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Critical Care Medicine


Answer 1
  1. Decrease the tidal volume to 342 mL (6 mL/kg IBW) and increase the respiratory rate to 34 breaths/min. To date, no pharmacologic intervention has been shown to reduce mortality in ARDS. The only therapy that has reduced mortality in ARDS is a careful ventilator strategy aimed at minimizing alveolar overdistension by limiting the patient’s tidal volume and plateau (or pause) pressure. In a study of groups ventilated with tidal volumes of 6 mL/kg IBW and those ventilated at 12 mL/kg IBW, the 6 mL/kg group (plateau airway pressures maintained > 30 cm H2O) had a 9% absolute reduction in mortality and spent approximately 2 fewer days on the ventilator.1 A corollary to the low tidal volume strategy is that Pco2 may rise due to the decreased minute ventilation. A strategy of “permissive hypercapnia,” in which the Pco2 is allowed to rise and induce a respiratory acidosis, has been shown to be safe in patients with ARDS.2 Increasing tidal volume and respiratory rate is incorrect because a tidal volume of 600 mL would be overdistending and injurious; the pH does not need to be normalized. Higher PEEP was not associated with an excess of barotraumas in a large randomized trial.3 Because there is no evidence to suggest that an Sao2 of 98% is superior to 89% and since oxygen therapy can induce lung injury, the goals for oxygenation in patients with ARDS are Sao2 of 88% or greater or Pao2 of 55 mm Hg or greater.1 While there is no absolute threshold for oxygen toxicity, most practitioners attempt to reduce the Fio2 below 60% when possible. Tidal volume should be based upon IBW, not actual body weight.

    REFERENCES
    1. Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. The Acute Respiratory Distress Syndrome Network. N Engl J Med 2000;342: 1301-8.

    2.  Hickling KG, Henderson SJ, Jackson R. Low mortality associated with low volume pressure limited ventilation with permissive hypercapnia in severe adult respiratory distress syndrome. Intensive Care Med 1990;16:372-7.

    3. Brower RG, Lanken PN, MacIntyre N, et al. Higher versus lower positive end-expiratory pressures in patients with the acute respiratory distress syndrome. N Engl J Med 2004; 351:327-36.

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