Low-Flow Inflation Technique

In the low-flow inflation technique [29, 30], a very low constant inspiratory flow is provided to the patient to generate a large total volume (Fig. 14.5b). The low flow causes a minimal but recognizable pressure decrease over the endotracheal tube, which means that the pressure-volume curve obtained under this “quasi-static” (dynamic, in truth) condition is shifted to the right with respect to a “true” static P-V curve (Fig. 14.5c). The amount of this shift depends on the resistance offered by the respiratory system, plus the resistance of the endotracheal tube, and the selected flow level. A high flow reduces the time required to obtain the curve, but it increases the shift. The slope of the curve, however, can be considered parallel with the true static P-V curve, under the hypothesis that airway resistance is constant throughout the inspiration, which is not true as resistance decreases with the lung volume. A low flow allows to reduce the shift, but the duration of the inspiration becomes long and the gas exchange artifacts above described for the super-syringe technique become significant. Other drawbacks of the low-flow inflation method are that this technique does not allow to obtain the deflation curve, it requires resetting the ventilator, and only volume-controlled ventilation can be used.