With respect to trans-pulmonary pressure (TPP):
Explain what is meant by the phrase "trans-pulmonary pressure (TPP)". (10% marks)
Describe the technique of measurement, including any limitations. (30% marks)
Discuss the rationale for its clinical use. (40% marks)
Briefly outline the evidence for its role in the management of patients with acute respiratory distress syndrome (ARDS). (20% marks)
TPP is the difference between the alveolar pressure (Palv) and pleural pressure (Ppl).
TPP is the net distending pressure applied to the lung.
Rationale for TPP measurements
By measuring TPP the effects of chest wall compliance are negated and a true measure of lung
distension is obtained. This may allow the safe tolerance of higher plateau pressures; with the
assumption that it is lung distension that is important in generating lung injury
Current therapies target Paw (<30 cmH2O) to minimise volutrauma or barotrauma. More accurate prevention of ventilator associated lung injury may be obtained by using TPP, e.g.:
In ventilated patients Ppl is estimated from oesphageal pressure (Pes.) with a thin wall latex
oesophageal ballon inserted via the NG or OG route. Its measurement is prone to error.
Measurement is automated on some ventilators.
Palv difficult to measure instantaneously during flow, but equalises to airway pressure at states of zero flow with airway occluded. Classically measured as inspiratory pause pressure after complete tidal volume.
Evidence, Talmor, NEJM 2008
61 patients ARDS / ALI – ARDSNet vs TPP targeted ventilation
No established role in general management.
May have a role in obesity, raised intra-abdominal pressure and air trapping.
Other techniques can compensate for inability to measure TPP e.g. best PEEP may be estimated by measuring respiratory compliance or oxygenation during a recruitment manoeuvre.
Amato’s re-analysis of the ARDS net data showed convincingly that total respiratory driving pressure (Pplat-PEEP) correlated most strongly with mortality. Total respiratory driving pressure may correlate with TPP.
Additional Examiners’ Comments:
Many candidates had little/no concept of either the utility or rationale for measuring transpulmonary pressure. Candidates confused terminology when discussing pleural pressure and alveolar pressure and could not give precise definitions
a) Explain what is meant by the phrase "trans-pulmonary pressure (TPP)"
The college defined TPP as "the difference between the alveolar pressure (Palv) and pleural pressure (Ppl)", or as "the net distending pressure applied to the lung". In fact this may be a sub-optimal definition according to Loring et al (2016), who would call the (Palv-Ppl) difference "elastic recoil pressure of the lung". However, in the ICU the TPP is usually measured in the absence of flow (i.e. in an inspiratory hold and at end-expiration). This means the pressure drop across the airway can be neglected (it is zero), and one does not need to consider it.
b) Describe the technique of measurement, including any limitations.
c) Discuss the rationale for its clinical use. (40% marks)
In management of ventilation:
d) Briefly outline the evidence for its role in the management of patients with acute respiratory distress syndrome (ARDS).
The college have quoted Talmor et al (2008): "Mechanical ventilation guided by esophageal pressure in acute lung injury." This was a randomised controlled study of 61 ARDS patients, of whom the TPP-guided group has better survival. The primary endpoint was oxygenation, and this too was better when PEEP was guided by TPP. Unfortunately the study sample was too small for the results to reach statistical significance. Apart from this study, the EpVent Trial (Fish et al, 2014) is under way and plans to enrol 200 patients. There seems to be little else.
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Sahetya, Sarina K., and Roy G. Brower. "The promises and problems of transpulmonary pressure measurements in acute respiratory distress syndrome." Current opinion in critical care 22.1 (2016): 7-13.
Loring, Stephen H., George P. Topulos, and Rolf D. Hubmayr. "Transpulmonary pressure: the importance of precise definitions and limiting assumptions." American journal of respiratory and critical care medicine194.12 (2016): 1452-1457.
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Eichler, Lars, et al. "Intraoperative Ventilation of Morbidly Obese Patients Guided by Transpulmonary Pressure." Obesity Surgery (2017): 1-8.
Fish, Emily, et al. "The Esophageal Pressure-Guided Ventilation 2 (EPVent2) trial protocol: a multicentre, randomised clinical trial of mechanical ventilation guided by transpulmonary pressure." BMJ open 4.10 (2014): e006356.
Rodriguez, Pablo O., et al. "Transpulmonary pressure and gas exchange during decremental PEEP titration in pulmonary ARDS patients." Respiratory Care 58.5 (2013): 754-763.
Terragni, Pier Paolo, et al. "Accuracy of plateau pressure and stress index to identify injurious ventilation in patients with acute respiratory distress syndrome." The Journal of the American Society of Anesthesiologists 119.4 (2013): 880-889.
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