Acute Lung Injury Pressure controlled ventilation

The advantages of using pressure controlled ventilaton
in acute lung injury (ALI):

1) Gas Distribution: acute lung injury is a heterogeneous disease process. Lung units are affected differently by disease. Some are effectively normal, some have low compliance, some have normal compliance but long time constants. Others are not involved in gas exchange. In volume ventilation, gas is preferentially delivered to more compliant lung units, causing overdistension and poor mixing. In pressure control, there is better distribution of gas to these differing lung units.

In the cartoon above, see, on the left side an injured lung segment: this contains normal alveoli (A1), non compliant alveoli (A2 e.g. consolidation) and alveoli with long time constants due, in this case, to proximal obstruction - such as a mucus plug or bronchial constriction. When a volume breath is delivered (with constant flow pattern) the gas passes down the path of least resistance into the most compliant alveoli - so there is relative overdistension of A1, A2 is inflated as expected, and A3 does not have time to inflate before the ventilator cycles off.

In the second cartoon, a pressure controlled breath is delivered. In this case there is better distribution of gas - because A1 will not overdistend to the same extent as before, and there is sufficient time for A3 to inflate.
Gas Distribution the main reason for using pressure control ventilation (along with variable flow in the spontaneously breathing patient. The same effect can actually be achieved in volume control - using low peak flow rates, decelerating flow patterns and an  inspiratory pause. However this requires a considerable amount of skill to apply than using pressure control. The major drawbacks of pressure control is changing tidal volume in relation to 1. changes in lung compliance, and 2. auto-PEEP.

2) Control of mean airway pressure: it is possible to increase the mean airway pressure, by varying the inspiratory time, without increasing the peak or plateau pressure. This facilitates maintaining oxygenation within a pressure limit, without overstretching the alveoli. However, if the prolongation of inspiration causes auto-PEEP, this advantage is lost. The remedy is to reduce the respiratory rate initially, and then to reduce the I:E. This concept, inverse ratio ventilation, is a key part of the open lung approach to ARDS and  is the basis of some modern pressure controlled modes - BiLEVEL/APRV.

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Copyright Patrick Neligan 2001-2002

Please note: these tutorials are for personal study purposes only.  They are not currently peer reviewed, and no responsibility will be taken for mistakes or inaccuracies. Reproduction of information is forbidden. All material is copyrighted by the GasWorks Group.