Transpulmonary Pressure Measurement: A Waste of Time?

I'm going to assume if you're reading this that you already know what transpulmonary pressure (P_L) is and how esophageal manometry works. I'll provide a brief description here, but if you're looking for an in-depth review , I'd recommend a few of the excellent papers already published. The somewhat short version is this: Esophageal manometry is a method for separating the pressure applied across the lung (P_L) from that created by the chest wall (P_CW). Esophageal pressure is used as a surrogate for pleural pressure (P_Pl), and subtracting P_Pl from the airway pressure (P_AW) measured by the ventilator equals P_L.

P_AW - P_Pl = P_L

Ever since the ARDSNet trial highlighted the clinical consequences of overdistending the lung, we've been trying to fine-tune mechanical ventilation strategies. ARDSNet helped highlight the reality that ventilator-induced lung injury (VILI), which is often insidious and all but invisible at the bedside, drives outcomes more than gas-exchange variables that are easy to measure. Since ARDSNet, we've been studying ventilator measures like positive end-expiratory pressure (PEEP) and driving pressure, and specific techniques like recruitment maneuvers. Unfortunately, we've failed to identify specific mechanical ventilation settings that build on the clinical improvements seen with low tidal volume ventilation.

There are many possible reasons why, but one of them is the failure to account for P_L. For example, because all PEEP strategies utilize P_AW to identify pressure targets while ignoring P_Pl, they fail to account for P_CW. Across patients, P_CW can vary significantly. For example, patients with obesity or intraabdominal hypertension experience much higher P_CW and require higher PEEP. This is in contrast to a thinner patient with isolated ARDS due to a direct respiratory insult. The thinner patient will have a much lower P_CW, so lower PEEP can still prevent atelectrauma at end expiration and higher PEEP poses greater risk for overdistention and volu- or barotrauma. It's under- and over-distention of the lung that causes VILI, not effects from the chest wall.

There are a few different methods for isolating P_L at the bedside, but esophageal manometry is the best studied. In 2008, a randomized trial published in The New England Journal of Medicine showed an improvement in gas exchange and a borderline mortality reduction using esophageal manometry to guide PEEP changes. Unfortunately, it was a small, single-center study, so enthusiasm was tepid at best.

In 2019, a second esophageal manometry trial was published. This one was multicenter, with a larger sample size. The targets for P_L were more conservative in the esophageal manometry arm and the PEEP goals in the control arm were more aggressive. As a result, the difference in PEEP settings between groups was small and P_L was quite similar. Not surprisingly, the results were a wash. Clinical outcomes between groups were painfully similar.

Three months ago, the authors of the second trial reanalyzed their data in an attempt to figure out what went wrong. They adjusted for extrapulmonary processes that may have driven clinical outcomes. The logic was that if pathology outside the lung drives the clinical course, it doesn't matter how the ventilator is managed, whereas if such patients can be eliminated from the analysis, only those who might benefit from a particular ventilator strategy would remain. Conveniently, patients managed via esophageal manometry did better after those with extrapulmonary processes that drove outcomes were excluded. The authors also determined that expiratory P_L should be set as close to zero as possible.

The authors of the reanalysis acknowledge that their paper should be taken for what it is: a post hoc examination of subgroups that weren't prespecified. If data are tortured long enough, they'll provide you with an answer of some sort. That doesn't mean the data have no value. The authors should be commended for doing a deep dive in order to better design future trials. They just can’t answer the question we really want to know: Is esophageal manometry helpful at the bedside?

Personally, I love the physiology behind the technique. In the academic world, understanding P_L is essential to conceptualizing VILI. Is it essential at the bedside? Data from LUNGSAFE imply that most intensivists don't think so, given how infrequently it's used. The existing trial data aren't likely to convince any skeptics. For now, we're forced to continue flailing at the bedside, searching for the elusive ventilator strategy (beyond low tidal volumes) that actually matters.

Aaron B. Holley, MD, is an associate professor of medicine at Uniformed Services University and program director of pulmonary and critical care medicine at Walter Reed National Military Medical Center. He covers a wide range of topics in pulmonary, critical care, and sleep medicine.

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