Rationale of inverse ratio ventilation

A low inspiratory flow, particularly with a decelerating flow pattern, will theoretically improve intrapulmonary distribution of gas and reduce dead-space, improve the ventilation-perfusion relationship, and enhance alveolar recruitment. Thus intrapulmonary shunting of blood is reduced and gas exchange improved while maintaining the same tidal volume as during conventional ventilation with an I:E ratio of 1:2 without increasing the excursion of volume and pressure ( Shantholz and Brower 1.9.94).

Intrapulmonary gas distribution depends not only on inspiratory flow rate but also on the heterogeneity of the time constants of different alveolar units. The distribution of ventilation during inverse ratio ventilation may preferentially reach those alveolar units with slow time constants (slow lung units), where the product of resistance and compliance is high (Sh§D!h.9lZ..,§n.d..,.Biow§L19.9.4). An experimental lung model has shown that, when applying the same level of total PEEP by using external PEEP or by inducing intrinsic PEEP (inverting the I:E ratio), the local lung unit end-expiratory pressure and volume differed. Slow lung units inflated more with intrinsic PEEP, while fast lung units inflated more with extrinsic PEEP.

Apart from the theoretical benefits to intrapulmonary gas distribution, the reduction in inspiratory flow rate during inverse ratio ventilation may reduce the stress on lung tissue when there are heterogeneous time constants. Although it has been claimed that pressure-controlled inverse ratio ventilation may reduce the risk of barotrauma compared with volume-controlled ventilation by diminishing peak airway pressure, the alveolar pressure is very similar in both modes when the total PEEP and minute ventilation are kept constant. Moreover, peak airway pressure is clearly irrelevant in this context because it mainly reflects the pressure dissipated to overcome the resistance of the endotracheal tube and central airways, and is unlikely to reflect alveolar distending pressure. To evaluate alveolar distending pressures, one should measure static end-inspiratory airway pressure during a zero flow period.

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