The Quality of Quality Metrics

To the Editor:

Perhaps the most important skills the respiratory care profession offers the medical community are those associated with life support: resuscitation and mechanical ventilation. We might wonder, therefore, why we have such high expectations¹ and yet so few metrics of quality performance in those areas. The study by Walsh et al² could be a landmark contribution in that direction. On the one hand, replication of their work will be a challenge for most institutions; what they have done involved special hardware (to port data from ventilators to the electronic health record) and custom web-based analytics software developed by their institution. Having attempted more modest projects that involve custom software and electronic health record integration,³ I can appreciate the formidable barriers to entry. On the other hand, Walsh et al² have provided a detailed set of rules that others can use to assemble quality metrics for the domains of mechanical ventilation, oxygenation, and ventilator-induced lung injury. Their definitions for rules-based algorithms might inform tools already developed in the emerging field of cognitive computing (eg, WatsonPaths).

However, before adopting all of the rules, I would question those related to ventilator-induced lung injury. Specifically, the paper² defined barotrauma-free as “peak inspiratory pressure (PIP) ≤ 30 cm H₂O” and volutrauma-free as “exhaled tidal volume > 4 mL/kg ideal body weight; < 8 mL/kg ideal body weight.” The results in the paper were reported as percentage of time spent in the “free” zones. First of all, these metrics were defined as measures reflecting ventilator settings, not lung trauma classifications as their names imply. Hence, they could be misleading. For example, someone reading only the abstract would probably conclude that freedom from the 2 types of traumas indicated actual patient outcomes. This could be avoided by simply naming these metrics in a way that reflects what they actually measure. For example instead of barotrauma-free, a more precise name might be low-risk PIP, and instead of volutrauma-free a better name would be low-risk tidal volume.

As the authors pointed out in the discussion, the quality of mechanical ventilation based on metrics like ventilator-associated pneumonia or incidence of pneumothoraces is an end point and hence is retrospective in nature. Although not explicitly stated as such, their study seemed to emphasize continuous, near real-time data analysis, rather than intermittent retrospective conclusions. In this context, monitoring PIP and tidal volume (V_T) makes sense. But the names of their associated metrics do not make sense.

As a minor point, the term barotrauma is an anachronism. It was apparently coined back in 1973 to indicate lung damage associated with high PIP. In 1992, Dreyfuss et al⁴ showed that it was a high V_T, not a high PIP, that was the major factor responsible for mechanical lung damage. Accordingly, they coined the more accurate term volutrauma.⁴ Gattinoni et al⁵ provide a very succinct and informative review. Even further, they point out that pressure change (stress) and volume change relative to a baseline (strain) are linearly related by a constant of proportionality, K (in this case defined as specific elastance): (1) where ΔP_tp is static transpulmonary pressure (obtained with an inspiratory hold) associated with V_T delivery (P_tp = pressure at airway opening minus pleural pressure⁶; ΔP_tp = elastance × V_T = one definition of driving pressure⁷) and ΔV is the V_T relative to baseline or resting lung volume, V₀. Actually the pressure change, ΔP_tp, is also measured relative to a baseline (ie, PEEP), corresponding to V₀ (although initially functional residual capacity was used as the reference lung volume in the equation⁸; hence, the reference pressure was ambient barometric pressure). The term K is specific elastance (the reciprocal of static compliance, often used to describe pediatric lung mechanics). It is defined by rearranging Equation 1 as follows. (2)

From Equation 2, we see that specific elastance has units of pressure (ie, volumes in the numerator and denominator cancel). Gattinoni et al are fond of saying “It follows that K … is the transpulmonary pressure recorded when V_T equals the resting volume, in other words, when the lung doubles its volume,”^5,8 yet I have never seen them explain exactly how it “follows.” The proof is that in Equation 2, if ΔV is set equal to V₀, then the term V₀/ΔV = 1, and hence K = ΔP_tp. In other words, K is equal to the change in transpulmonary pressure required to make ΔV = V₀ and hence the end-inspiratory volume = 2V₀, or double the initial lung volume.

The point being, as Gattinoni et al stated,⁵ when ΔP is equated to ΔV (Equation 1), “The distinction between volutrauma and barotrauma then vanishes.” Hence, they should not be conceived of as 2 different metrics and certainly not defined as ventilator variables instead of patient outcomes, yet it is confusing to use the terms interchangeably when we want to refer to the strain-related cause of ventilator-induced lung injury.

In summary, I am suggesting that we use the term terms low-risk PIP instead of barotrauma-free and low-risk tidal volume instead of volutrauma-free in defining quality metrics of mechanical ventilation. Furthermore, I suggest that we use the term volutrauma (as a patient outcome quality descriptor) instead of barotrauma because volutrauma is more obviously associated with the measurement of V_T, in contrast to barotrauma, which is easily mistaken to be associated with measurement of PIP instead of ΔP_tp (which is also less convenient to measure than V_T).

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