Transpulmonary pressure (PL), the distending force of the lung, is the difference between airway (
PAW) and pleural pressure (
PPL), with
PPL estimated by esophageal pressure (
PES) [
21,
22]. During controlled mechanical ventilation,
PPL varies from non-dependent to dependent lung regions of the lung [
23]. The absolute
PL gradient in the supine position primarily depends on lung weight as well as shape and mechanical properties of lung and chest wall.
PES is a reasonable estimate of
PPL in the zone between the non-dependent and dependent lung regions. In ARDS, the superimposed pressure from non-dependent to dependent lung regions is 10 cmH
2O, on average [
24]; thus,
PPL is roughly
PES + 5 cmH
2O in dependent lung regions and
PES-5 cmH
2O in non-dependent lung regions. When interpreting
PL from
PES measurements, the absolute difference (not corrected) between
PAW and
PES at end-inspiration or end-expiration represents the P
L in the middle lung, and the difference between end-inspiration and end-expiration in
PPL (Δ
PPL) approximates Δ
PES. Elastance of respiratory system and chest wall may vary unpredictably and with changes in PEEP. In obese patients or those with increased intraabdominal pressure (
PPLAT above 27 cmH
2O), a simplified formula may help estimate the required correction of
PPLAT:
PPLAT target + (intraabdominal pressure-13 cmH
2O)/2 [
25,
26]. In mechanically ventilated non-obese patients, the average intraabdominal pressure is 13 cmH
2O and half of intraabdominal pressure is transmitted to the thoracic cavity [
27]. The following parameters have been suggested as potential targets for individualized mechanical ventilation when using
PL [
28,
29]: (1) end-inspiratory
PL (non-dependent lung) below 15–20 cmH
2O; (2) Δ
PL below 10–15 cmH
2O; (3) PEEP set at end-expiratory
PL (dependent lung) equal to 0–6 cmH
2O; and (4) P
L during recruitment maneuvers not to exceed 25 cmH
2O [
29,
30]. To date, RCTs evaluating the role of individualized PEEP set according to
PL at end-expiration and compared with low or high Pa
O2/F
iO2 table have not shown beneficial effects on outcomes [
31,
32].