Permissive hypercapnia.
There has been increasing recognition that mechanical ventilation can cause acute parenchymal lung injury (ventilator-induced lung injury, or VILI) in addition to the more widely recognized forms of barotrauma. Furthermore, in patients with acute lung injury, this type of injury may cause considerable morbidity and mortality. Subsequently, the goals of mechanical ventilation have been altered to avoid this outcome. In patients with relatively normal lungs who are receiving mechanical ventilation because of neuromuscular dysfunction or impaired conscious level or for short-term postoperative support, maintaining normal blood-gas tensions without risk of VILI is usually easy. In patients with acute asthma, chronic obstructive pulmonary disease, or acute lung injury, however, accepting abnormal blood-gas tensions, particularly an elevated PaCO2 (permissive hypercapnia), to improve survival and reduce complications is frequently necessary. Extensive experience has shown that ventilated patients usually tolerate moderate hypercapnia and frequently some degree of hypoxemia in the absence of shock, anemia, severe cardiac disease, or other contraindications.