Question 25

College answer

TPP
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.:

• Limit recruitment maneuvers to TPP 25 cmH2O
• Setting PEEP to TPP 0-10 cmH2O
• Limiting volutrauma by setting VT to a TPP 25 cmH2O
• Determination of respiratory muscle work in spontaneous ventilation
• Assessment of ventilator dys-synchrony
• Estimation of auto-PEEP in spontaneously breathing patients

Measurement
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.

• Malposition – gastric (one of third balloon placements in study below challenging)
• Positioning: supine vs erect (addition of mediastinal weight)
• Assumption that pleural pressures even through the chest
• Extrinsic factors – obesity, rising intra-abdominal pressure

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

• TPP group had higher PEEP, better oxygenation, higher Pplat
• Trends to better compliance, better mortality

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

Discussion

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. 

Technique:

• Insert the oesophageal manometer into the patient up to around 60cm 
• Inflate the balloon with ~ 0.5ml of air
• Transduce the pressure
• Ballot the stomach: a properly positioned transduced catheter will "feel" your abdominal poking
• Withdraw the catheter into the oesophagus (to a depth of around 40cm)
• Confirm placement with "cardiac oscillations" on the monitor

c) Discuss the rationale for its clinical use.  (40% marks)

In general:

• Airway pressure alone may be misleading
• TPP offers a more accurate asssement of stress upon the lung parenchyma
• TPP has the advantage of separating chest wall compliance from lung compliance.
• Customisation of PEEP and VT settings is possible, particularly for the morbidly obese and patients with high abdominal compartment pressure. 
• The measurement of TPP by oesophageal manometry is fairly non-invasive

In management of ventilation:

• Perform smarter recruitment manoevres, eg. using TPP of 25
• Set the PEEP in ARDS to prevent atelectasis (TPP 0-10)
• Set  the VT in ARDS to prevent volutrauma (TPP <25)
• Set the ventilator in morbidly obese patients, those with abdominal compartment syndrome or with some sort of rapidly changing abdominal pressure (pregnant, undergoing laparsocopy, etc).
• Detect "respiratory entrainment" or reverse triggering, a form of patient-ventilator dyssynchrony where the mandatory inspiration causes the patient's respiratory muscles to contract, as if in protest. 
• Detect ineffective respiratory efforts
• Improve the synchrony of SIMV with weak patient efforts
• Improve the cohesion between ventilator inspiratory time and the patients respiratory time
• Auto-PEEP can be measured using this method 
• It may provide a simple measure of patient effort in weaning from mechanical 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.