The authors respond

In Reply:

We appreciate the well-intentioned interest of Dr Aşar and colleagues, who express concern regarding the energy nomenclature that we used in our article.¹ Dr Aşar is correct that there exists considerable variation and, as yet, no broadly accepted definitions in this emerging field directed at the subcomponents of power. However, we respectfully but strongly disagree with several important misconceptions contained within their letter.

First, the term dynamic compliance should not be generally understood to be determined by the difference of peak pressure (which includes the nonelastic, flow resistive element) and PEEP. In fact, the traditional physiologic definition of dynamic compliance is compliance estimated during uninterrupted rhythmic breathing as the slope of the pressure volume loop that links the zero flow points at the extremes of the ongoing tidal cycle.² This dynamic compliance differs from (and invariably is less than) the static compliance, which is determined after an occlusion-imposed pause, as in bedside practice. Sustained no-flow conditions allow for stress relaxation and flow redistribution to occur, dropping the recorded value to a plateau pressure that underestimates the zero-flow end-inspiratory pressure that occurs transiently during tidal breathing.

Second, the conceptual basis for the concern regarding inclusion of PEEP in the dynamic power of inflation has been the subject of recent but now resolved debate. When no gas is flowing, PEEP is indeed a static pressure. But PEEP during active inflation is an important component of the applied pressure that must be generated anew with each micro-increment of inflation volume. Therefore, PEEP is a subcomponent of total dynamic energy, joining the pressure related to volume in excess of PEEP (the driving pressure), whose product with volume we term the driving power.

Third, the biologic impact of driving power depends upon the pressure platform (PEEP) from which it begins.^3,4 Along this line, the figure provided by Aşar et al seems to invite confusion rather than clarification. In concept, there can be no “PEEP power” block (as labeled) because PEEP is a static pressure, and power by definition is a dynamic entity. Therefore power, a pressure-flow product, may include a PEEP component during the process of inflation, but “PEEP power” cannot stand alone.

Fourth, what Dr Aşar and colleagues believe everyone knows and accepts is not an accurate assumption. Although he might wish otherwise, the term dynamic mechanical power, as defined in his self-cited paper, is not generally accepted by the academic community engaged in studying these questions and developing this emerging field. Indeed, the publication date of April 2020 hardly allows for the evaluation of its conceptual merit, let alone its adoption for generalized use.

Finally, the paper currently under discussion is not our first to use these specific terms nor to examine the concepts that underpin the components of inflation power.^3,6

In the end, although we consider these complaints to be invalid, we do agree that a consistent nomenclature for power components is needed to facilitate communication as research into the emerging energetics of ventilation proceeds. Definitions should be grounded on a firm understanding of the underlying energetics and physiology.

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