Rationale and literature findings
Although high tidal volumes (>10 ml/kg) and elevated airway plateau pressures (Pplat) may increase the amount of recruited volume at end expiration [
26,
27], large tidal volume ventilation can result in overdistension and excessive lung stress, especially in ARDS patients [
28]. In a seminal prospective RCT by the ARDS Network, a ventilatory strategy targeting a tidal volume of 6 ml/kg PBW and Pplat ≤ 30 cmH
2O was associated with reduced mortality in patients with ARDS compared with a strategy targeting a tidal volume of 12 ml/kg PBW and Pplat ≤ 50 cmH
2O [
6]. Mechanical ventilation with excessive tidal volumes can induce a systemic and pulmonary inflammatory cytokine response that may be attenuated by a lung-protective strategy [
29]. However, although setting the tidal volume according to PBW is an easy way to initiate protective ventilation, this strategy can result in different levels of lung stress and strain according to the amount and distribution of aerated lung tissue [
30].
Some studies have suggested that tidal volumes even less than 6 ml/kg may be preferable [
31], but higher PEEP levels may then be necessary to maintain oxygenation [
32]. In one study, the combination of lower tidal volume and higher PEEP significantly reduced hospital mortality compared to higher tidal volume and lower PEEP [
33]. The coexistence of severe acidosis may prevent strict adherence to these objectives unless addressed by other measures, such as the concurrent use of extracorporeal life support. Large increases in chest wall stiffness may cause Pplat to exceed the recommended upper limit of 30 cmH
2O, even when lung stretch is not excessive.
Unfortunately, the evidence supporting lower tidal volumes is not always applied, with a recent large international survey showing that tidal volume was kept at < 7 ml/kg PBW in only about 50% of patients with ARDS [
3]. It has been suggested that tidal volume should be titrated according to the PBW and not to the ideal body weight (IBW) because of a better relationship, in healthy subjects, of PBW with lung size [
34]. However, in patients with ARDS, the proportion of the lung available for ventilation is markedly decreased, which is reflected by low respiratory-system compliance [
35]. Therefore, it was recently suggested that tidal volume should be scaled to compliance using the driving pressure (∆
P = Pplat – PEEP). Indeed driving pressure is the ratio of tidal volume to compliance, the latter indicating the “functional” size of the lung. Driving pressure predicts outcomes better than any other ventilatory parameters in patients with ARDS, with values exceeding 15 cmH
2O of particular concern [
36]. Thus, observation of a low driving pressure may reinforce the relaxation of strict tidal volume or Pplat targets in patients with conflicting clinical priorities (e.g., a patient with severe acidosis and high PEEP requirements).