During controlled mechanical ventilation, the machine provides a fixed number of breaths per minute and remains totally unresponsive to the patient's efforts to alter frequency. This 'lock-out' mode demands constant vigilance to make appropriate adjustments for changes in ventilatory requirements and is used only in situations where pH and/or Paco2 must be tightly controlled. Most patients require sedation to ensure comfort and ablate breathing efforts. Under these conditions, assist-control has similar capability and offers additional advantages.
When the respiratory system is inflated passively, the mean airway pressure (an easily measured analog of mean alveolar pressure) becomes a valuable indicator of average lung volume. Increments in mean airway pressure tend to raise pleural pressure and impede venous return. Moreover, in patients with acute lung injury, mean airway pressure tends to reflect lung size and the efficiency of arterial oxygenation.
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