March 2002 / Volume 47 / Number 2 / Page 266
Setting the Frequency-Tidal Volume Pattern
IntroductionAlveolar (and thus arterial) PO2 and PCO2 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 H2O, 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 PaO2 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. PO2 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 (VT), a VT 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 if 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 VT 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 VT 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.
Balancing Minute Ventilation Needs and Overstretch Injury
Oxygen Demands, Carbon Dioxide Clearance, and Minute Ventilation
Clinically Balancing the Minute Ventilation Need with the Risk of Overstretch
Clinical Practice: Setting the Frequency and Tidal Volume
Normal Ventilatory Pattern and the Minimal Work Concept
Frequency-Tidal Volume Setting in Patients with Lung Injury
Inspiratory Time and Inspiratory-Time-to-Expiratory-Time Ratio
Can Flow Patterns Affect Injury?
Invasive positive-pressure ventilation provides ventilatory support through periodic tidal inflations of the lung. Issues that need to be considered in setting frequency and the volume of each inflation include the level of minute ventilation support required, the potential for stretch injury in the lung, the potential for intrinsic positive end-expiratory pressure (PEEP) buildup, and the effects of the flow pattern and duration on gas exchange and comfort. Optimal setting of the frequency-tidal volume (f-VT) pattern often involves balancing these sometimes competing clinical goals.
The entire text of this article is available in the printed version of the March 2002 RESPIRATORY CARE.