in the previous issue of critical care lu and coworkers , a group of well known scientists with long - term expertise in this area , address the issue of quantifying alveolar recruitment induced by the application of positive end - expiratory pressure ( peep ) in patients with acute respiratory distress syndrome ( ards ) . the current ' gold standard ' for this measurement is the computed tomography ( ct ) method [ 2 - 4 ] , in which an end - expiratory spiral ct scan of the whole lung obtained at a given peep level is compared with one obtained at zero end - expiratory pressure ( zeep ) . although repeatedly validated , this method requires considerable expertise and exposes the patient to risks associated with transporting them from the intensive care unit to the imaging facility . in their study , lu and coworkers compared the ct method with the pressure - volume ( p - v ) curve method , which , although currently used only in clinical research , could potentially be implemented at the bedside . first , from a scientific point of view , the approach is of great value . the p - v method has been used in several studies to evaluate the physiological effects of different ventilatory strategies in patients with ards [ 5 - 7 ] . according to the data presented by lu and coworkers , the alveolar recruitment achieved with this method tightly correlates with that obtained using the ' gold standard ' , but the authors point out that the broad limits of agreement between the two methods indicate that they are not interchangeable . however , in evaluating the results , one should keep in mind that their study design could have led to a bias against the p - v method . indeed , in 12 out of 19 patients the p - v curves at zeep were measured immediately after discontinuation of peep ( principally for safety reasons , as suggested by the attending physician ) . in contrast , in the majority of previous studies the p - v curves at zeep were recorded after a 1530 min period of mechanical ventilation at zeep , allowing complete lung de - recruitment to take place . the strong rationale behind this approach is that the shape of the p - v curve at zeep may differ substantially depending on whether it is measured after a period of zeep ventilation or immediately after removal of peep . one may argue that in some patients a brief period of zeep ventilation is not clinically acceptable because of the resulting potential for hypoxia ; undoubtedly this represents a limitation of the p - v method . nevertheless , the difference between the original p - v method and the modified version used by lu and coworkers in more than half of their patients might partly account for the broad limit of agreement with the ' gold standard ' method . the second important message of the report pertains to the potential usefulness of the p - v method in clinical practice . the p - v curve method requires the construction of two static p - v curves ( at peep and at zeep ) . these two curves must be plotted in the same volume - pressure coordinate system in order to relate both of them to the functional residual capacity ( frc ) of the respiratory system at the time of testing . consequently , the difference between the end - expiratory lung volume during mechanical ventilation at peep and the frc must be assessed . this is achieved by disconnecting the patient from the ventilator and measuring the exhaled volume during a prolonged expiration at atmospheric pressure . performing all of these measurements is at present virtually impossible at the bedside , but it could be achieved through relatively simple software able to adequately control the ventilator . interestingly , the option of measuring frc with the inert gas wash in - wash out technique is now commercially available , which may permit noninvasive estimation of the end - expiratory lung volume . should the measurement of peep - induced alveolar recruitment be implemented in the next generation of mechanical ventilators ? in other words , do we really need to measure peep - induced alveolar recruitment at the bedside ? the ' cornerstone ' ards network protocol using low tidal volume ventilation does not require measurement of respiratory mechanics except for plateau pressure recording . inspired oxygen fraction and peep are set according to an empiric table , aiming at the lowest peep level compatible with a blood oxygenation target . a subsequent ards network study , comparing the traditional lower peep strategy with higher peep , was inconclusive . in that study both the lower and higher peep strategies were table based . a recent study   challenged this approach , demonstrating that empirical peep setting frequently fails to induce alveolar recruitment and may increase the risk for alveolar over - inflation . furthermore , a subsequent randomized multicentre trial , confirming previous findings , clearly indicated that a physiological peep setting strategy , based on p - v curve measurements , may reduce mortality in ards patients . in this regard , the implementation of bedside measurement of p - v curves and alveolar recruitment could facilitate development of clinically applicable protocols for approaching measurement of respiratory mechanics and its interpretation . this could improve our clinical awareness of the complexity of the ards scenario while we await a definitive evidence - based approach to the peep titration dilemma . ards = acute respiratory distress syndrome ; ct = computed tomography ; frc = functional residual capacity ; peep = positive end - expiratory pressure ; p - v = pressure - volume ; zeep = zero end - expiratory pressure .
for positive end - expiratory pressure ( peep ) to have lung protective efficacy in patients with acute respiratory distress syndrome , it must increase the end - expiratory lung volume through alveolar recruitment while avoiding lung over - inflation . 
 peep may increase the end - expiratory lung volume either by increasing the proportion of aerated alveoli at end - expiration or by further inflating already ventilated lung regions . 
 the optimal peep regimen is still a matter of debate . in theory , 
 the ability to measure of peep - induced alveolar recruitment would be extremely useful in titrating peep at the bedside . 
 however , until now this measurement has been confined to clinical research settings . 
 interesting work by lu and coworkers , published in the previous issue of this journal , deals with the problem of measuring peep - induced alveolar recruitment . 
 the ' gold standard ' technique ( i.e. the computed tomography method ) is compared with the pressure - volume curve method . because implementation of the latter method at the bedside would be relatively simple , that report , in addition to its intrinsic scientific value , may have important clinical implications .