Volume-controlled modes of ventilation would appear to be ideal since they guarantee the total volume delivered to the patient, ensuring the level of alveolar ventilation (Table..!). However, in recent years, there has been increasing focus on the potential detrimental consequences of mechanical ventilation and specifically on ventilation-induced lung injury. A key factor augmenting lung injury is likely to be the degree of end-inspiratory lung stretch. In patients with acute respiratory distress syndrome there is marked regional heterogeneity in compliance, with preferential ventilation to the more compliant regions. In volume-controlled ventilation there is no specific control of regional inflation and the more compliant regions may become overdistended, causing lung injury. This may be avoided by limiting airway pressures or decreasing tidal volumes. The use of pressure-controlled ventilation provides an upper limit to alveolar pressures even if the respiratory system mechanics change (Slutsky.1993). Pressure-controlled ventilation appears to be well suited to this pressure-targeted lung-protective strategy.
Table 1 Comparison of volume-controlled and pressure-controlled ventilation
Pressure-controlled ventilation has also been used with a prolonged inspiratory time (see discussion of iQyerse ratio. ventilation below) in an attempt to improve oxygenation. The rationale underlying this approach is that with inverse ratio ventilation it is possible to increase mean airway pressure, while minimizing end-inspiratory pressures. However, not all patients benefit and identifying those who do is difficult. No controlled study has demonstrated an advantage, and therefore no clear indications exist for the use of this mode of ventilation. It is recommended in situations of inadequate oxygenation prior to considering extracorporeal support. Inverse ratio ventilation is most likely to be beneficial early in the disease process when there are still recruitable alveoli. Common criteria include the inability to provide acceptable oxygenation despite a PEEP greater than 10 cmH 2O and Fio2 greater than 0.80. Some patients do not respond to pressure-controlled inverse ratio ventilation; it has been suggested that this may occur when static thoracic compliance is less than 25 ml/cmH 2O. It is not clear that inverse ratio ventilation offers any distinct advantages over other approaches that increase mean airway pressure. Pressure-preset ventilation is used almost uniformly in neonatology, but is employed far less commonly in adult patients.
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