Is mechanical power the final word on ventilator-induced lung injury?-no

Abstract

Despite being a promising idea that combines several variables related to ventilator-induced lung injury (VILI), the concept of mechanical power (MP) carries a number of limitations, leaves several open questions, lacks proper modelling of positive end-expiratory pressure (PEEP) effects and, more importantly, does not respect the amount of lung tissue subjected to MP. First, the assessment of MP as a measure for development of VILI would have the highest relevance when volume displacement and related pressure changes are measured directly within the lung. Thus, ideally the relationship between MP delivered to the total respiratory system, and that delivered to lung tissue is discerned. Second, MP as defined today relates to the inspiratory phase only, and it is very possible that the expiratory phase will also play a role. Third, the calculation of MP during spontaneous breathing is challenging as airway pressure, flow and esophageal pressure are affected counter-directionally and simultaneously overlapping by the action of the ventilator and the respiratory muscles. Fourth, in its current form, MP is modelled with a positive linear relationship with PEEP, which is based on incorrect mathematical modelling to integrate the role of PEEP into MP. Fifth, the present equation used to calculate MP is qualitatively in disagreement with clinical data on VILI. The reduction of MP to its elastic part, might not only result in a higher association with VILI, but also amplifies an indirect U-shaped relationship with PEEP.

Keywords: Mechanical power (MP); driving pressure; mechanical ventilation; tidal volume; ventilator-induced lung injury (VILI).

Figure 1

Change of lung volume during tidal ventilation with positive end-expiratory pressure (PEEP), driving pressure (∆Paw) and tidal volume (VT). elastic mechanical work (MW) and resistive mw marked with dotted and crossed pattern areas; airway pressure does not change from zero to PEEP during tidal ventilation and thus no PEEP related mechanical work is present (area is zero, indicated by the red double arrow).