Monitoring-based Ventilation Strategies in ARDS
Je Hyeong Kim1
1Department of Critical Care Medicine, Korea University Ansan Hospital, Republic of Korea
Despite development in intensive care medicine after first description of acute respiratory distress syndrome (ARDS), it was recently reported that hospital mortality in severe ARDS was 46.1%. Most deaths of ARDS patients is attributable to multiple organ dysfunction syndrome (MODS) rather than primary respiratory failure. This is mainly due to the ventilator-induced lung injury (VILI) which is the additional inflammatory damage caused by mechanical ventilation in ARDS. VILI plays a pivotal role in the initiation and/or propagation of a systemic inflammatory response that leads to MODS in majority of patients
The fundamental cause of development of VILI is the heterogeneity in acute lung inflammation of ARDS. In ARDS lung, severe inflammation with consolidation and/or atelectasis is present in the dependent part, but the non-dependent part is relatively normal. If mechanical ventilation with traditional high tidal volume without positive end-expiratory pressure (PEEP) is applied, non-dependent part of the lung is damaged by physical stretch injury and dependent part of the lung is injured by shearing force caused by the reopening and collapse of alveoli.
Lung protective ventilation (LPV) strategies are the measures to prevent or minimize VILI, and include low tidal volume ventilation, high PEEP, prone position ventilation, and alveolar recruitment maneuver. Excepting low tidal volume ventilation and prone position ventilation strategies, the clinical evidences of high PEEP and alveolar recruitment maneuver strategies are controversial and additional confirmatory trials are needed.
Among LPV strategies, high PEEP strategy could be optimized with a monitoring-based approach. High PEEP can cause detrimental effects such as end-inspiratory alveolar overdistention, increased in intrapulmonary shunt and dead space, and cor pulmonale. Therefore, decision of optimal PEEP is clinically important, and esophageal pressure monitoring is the best way to decide. In the EPVent trial, a pilot study, the decision of PEEP with esophageal pressure measurement resulted in significantly higher oxygenation and transpulmonary end-expiratory pressure and decreasing trends in 28-day mortality. Based on these results, the EPVent-2 trial was performed and reported recently. However, esophageal pressure guided PEEP resulted in no significant difference in death and days free from mechanical ventilation.
Recently, there was an important progress in monitoring-based ventilation strategies in ARDS. It is driving pressure (DP), in which tidal volume is intrinsically normalized to functional lung size. DP can be routinely calculated for patients who are not making inspiratory efforts as the plateau pressure minus extrinsic PEEP. According to the multilevel mediation analysis to analyze individual data from 3,562 patients with ARDS enrolled in nine previously reported randomized trials, decreases in DP owing to changes in ventilator settings were strongly associated with increased survival. Currently recommended safe limits of DP is 15 cmH2O. In ARDS patients with low tidal volume, if the DP is above 15 cmH2O after PEEP titration, tidal volume should be reduced that the DP is below 15 cmH2O. However, the effect of DP was suggested from retrospective analyses through a very sophisticated statistical procedure, clinical trials need to be designed in which ventilator changes are linked to achieve changes in DP.