A new review published in the American Journal of Respiratory and Critical Care Medicine seeks to bring clarity to an important physiologic term. Within the realm of pulmonary, sleep, and critical care medicine, the transpulmonary pressure (Ptp ), measured using an esophageal catheter, is used to detect respiratory effort during sleep and to manage the mechanically ventilated patient in the intensive care unit (ICU). In the ICU, Ptp provides an estimate of the stress imposed on the lung by positive pressure breaths.
Loring and colleagues take issue with the fact that clinicians and researchers use Ptp to represent two different measurements. Technically, the Ptp is the pressure drop across the entire respiratory system—from the mouth to the pleural space (Ptp = pressure at the mouth [Pao ] – pleural pressure [Ppl ]). In the sleep lab setting, clinicians are interested in abnormal swings in the esophageal pressure owing to increased respiratory system resistance (Rs ). Breath-to-breath increases in the Ptp during sleep are caused by partial collapse of the upper airway, which leads to an increase in Rs and a larger pressure drop across the system. Referring to the Ptp in this setting is to use it as was intended.
We invoke the Ptp incorrectly in the ICU. Typically, when we use the term, we are not referring to the pressure drop across the respiratory system but rather to the pressure that the alveoli are subjected to (Ptp = Palv – Ppl ). This is more accurately called the elastic recoil pressure of the lung (Pel[L]). It is the Pel(L) in which we are interested when we are setting positive end-expiratory pressure and managing tidal volume with the goal of minimizing ventilator-induced lung injury.
The article provides a great lesson in physiology and properly defines relevant terms. However, Loring and colleagues fail to convince me that the semantic issues they have identified have a meaningful impact on patient care. Whether we call it Ptp or Pel(L) when we are trying to reduce stress on the lung in the ICU, we know what we are doing. The authors imply that using the wrong term is synonymous with failure to understand that an esophageal catheter is required to quantify the contribution of the chest wall to airway pressure.
As an example, Loring and colleagues cite a recent patient-level meta-analysis that assessed the impact of driving pressure on mortality, commenting that using an esophageal catheter to assess the contribution from the chest wall to driving pressure would more accurately measure stress on the lung. This is true, of course. It hardly negates the findings from the study, however, and it's unlikely that the meta-analysis investigators were unaware of the risks and benefits of using an esophageal catheter. Rather, they dealt with the data at hand, which is what clinicians do at the bedside. The physiology lesson was helpful, but Loring and colleagues overestimate the effect of using a technically incorrect term.
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Cite this: What Is Transpulmonary Pressure, Really? - Medscape - Sep 30, 2016.