Basics of transpulmonary pressure: towards a better surrogate of lung stress

Now, let’s extend this idea to the whole lung in a clinical scenario. Say you are checking Pplat in a mechanically ventilated patient. To do this, you have to do an end-inspiratory hold (zero airflow) so that any resistive pressure is eliminated. Similarly, we perform an expiratory hold to determine total PEEP. Hence, while checking Pplat and PEEP, the airway pressure (Paw) read on the ventilator is a reflection of alveolar pressure (Palv). But remember that it’s the transalveolar pressure (Ptm) that determines stress, not Palv. You can now deduce that if you want to determine lung stress, the key piece of information missing here is the value of Ppl.

Definitions of transmural pressures.

There’s a lot of confusion in terminology and it’s important we all speak the same language. 

Transmural pressure: A generic term reflecting the difference between intramural and extramural pressures in an elastic chamber. It is used in both cardiovascular and pulmonary physiology.

Transalveolar pressure: Transmural pressure of a single alveolus. This reflects alveolar wall stress but is not clinically relevant since the lung is composed of several alveoli/lung units. We need a special name for ‘pressure across the lung parenchyma’.

Transpulmonary pressure: This is, in fact, the difference in pressure at the airway opening (Pao or Paw) and the pleural pressure (Ppl). Note that this includes the pressure difference across the airways, which is the resistive pressure. In conditions of zero airflow (e.g., while checking Pplat and total PEEP), resistive pressure is zero, and transpulmonary pressure indeed reflects the pressure across the lung parenchyma: the true surrogate of lung stress. For better clarity, these should be specifically referred to as end-inspiratory transpulmonary pressure (Pplat – Ppl) & end-expiratory transpulmonary pressure (totalPEEP – Ppl).

(P.S.: If you read classic texts on respiratory mechanics, the pressure across the lung parenchyma is referred to as ‘elastic recoil pressure of the lung’ or Pel(L). Stephen Loring et al have written an entire article addressing this point!)

Clinical correlates

It should become clear from the discussion so far that one cannot purely rely on PEEP and Pplat as indices of lung stress. The knowledge of Ppl becomes extremely relevant here. E.g., if two patients have the same Pplat but very different Ppl, their end-inspiratory transpulmonary pressure (lung stress) would vary significantly. In a healthy supine subject, end-expiratory Ppl is quite close to zero. So, what are some of the patient populations where Ppl varies significantly from the average? The two important ones are (a) Morbidly obese patients, and (b) Patients with abdominal compartment syndrome.

In morbidly obese patients, excessive weight of the chest wall increases the pleural pressure. This effect is especially magnified in a recumbent position. In fact, Ppl can be significantly reduced in obese patients if they are placed in a sitting or reverse trendelenberg position.   In abdominal compartment syndrome, the high intraabdominal pressure pushes against the diaphragm. Remember that from the perspective of respiratory mechanics, diaphragm is a part of the ‘chest wall’. A hypothetical comparison is presented in Figure 3. (These numbers are very realistic and not an exaggeration!)