Transpulmonary pressure monitoring in critically ill patients: pros and cons—correction of description of the non-invasive PEEP-step method for separation of lung and chest wall mechanics

Abstract

In a recent pro/con review on transpulmonary pressure by Ball, Talmor and Pelosi [1], the authors describe in detail positioning, inflation, and calibration of the esophageal balloon catheter and different interpretations of absolute esophageal and transpulmonary pressure measurements. They also briefly describe the only method that does not require esophageal pressure for separation of lung and chest wall mechanics, the PEEP-step method (PSM). However, they dismiss PSM invoking completely erroneous assumptions that the method “assumes implicitly that the end-expiratory transpulmonary pressure estimated with esophageal manometry is zero regardless of the applied PEEP level”. But PEEP causes an increase in EELV, and during inflation of the lung, transpulmonary pressure increases in relation to the volume inflated and the elastic properties of the lung, ΔV × EL. In five successful PSM validation studies, based strictly on tidal airway and esophageal pressure variations [2,3,4,5,6], we have shown that calculated end-expiratory transpulmonary pressure (PLEE) increases as much as PEEP (= PAWEE) is increased. Consequently, the change in end-expiratory esophageal pressure, calculated as ΔPAWEE - ΔPLEE , is zero, which proves that the chest wall does not impede PEEP inflation and therefore lung elastance can be determined as ΔPEEP/ΔEELV. Thus, it is not transpulmonary pressure, but tidally calculated esophageal pressure that remains zero and the dismissal statement is completely erroneous and misleading.

In data on absolute esophageal and transpulmonary pressure from the Brochard group, analysis of tidal variation in esophageal and transpulmonary pressure fully confirms the validity of PSM (for details, see, Figs. S2, S3, S4 in e-supplement).

Below, I give a correct description of the background physiology, validation, mathematical derivation and measurement procedure of the PEEP-step method.

Background physiology

The PEEP step method (PSM) is a non-invasive, esophageal pressure free method for separation of lung and chest wall mechanics, based on the physiological conditions at functional residual capacity (FRC), where the contra-directional forces of the elastic recoil of the lung, striving to lower lung volume, and the rib cage spring out force, striving to expand the chest wall, balance each other. Thus, the chest wall complex does not lean on, or squeeze the lung at end-expiration at FRC. In case of a pneumothorax, the chest wall expands to 70–80% of total lung capacity (TLC). If end-expiratory lung volume instead is increased by PEEP, the rib cage spring out force will move the chest wall complex outwards in parallel with the lung volume increase, i.e., the ΔPEEP (= ΔPAWEE) of the ventilator only has to overcome the recoil of the lung. Consequently, the end-expiratory transpulmonary pressure (PLEE) will increase as much as PEEP is increased and EELV will increase in relation to the size of ΔPEEP and the elastic properties of the lung only, ΔPEEP/EL. Thus, if ΔEELV is determined by the ventilator pneumotachograph as the cumulative difference in expiratory tidal volume between PEEP levels, lung elastance can be calculated as ΔPEEP/ΔEELV. (for details on determination of ΔEELV by cumulative expiratory tidal volume, see e-supplement).

Validation

The effect of the expansive chest wall off-loading the chest wall from the lung during PEEP inflation was confirmed by comparing ΔEELV measured by pneumotachograph of the ventilator with ΔEELV calculated as ΔPEEP/EL, where EL is determined by esophageal pressure as the difference between tidal airway and esophageal pressure variations divided by the tidal volume, (ΔPAW-ΔPES)/VT). Cumulative measured ΔEELV and cumulative calculated ΔEELV = ΔPEEP/EL in pooled raw data from [3, 5, 6] correlated along the line of identity, y = 1.03x, R² = 0.86, i.e., showing that the chest wall does not impede PEEP inflation.

To show that this correlation is not a result of faulty tidal esophageal pressure measurements by the PSM group, the correlation between cumulative measured ΔEELV versus cumulative ΔEELV calculated as ΔPEEP/EL, and the correlation between cumulative ΔPEEP and cumulative ΔPLEE calculated as ΔEELVxEL, on mean data from the PSM validation studies, were combined with data from studies with first name Katz [7, 8], Falke [9], Garnero [10], Pelosi [11], and Gattinoni [12]. The two plots showed correlation along the line of identity (Figs. 1 and 2).

Fig. 1

Left panel: Correlation plot of cumulative ΔEELV measured PEEP step by PEEP step by the ventilator pneumotachograph versus cumulative ΔEELV calculated PEEP step by PEEP step as ΔPEEP/EL in Katz I [7], Katz II [8], Falke [9], Persson [5], Lundin [3], Stenqvist [6], Garnero [10], Pelosi [11], Gattinoni [12]. EL in these studies ranged from 9 to 43 cmH₂O/L and the ratio of EL/ERS ranged from 0.28 to 0.95. As the change in EELV is ΔPEEP/EL, lung elastance can be determined as ΔPEEP/ΔEELV, without esophageal pressure, by increasing PEEP and determine ΔEELV. Right panel: Correlation plot between cumulative ΔPEEP and cumulative ΔPLEE calculated as ΔEELV x EL, PEEP step by PEEP step in [3, 5,6,7,8,9,10,11,12], showing that PLEE increases as much as PEEP is increased. The transpulmonary driving pressure (ΔPL) of a tidal volume equal to ΔEELV is equal to ΔPEEP

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Fig. 2

Left panel: 44 PEEP steps with average ΔPEEP of 5.1 cmH₂O, in the same studies as listed in Fig. 1. The increase in end-expiratory transpulmonary pressure is calculated as ΔEELV × EL, where EL = (ΔPAW − ΔPES)/VT. The change in end-expiratory esophageal pressure (ΔPPLEE) is calculated as ΔPEEP − ΔPLEE. Right panel: ΔPEEP compared to transpulmonary driving pressure (ΔPLconv determined by esophageal pressure) for a tidal volume equal to the PEEP-induced change in end-expiratory lung volume (VT = ΔEELV). Values divided into three groups according to the size ΔEELV and corresponding tidal volume. From validation study in supposedly lung healthy in the OR, with permission from Elsevier [5]

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Tidal pressure variations equations for separation of lung and chest wall mechanics used for validation of PSM

Airway driving pressure = ΔPAW

Esophageal driving pressure = ΔPES

Transpulmonary driving pressure = ΔPAW − ΔPES = ΔPL

Respiratory system elastance: ERS = ΔPAW/VT

Chest wall elastance: ECW = ΔPES/VT

Lung elastance: EL = ΔPL/VT = ERS − ECW

End-expiratory airway pressure: PAWEE = PEEP

End-expiratory lung volume change: ΔEELV = ΔPEEP/EL

End-expiratory transpulmonary pressure change: ΔPLEE = ΔEELV × EL

End-expiratory esophageal pressure change: ΔPESEE = ΔPAWEE − ΔPLEE

The mathematical derivation

PES based lung elastance is

$${\text{EL}} = \Delta {\text{PL}}/{\text{VT}}$$

and PEEP based lung elastance is

$${\text{EL}} = \Delta {\text{PEEP}}/\Delta {\text{EELV}}$$

Thus

$$\Delta {\text{PL}}/{\text{VT}} = \Delta {\text{PEEP}}/\Delta {\text{EELV}}$$

when \(\Delta {\text{EELV}} = {\text{VT}}\)

$$\Delta {\text{PL}} = \Delta {\text{PEEP}}$$

Thus, the transpulmonary driving pressure of a tidal volume equal ΔEELV is equal to ΔPEEP. As also the increase in end-expiratory transpulmonary pressure (ΔPLEE) is equal to ΔPEEP, the transpulmonary pressure at a certain lung volume is the same, irrespective of whether this volume has been reached by tidal or PEEP inflation.

The increase in EELV during the first expiration after increasing PEEP, is.

ΔPEEP/ERS [13].

Total ΔEELV is

$$\Delta {\text{PEEP}}/{\text{EL}}$$

The difference between these two volumes constitutes the second phase multi-breath phase of PEEP inflation, which is

$$\left( {\Delta {\text{PEEP }} \times {\text{ ECW}}} \right)/\left( {{\text{ERS }} \times {\text{ EL}}} \right).$$

The increase in end-expiratory esophageal pressure during the first expiration is the first expiration volume times chest wall elastance,

$${\text{ECW}}\; \times \, \Delta {\text{PEEP/ERS}}$$

The corresponding increase in end-expiratory transpulmonary pressure is

$${\text{EL }} \times \, \Delta {\text{PEEP/ERS}}$$

The increase in end-expiratory transpulmonary pressure during the second phase PEEP inflation is

$${\text{EL }} \times \, \left( {\Delta {\text{PEEP }} \times {\text{ ECW}}} \right)/\left( {{\text{ERS }} \times {\text{ EL}}} \right) \, = {\text{ECW }} \times \, \Delta {\text{PEEP/ERS}}$$

Thus, the increase in transpulmonary pressure during the second phase multi-breath increase in ΔEELV, is equal to the increase in end-expiratory esophageal pressure during the first expiration after increasing PEEP. This proves that the chest wall is off-loaded from the lung at end-expiration during PEEP inflation due to the spring out force of the rib cage.

The mathematical derivation is summarized in Figs. 3 and S1 of e-supplement.

Fig. 3

Left panel: A PEEP step in isolated lung. Airway pressure (red arrow) is equal to transpulmonary pressure (blue arrow). The tidal volume from low PEEP is 500 ml and the ∆PAW is 10 cmH₂O. Increasing PEEP with 10 cmH₂O, results in an increase in EELV with 500 ml. The end-inspiratory transpulmonary (= airway) P/V point from the low PEEP is equal to the end-expiratory transpulmonary (= airway) P/V point at the high PEEP (indicated by blue ring). Lung elastance is ∆PAW/VT and ∆PEEP/∆EELV in an isolated lung. Right panel: PEEP step in situ. The tidal volume from the low PEEP level is equal to the PEEP induced end-expiratory lung volume change. The end-inspiratory airway plateau pressure from ZEEP is right shifted (black arrow) from the position in isolated lung (blue ring). The difference between end-inspiratory plateau pressure of the tidal volume from the low PEEP level and the end-expiratory transpulmonary pressure at the high PEEP level is equal to tidal pleural pressure variation (green arrow, ΔPPL). The transpulmonary plateau pressure of the tidal volume from ZEEP is equal to the end-expiratory transpulmonary pressure at the high PEEP, 10 cmH₂O

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PEEP is increased by a default value of 70% of the airway driving pressure at baseline PEEP as the average ratio of EL/ERS (ΔPL/ΔAW) is ≈ 0.70, and ΔEELV will therefore on average be equal to the tidal volume. ΔEELV is determined as the cumulative difference in expiratory tidal volume between the PEEP levels and lung elastance is calculated as ΔPEEP/ΔEELV. Transpulmonary driving pressure is calculated as PSM derived EL times tidal volume. The whole procedure is 1.5–2 min (Fig. 3).

To account for the non-linearity of the lung pressure/volume curve, a two PEEP-step procedure is required to assess the curve from end-expiration at baseline PEEP to end-inspiratory plateau pressure at the highest PEEP level. A two-degree polynomial is fitted to three end-expiratory airway P/V points and the end-inspiratory transpulmonary P/V point at the highest PEEP level of the procedure is estimated based on tidal pleural pressure variation extrapolated from the two lower PEEP (Fig. 4).

Fig. 4

Left panel: The lung P/V curve (blue) determined by a two PEEP-step procedure. Tidal airway P/V curves at three PEEP levels (red arrows). The pressure difference between the end-inspiratory airway plateau pressure and the transpulmonary pressure at the same volume level, i.e., the tidal variation in pleural pressure, is determined for each of the two lowest PEEP levels. The transpulmonary plateau pressure at the highest PEEP level is estimated by extrapolation the tidal pleural pressure variations at the two lower PEEP levels. The transpulmonary plateau pressure at the highest PEEP level is then calculated as the airway plateau pressure minus the extrapolated pleural pressure variation [14]. The difference between estimated transpulmonary plateau pressure and plateau pressure by esophageal pressure measurements were 0.2 ± 1.4 and 0.1 ± 0.8 cmH₂O in ARF patients in the ICU and lung heathy in the OR, respectively [14, 15]. Right panel: Optimal PEEP, i.e. the PEEP level providing the lowest transpulmonary driving pressure, can be determined by identifying the steepest point of the curve as the root of the second derivative of the polynomial and distributing the requested tidal volume symmetrically around this P/V point [14]. The pressure corresponding to the end-expiratory volume can be calculated from the polynomial. This now calculated pressure is the optimal mechanical PEEP

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As a tidal lung P/V curve, irrespective of volume and PEEP level, is superimposed on the total lung P/V curve, the transpulmonary driving pressure and plateau pressure of any combination of PEEP and tidal volume can be calculated from the equation for the lung P/V curve. This makes it possible to estimate the mechanical consequences of any combination of PEEP and tidal volume and identify risk of ventilator induced lung injury (VILI) and when more aggressive settings can be used to avoid ECMO treatment without risking VILI (Fig. 5).

Fig. 5

Best fit lung P/V curve (light gray) of PEEP non-responder and responder with overall lung compliance (CLoa) of 54 and 112 ml/cmH₂O, respectively [14]. Tidal lung P/V curves (blue arrows) for a VT of 6 ml/kg IBW (500 ml in a patient with 70 kg IBW) at PEEP 8 and 13 cmH₂O superimposed on total lung P/V curve (light grey curve). In the non-responder, transpulmonary driving pressure increases to close to the upper safety limit when PEEP is increased by 5 cmH₂O. In the responder, transpulmonary driving pressure falls from a moderate level to a low level as lung compliance increases with increasing PEEP

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PEEP-step method to determine transpulmonary pressure

Pro: Validated and mathematically derived background physiology. Non-invasive, full PEEEP-trial, including ΔEELV, and optimal PEEP with lowest transpulmonary driving pressure determined in 3–4 min by a two PEEP-step procedure

Con: Transpulmonary plateau pressure at highest PEEP level estimated by extrapolation of tidal pleural pressure variations from lower PEEP levels

No datasets were generated or analysed during the current study.

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1. Sahlgrenska Academy, Gothenburg University, Gothenburg, Sweden

   Ola Stenqvist

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OS wrote the manuscript and prepared the figures.

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Correspondence to Ola Stenqvist.

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OS holds shares in the Lung Barometry AB (LBAB), which owns the commercial rights to the PSM technology. LBAB has recently entered into an agreement to commercialize the PEEP-step method with a Med Tech company.

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Stenqvist, O. Transpulmonary pressure monitoring in critically ill patients: pros and cons—correction of description of the non-invasive PEEP-step method for separation of lung and chest wall mechanics. Crit Care 28, 355 (2024). https://doi.org/10.1186/s13054-024-05125-5

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• Received: 30 September 2024

• Accepted: 06 October 2024

• Published: 04 November 2024

• DOI: https://doi.org/10.1186/s13054-024-05125-5

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