Assessment of respiratory muscle activity during PSV, can be achieved by measurements of WOB  and PTP . However, both WOB and PTP require the correct positioning of an esophageal balloon to estimate pleural pressure, and, although extensively used in investigational studies, neither is commonly used in clinical practice. P0.1 has shown good correlation with WOB during PSV , and may estimate the respiratory effort. Alternatively, an index of muscular activity, which we have named pressure muscle index (PMI), can be easily obtained during PSV. PMI is measured as the difference between the end-inspiratory occlusion plateau pressure and the airway pressure before the occlusion (PEEP + pressure support), obtained from the ventilator display during an end-inspiratory occlusion (Fig. 1) . PMI has shown a good correlation with PTP (p<0.01). Despite the underlying assumptions and many simplifications required, PMI maybe a sensitive estimation of respiratory effort and could be a useful parameter in setting the level of pressure support.
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