LUNG PROTECTIVE STRATEGIES

Lung Protective Strategies

Lung Protective Strategies

Introduction

The premise that a successful lung protective strategy is predicated exclusively on the application of a specific low tidal volume (Vt) is incomplete. In the most widely quoted Acute Respiratory Distress Syndrome Network (ARDS-net) study, it is true that Vts were limited in the experimental group; however, plateau pressure (Ppl) was a significant determinant of how the Vt was adjusted in both the control and experimental groups. In this work, Vt was titrated to a Ppl and patient dyspnea, with an allowable Vt of up to 8 mL/kg (ideal body weight), with Ppl remaining below 30 cm H2O. The Vts (±SD) at days 1, 2, and 7 were 6.2 ± 0.9, 6.2 ± 1.1, and 6.5 ± 1.4, respectively. Considering the SDs, patients were treated with Vts exceeding 7 mL/kg and some approaching 8 mL/kg.

Discussion

Lung pathology in ARDS is far from homogeneous, different lung regions will have variable compliance characteristics and different time constants and will expand differentially depending upon their individual characteristics. As Gattinoni et al point out in their recent and rather cogent review of the anatomical and physiologic framework of ventilator-induced lung injury (VILI), “high tidal volume induces VILI by augmenting the pressure heterogeneity at the interface between open and constantly closed units VILI occur only when a given threshold is exceeded.” These authors also emphasize that lung expansion force can be divided into 3 components: (1) the force needed to overcome alveolar surface tension, (2) the force required to distend the lung fibrous tension, and (3) the force necessary to expand the chest wall. Hence, the safety or harmful effects of a particular Vt or airway pressure are dependent on several factors, including the underlying character of the lung and the chest wall. Providing that ventilation is delivered within the physiologic limits of the lung and the total lung capacity is not exceeded, mechanical ventilation is likely to be safe.

Transpulmonary pressure is arguably the most effective means by which to assess safe ventilator limits; however, it will often be difficult to determine this measurement, and there will likely be regional lung differences. It would seem that avoidance of arbitrary Vt parameters and targeting maintenance of the easily measurable Ppl at reasonable levels (generally, =30 cm H2O) should make injurious overdistension less likely.

In the current study, the authors accept a Vt of less than 6.5 mL/kg (ideal body weight and corrected for circuit compliance) as adherence to a lung protective strategy; however, we are not given intraoperative Ppl because this parameter was not recorded. Based on the pre- and postoperative Ppl values, Vts higher than the prescribed 6.5 mL/kg (corrected) might likely have remained within the Ppl parameters of the ARDS-net study ; hence, these volumes may have been tolerable without exacerbating lung injury. Thus, current expectations for ventilator management may not actually have been violated intraoperatively and might explain the lack of a negative effect of “higher Vt” ventilation.

In addition, there are generally two factors that determine the extent ...