Fast and Slow Compliance: Time, in Addition to Pressure and Volume, is a Key Factor for Lung Mechanics

Static lung mechanics are considered state of the art in spite of the fact that they only provide a narrow view and do not represent the mechanical behavior of the lung during on-going tidal ventilation. Static measurements are usually cumbersome to perform and are uncommon in clinical practice. There is now ample proof of the importance of choosing a protective ventilatory strategy, which has been defined as ventilating with pressures between the lower and upper inflection point (LIP, UIP) [1, 2]. Determination of these two inflection points demands static or at least quasi static measurements. The definition of true static conditions is that a sufficiently long end-inspiratory and end-expiratory pause is used to not only stop gas flow in the airways, but also equilibrate visco-elastic forces of the lung tissue. It has been shown that this equilibration time is short and the tracheal pressure decreased as little as ∼ 2 cmH2O during the five seconds after instigation of an end-inspiratory pause [3]. This pressure fall is small compared to the pressure fall that occurs within milliseconds immediately after closing the inspiratory valve of the ventilator. The initial pressure drop is a result of obtaining no-flow conditions in the patient’s airways and the time is correlated to the endotracheal tube and patient airway resistance (R in cmH2O/L/s), the breathing circuit compliance (C in l/cmH2O) and the flow immediately before closing the valve: t = time constant = R × C In a typical case, the breathing circuit has a compliance of 0.5 × 10−3 l/cmH2O and a tube resistance of 6 cmH2O/l/s which gives a time constant of 3 ms. In this case the flow will decrease by 95% in three time constants, i.e., ∼ 10 ms.