Use of pressure volume curves

Since respiratory muscle activity can alter intrathoracic pressure, the pressure volume curve is more easily obtained in the relaxed, fully ventilated patient. Both static and dynamic respiratory system compliance can be determined as the slope of the linear portion of the curve, i.e. where incremental pressure inflates the lungs. Below the lower inflexion zone the small airways are closed and expiration does not reach functional residual capacity. The lower inflexion zone therefore represents the appropriate setting for external PEEP to avoid gas trapping. Above the upper inflexion zone the lungs cannot inflate further. The upper inflexion zone therefore represents the maximum setting for peak airway pressure.

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