use low TV and FiO2 on ventilator for ECMO patients

Over four decades ago, the first large trial of extracorporeal membrane oxygenation (ECMO) in severe hypoxemic respiratory failure showed no benefit. This trial was conducted during a time when the effects of excessive pressure and volume on the development of ventilator-induced lung injury (VILI) were largely unappreciated. As a consequence, both ECMO and control patients were exposed to tidal volumes usually in excess of 12 to 15 mL/kg ideal body weight and peak pressures in excess of 35 cm H2O—conditions now known to be significant risks for VILI. Today, it is widely recognized that a major benefit of venovenous (VV) ECMO may be the ability to reduce mechanical ventilator settings to “safe” levels—generally FiO2, plateau pressure, and tidal volume (VT) as low as possible. To accomplish this, the two largest ECMO trials to date (EOLIA and LIFEGARDS) both were able to decrease VT significantly to levels approaching anatomic dead space (3-4 mL/kg ideal body weight), with plateau pressure reductions of 5 to 8 cm H2O. Positive end-expiratory pressure (PEEP), however, was maintained to prevent collapse-reopening injury.

Reducing the VT to levels approaching anatomic dead space is made possible because VV ECMO is very effective at clearing CO2. Indeed, some have speculated that a mechanical VT may not be needed at all—simply rely on CPAP and spontaneous breathing efforts. Data supporting this approach, however, are limited. In these trials, the independent effect of a sustained reduction in VT on a beneficial outcome was not specifically assessed. However, the conceptual benefits of very low VT management are thought to be severalfold. Tidal (dynamic) stress and strain on alveolar structures are minimized, thus reducing VILI. The end-inspiratory maximal (static) stress and strain (also VILI risk factors) are also reduced. The driving pressure (end-inspiratory plateau pressure − PEEP), a reflection of compliance during the tidal breath delivery, may also be reduced (better compliance) with VT reduction if end-inspiratory overdistension was occurring. Indeed, lower driving pressures were observed in both in EOLIA and LIFEGARDS.

An important reminder—VV ECMO use does not obviate the need to continue other conventional management strategies. VV ECMO does not supply all of the oxygenation needed to support patients in severe respiratory failure—PEEP and supplemental O2 are still needed and should be set to balance derecruitment/overdistension along with minimizing oxygen exposure. Additionally, proning and neuromuscular blockade as appropriate, ventilator synchrony adjustments, sedation, and fluid management protocols may all still be needed.1

Footnotes

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