Ventilator - Induced Lung Injury

By Neil R. MacIntyre, MD, FCCP

Determining Underrecruitment and Overdistention in the Lung

Conceptually, the upper and lower inflection points on a static pressure volume (PV) plot might be the best guide to determining lung recruitment and overdistention.^57,59 These inflection points were initially thought to represent the attainment of optimal recruitment (rise in compliance at the lower inflection point) and the development of overdistention (reduction in compliance at the upper inflection point). However, the "whole lung" PV plot actually reflects an amalgam of the mechanical properties of numerous lung regions with potentially widely varying regional PV relationships. Indeed, recruitment of alveoli may well be occurring throughout much of the steep part of the whole lung PV plot. It thus may be overly simplistic to assume that the measured lower and upper inflection points represent the ideal points to set PEEP and VT.⁶⁰

Another concern with the static PV plot is that it is technically difficult to perform and often requires heavy sedation or paralysis. A new technique, the single-breath "slow flow" pressure volume measurement (so called because the slow inspiratory flow minimizes resistive pressures so that a single dynamic measurement can approximate the true static plot) may make this mechanical assessment more clinically useful.⁶¹ As noted below, the PV curve must take into account the effect of an abnormal chest wall compliance when it exists.⁶²

Even without complex PV assessments, clinicians can still use routinely monitored parameters to assess the risk of overdistention and underrecruitment. Conceptually, overdistention is likely to occur when lung regions are subjected to transalveolar pressures exceeding the normal physiologic maximum of 30 to 35 cm H₂O. In mechanically ventilated patients, the end-inspiratory intra-alveolar pressure (reflected in the "plateau" airway pressure under no-flow conditions) is a reasonable approximation of transalveolar pressure if chest wall compliance is near normal (ie, the pleural pressure on the other side of the alveolar walls is so low that it can be ignored). In patients with abnormal chest wall compliance (eg, bindings, obesity), however, the plateau pressure may grossly overestimate transalveolar pressure.⁶² An esophageal balloon to measure pleural pressure can be helpful under these conditions. These chest wall considerations also apply to the PV maneuver as described above.

Determining adequate recruitment may be more problematic. Static compliance improvements from changes in ventilator settings correlate with improved recruitment, but these measurements are time-consuming and may require patient sedation/paralysis.⁵⁴ Fortunately, gas exchange improvements also generally correlate with improved recruitment⁵⁴ and the ratio between PaO₂ and the fraction of inspired oxygen (FIO₂) is often used as a surrogate for recruitment assessment. It must be remembered, however, that pressures required for recruitment of the sickest regions may produce overdistention in healthier regions. Aggressive recruitment strategies with positive airway pressure must thus be balanced against the risk of producing overdistention injury.

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