Ventilation Strategies: Tidal Volume and PEEP

Abstract

Acute respiratory distress syndrome (ARDS), defined as an increment in the lung alveolar-capillary membrane permeability causing a pulmonary edema rich in proteins, has been recently reclassified as mild, moderate, and severe according to Berlin definition. The extravasation of plasma inside the alveolar space turns an air-filled lungs into a heavy high-osmotic pressure liquid-filled lungs. As a consequence, the higher weight of the lungs under the action of the gravity force predisposes the lowermost lung regions to collapse provoking a higher intrapulmonary shunt, a refractory hypoxemia, and a decrease in lung compliance. The functional alterations of the respiratory system are expressed by a decrease in the functional residual capacity (FRC) and a shift of the respiratory system pressure-volume curve down and to the right. It is well documented that lower tidal volumes (6 mL/kg of predicted body weight) compared to higher tidal volumes (12 mL/kg of predicted body weight) associated with PEEP levels titrated by a PEEP/FIO2 table reduced mortality in a randomized, clinical trial that analyzed 861 ARDS patients (ARMA trial). So, it is important to adjust tidal volume to lung size that depends on the height and sex but, more importantly, to adjust the tidal volume to functional lung size that depends on the ARDS severity (lung compliance), sex, height, and chest wall compliance during controlled ventilation. During assisted ventilation another two factors must be added to the interaction between the FRC and above tidal ventilation inside the thoracic cage: the patient’s negative inspiratory efforts and the pressures that resulted from the desynchronization between the patient and the mechanical ventilator. Recently, Amato and colleagues showed that driving pressure (ΔP=VT/CRS), in which VT is intrinsically normalized to functional lung size (instead of predicted lung size in healthy persons), would be an index more strongly associated with survival than VT or PEEP in patients who are not actively breathing. Recently, three large clinical trials, including ARDS patients ventilated with low tidal volume, have compared different PEEP strategies (high vs. low), but none of them could show a significant difference in mortality. Moreover, a recent meta-analysis has pooled those trials, revealing some combined benefit of the high PEEP strategy; still, the survival benefit was modest and limited to the subgroup of ARDS patients with PaO2/FiO2 <200. Matos and colleagues showed in 51 ARDS patients that keep sufficient PEEP levels after maximal recruitment maneuvers is crucial in ARDS patients. Evidence showed that should be an interaction between high PEEP and low tidal volume during invasive mechanical ventilation in ARDS patients, and static pressure-volume curve of the respiratory system should be the best window to ventilate them. Finally, a recent meta-analysis showed that in the era of low tidal ventilation, prone position improved survival in moderate/severe ARDS patients. If PEEP titration during prone position should improve, survival in moderate/severe ARDS is still a matter of debate.