Volume-controlled mechanical ventilation is delivered with a constant inspiratory flow, resulting in increasing airway pressure through inspiration. To maintain this fixed rate of gas flow the pressure must rise through inspiration. The actual preset tidal volume remains constant as lung compliance and resistance change. The inspiratory flow rate alters the velocity with which gas flow is delivered (Figure 1). Ventilation with a high inspiratory flow delivers the pre-selected tidal volume more quickly. If the ventilator is time cycled between inspiration and expiration and the tidal volume has been delivered before all the time allowed for inspiration has elapsed, an inspiratory pause occurs and the pressure drops below the peak inspiratory pressure. There is no fresh gas flow during this inspiratory pause. High inspiratory flow during volume-controlled ventilation has detrimental effects on lung ventilation. Therefore, low inspiratory flow rates should be used to keep the peak ventilatory pressure as low as possible. This ensures more homogeneous ventilation.
The risk of lung injury can be reduced by using pressure-limited ventilation (Figure 2). In older ventilators, pressure limitation stops the inspiratory flow, resulting in a reduction in target tidal volume. In more modern ventilators, once the pressure limit is reached, the flow decelerates to maintain the peak pressure at the pressure limit for the rest of the breath. This ensures the tidal volume delivered is as close to the target tidal volume as possible for the set pressure limit. A pressure limit of 30–35 cm H2O is appropriate in adults.
Insert Figure 2
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