Closing volume is the volume of lung inflated when small airways in the dependent parts of the lung begin to collapse during expiration. In normal health, closing volume is less than FRC and accounts for the residual volume (RV) of the lung at the end of expiration. If closing volume encroaches on FRC, airway closure may occur during normal expiration and decrease ventilation to areas distal to the closure, worsening the ventilation/perfusion relationship (V/Q). FRC usually remains greater than closing volume, but closing volume slowly increases with age. In the supine position, closing volume exceeds FRC by the mid-40s, and in the erect position by 60 years of age. The reduction in FRC during general anaesthesia reduces FRC below closing volume even earlier, so young patients may have increased V/Q mismatch.
Compliance describes the elastic properties of various parts of the respiratory system. Compliance represents a volume change per unit change in pressure (200 ml/cm H2O in the normal lung). The total respiratory compliance consists of combined lung and chest wall compliance and is normally 70–80 ml/cm H2O.
Static compliance (alveolar stretchability) is measured when there is no flow activity at the end of inspiration (Figure 8).
Dynamic compliance describes the change in volume as pressure changes during actual gas flow through the respiratory cycle (Figure 9).
The static compliance curve can be used to select the ideal level of PEEP for a patient in the ICU (Figure 8). This level of PEEP corresponds to a point on the favourable part of the pressure–volume curve for alveoli, maximizing oxygenation and minimizing over-distension. PEEP should be increased to the critical opening pressure for most of the alveoli (the lower inflection point) at which point most of the collapsed alveoli open and the lung becomes more compliant. Over-distension of alveoli occurs at the flatter and top part of the curve (upper inflection point) and the lung becomes less compliant. The advantages of using PEEP are given in Figure 10.
The time constant of a ‘lung compartment’ is a function of its resistance and compliance. The time constant (tau) expresses how quickly a compartment can react to an alteration of pressure and gives an indication of the filling or emptying velocity of a lung compartment. The lung consists of a large number of compartments with variable time constants. This heterogeneity is often exaggerated with lung disease (e.g. pneumonia, pulmonary fibrosis). The more inhomogeneous the lung ventilation, the wider the spectrum of regional time constants. This causes variation in the filling and emptying periods and the filling volumes for individual compartments. At a given pressure, a compartment with high resistance and good compliance fills slowly with a resulting large volume (asthma). Conversely, a compartment with poor compliance and low resistance fills quickly, resulting in a smaller volume (e.g. pulmonary fibrosis).
During volume-controlled ventilation with an end-inspiratory pause, pendelluft arises between compartments with different time constants. The greatest part of the inspired volume is taken in by the compartment with the quickest time constant. During the end-inspiratory pause the lung redistributes its volume into various compartments depending on the different time constants of the alveolar units. Inhaled gas that has already taken part in gas exchange flows from the quicker to the slower compartments (pendelluft). This is not important in the normal lung, but it may play a role in reducing overall lung compliance and oxygenation in chronic obstructive pulmonary disease (COPD).
In adult respiratory distress syndrome (ARDS) it may be difficult for inflated lung units to equilibrate with stiff lung units during an inspiratory pause (Laplace equation P=4T/r, where P is the pressure, T is the surface tension and r is the radius of the unit). The effects of surface tension are important when surfactant is depleted by disease and small lung units empty into larger ones.
Work of breathing
The work of breathing (Figure 9) is the work required to move the chest wall and lungs during inspiration and expiration. In this context, it is most convenient to measure work as the product of pressure and volume. This can be explained by considering the dynamic pressure–volume relationship in quiet inspiration and quiet expiration. In spontaneous respiration, the work of breathing involves the work necessary to overcome elastic forces and the work to overcome the non-elastic forces or the flow resistance of the airways.
The area on the right of the gradient line in Figure 9 represents the work expended to overcome airway resistance during inspiration. The area to the left of the line relates to the work expended to overcome elastic forces during passive expiration in the normal lung. In obstructive ventilation disorders, more work is needed to overcome the flow resistance, particularly if positive intrapleural pressures are generated in expiration (Figure 9). In restrictive disorders, more elastic respiratory work is required during inspiration. In the ICU, respiratory work is further increased in intubated patients because of the increased flow resistance of the tracheal tube and ventilator tubing.
© 2003 The Medicine Publishing Company Ltd