Pulmonary Compliance | lung Compliance | Physiology
Pulmonary compliance
Pulmonary compliance is defined as the change in lung volume per unit change in pressure.Dynamic compliance is the volume change divided by the peak inspiratory transthoracic pressure. Static compliance is the volume change divided by the plateau inspiratory pressure. With the initiation of an inspiratory breath the transthoracic pressure gradient increases to a peak value. This increase is a function of elastic resistance of the lung and chest wall as well as airway resistance. The pressure then falls to a plateau level as the gas redistributes in alveoli. Consequently dynamic compliance is always lower than static compliance. Ventilation normally occurs in the steep portion of the curve, whereas large changes in volume occur in response to small changes in pressure. However at low and high volumes, large changes in pressure result in minimal changes in volume. In diseased lungs in which compliance has dropped into the flat portion of the curve, the goal of mechanical ventilation is to return it to the steep portion. Excessive pressure applied by the ventilator results in ventilation at the top of the curve where the process once again becomes inefficient
Changes in lung volume and pleural pressure during a normal breathing cycle, which reflect the elastic and flow-resistant properties of the lung, are displayed as a pressure-volume loop The slope of the line that connects the end-expiratory and end-inspiratory points in the figure provides a measure of the dynamic compliance of the lung. The area that falls between this line and the curved lines to the right and left represents the additional work required to overcome flow resistance during inspiration and expiration, respectively.