Setting the frequency-tidal volume pattern.
Alveolar (and thus arterial) P(O2) and P(CO2) clearly depend on minute ventilation. However, we need to balance gas exchange goals against the risk of overstretching, especially of the healthier regions of the lung. The plateau pressure is probably the best easily-obtained marker of the risk of stretch in the lung, and a commonly quoted threshold is 30--35 cm H(2)O, the normal maximum transalveolar pressure at total lung capacity. In establishing the proper balance of stretch versus gas exchange, we need to address what levels of pH and P(aO2) we consider acceptable. There are no good data to guide us on the lowest tolerable pH, but 7.2 is commonly quoted in the literature, and 7.15 was the lower limit of acceptability in the ARDS (acute respiratory distress syndrome) Network trial. P(O2) levels as low as 55 mm Hg may be well tolerated, provided there is reasonable oxygen delivery. In distributing the desired minute volume between respiratory frequency and tidal volume (V(T)), a V(T) of 6 mL/kg ideal body weight has been shown to improve ARDS outcome, compared to 12 mL/kg. Thus, 6 mL/kg should be the "start point." Adjustments upward could be considered the plateau pressure is acceptable, in order to improve gas exchange or comfort. Conversely, downward adjustments should be considered if the plateau pressure is high and the gas exchange is acceptable. Frequency is adjusted for the desired minute ventilation. It must be recognized, however, that as frequency (and minute ventilation) increases, the risk of air trapping and intrinsic positive end-expiratory pressure (PEEP) increases. Just like applied PEEP, intrinsic PEEP increases the baseline pressure and stretch upon which the V(T) is delivered. The end-inspiratory stretch increases accordingly. The shape and duration of the flow pattern may affect gas mixing, recruitment, cardiac function, intrinsic PEEP buildup, and patient comfort. It is also conceivable that certain flow patterns can produce an acceleration injury. Although small clinical trials using physiologic end points espouse certain flow patterns, there are no good outcome data at present supporting any particular approach. Some authors suggest that high-frequency ventilation (HFV) might be considered an "ultimate" lung-protective strategy. HFV creates considerable intrinsic PEEP, which, when coupled with sustained inflation maneuvers, can provide substantial alveolar recruitment. In addition, the small V(T) of HFV prevents excessive end-inspiratory distention. Although considerable clinical data support the use of HFV in pediatric patients at risk for ventilator-induced lung injury, there are few data from adults. Whether HFV will prove valuable in well-designed open lung strategies in the adult population still has to be determined.
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