forms of acute lung failure.

• In severe lung edema most alveolar spaces are collapsed in paravertebral areas and there is a biphasic P-V relationship, corresponding to an important recruitment occurring at a certain level of PEEP in these regions. This 'critical' level of PEEP corresponds to the lower inflection point of the P-V curve (Pflex).

• PEEP is not solely responsible for the recruitment of alveolar space; tidal volume and end-inspiratory airway pressure also contribute to this effect.

• At zero or low levels of PEEP, mechanical tidal ventilation is preferentially distributed to non-dependent lung zones. With increasing PEEP, more tidal volume is shifted to dependent regions in the supine patient.

• When alveolar collapse occurs during expiration, the next positive pressure breath must 'snap open' this alveolar unit before gas can enter. Repetitive collapsing and snapping open appears to potentiate the shear stress and parenchymal injury from mechanical ventilation. PEEP seems to decrease the severity of ventilator-induced lung damage.

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