Inflation and deflation limbs of the PV curve : how to use them

In our opinion the best way to deal with the PEEP issue is to consider first the volume-pressure curve of the respiratory system and, second, of the lung. It must be noted that at the same pressure the inspiratory volume is lower than the expiratory one and, conversely, the pressure required to reach a given volume is greater along the inspiratory limb. This extra-pressure is physiologically spent to overcome the forces which link together the water molecules in the inner surface of the alveoli, to slide each other the different strata of the extracellular matrix (tissue resistance) and, when atelectasis are present to open them (recruitment). It is important to stress that the opening pressure of the different lung regions which are collapsed, for a variety of reasons, span along the whole volume-pressure curve, as consistently shown in experimental and human ARDS. It worth noting that at the inflation of 30 cmH2O a consistent part of the potentially recruitable lung, which may be estimated from 15 to 30%, remains closed, as the opening pressures of these regions span from30 to 45-60 cmH2O. Therefore, it is intuitive, but often forgotten, that tailoring PEEP on the inflation limb of the volume-pressure curve, or, more in general, on the inspiration, may be misleading, as PEEP is the pressure applied during the expiratory phase. In presence of recruitable regions the inspiratory pressures almost always lead to improvement of gas exchange as, at least during the inspiratory time, more lung volume is available and less blood is flowing through it. During the deflation less pressure is necessary to maintain a given volume as the extra-pressure required to maintain the same volume during inspiration is no longer necessary. Therefore observing the shape of the volume-pressure curve it is possible to observe that the compliance at the beginning of the deflation is usually very low (small change in volume result in great increase in pressure) due to the possible overdistension of the lung in these pressure-volume regions. The phenomenon is greater higher is the end-inspiratory pressure used when building the volume-pressure curve. Accordingly the increase in compliance during deflation is a normal phenomenon not necessarily linked to recruitment and de-recruitment. The derecruitment in ARDS, as assessed by loss of aeration with the CT scan, begins at pressures close to 20 cmH2O and then progressively increases. Note that, while the inflation/recruitment curves are very similar the deflation/derecruitment curves progressively diverges, being the derecruitment (complete loss of gas in the alveolar units)lower than the aeration, i.e. decrease of gas within the pulmonary units. It must be noted that when the compliance is measured there is a shift from the expiratory to the inspiratory volume/pressure curve . During the deflation for a given decrease of pressure, as an example 5 cmH2O, the computed compliance increases any time the inspiratory and expiratory limbs progressively diverge, remains constant if the 2 limbs run parallel and decrease again when the 2 limbs converge. It must be noted that this behavior is exactly the opposite if we progressively increase the PEEP, increases when they diverge, is equal when they are parallel and decreases when they converge.