Can we estimate transpulmonary pressure without an esophageal balloon?-yes

Abstract

A protective ventilation strategy is based on separation of lung and chest wall mechanics and determination of transpulmonary pressure. So far, this has required esophageal pressure measurement, which is cumbersome, rarely used clinically and associated with lack of consensus on the interpretation of measurements. We have developed an alternative method based on a positive end expiratory pressure (PEEP) step procedure where the PEEP-induced change in end-expiratory lung volume is determined by the ventilator pneumotachograph. In pigs, lung healthy patients and acute lung injury (ALI) patients, it has been verified that the determinants of the change in end-expiratory lung volume following a PEEP change are the size of the PEEP step and the elastic properties of the lung, ∆PEEP × Clung. As a consequence, lung compliance can be calculated as the change in end-expiratory lung volume divided by the change in PEEP and esophageal pressure measurements are not needed. When lung compliance is determined in this way, transpulmonary driving pressure can be calculated on a breath-by-breath basis. As the end-expiratory transpulmonary pressure increases as much as PEEP is increased, it is also possible to determine the end-inspiratory transpulmonary pressure at any PEEP level. Thus, the most crucial factors of ventilator induced lung injury can be determined by a simple PEEP step procedure. The measurement procedure can be repeated with short intervals, which makes it possible to follow the course of the lung disease closely. By the PEEP step procedure we may also obtain information (decision support) on the mechanical consequences of changes in PEEP and tidal volume performed to improve oxygenation and/or carbon dioxide removal.

Keywords: Lung compliance; end-expiratory lung volume change; esophageal pressure; positive end expiratory pressure (PEEP); transpulmonary pressure.

Conflict of interest statement

Conflicts of Interest: S Lundin and O Stenqvist are shareholders in The Lung Barometry Sweden AB. P Persson has no conflicts of interest to declare.

Figures

Figure 1

Airway pressure and lung volume in a patient with healthy lungs (15). Airway driving pressure (ΔPAW) at baseline PEEP was ≈6.9 cmH2O with a tidal volume of ≈375 mL. A PEEP-increase of only 3.8 cmH2O resulted in a successive increase in end-expiratory lung volume, EELV, with 383 mL.

Figure 2

PEEP inflation in lung healthy (left panel) and in ARDS patient (right panel) (with permission from Stahl et al. Acta Anaesthesiol Scand) (35). PEEP was increased in steps of 2 cmH2O until end-expiratory airway pressure reached 45–50 cmH2O. Black arrows: Airway (respiratory system) P/V curves. The blue line connecting the end-expiratory airway P/V points does not follow the respiratory system PV-curve. Note that the inspiratory capacity at end-inspiration of the highest PEEP level is 4,600 mL at an end-expiratory airway pressure of 30 cmH2O in lung healthy patients, but only 1,600 mL at an end-expiratory airway pressure of 20, a “baby lung” in the ARDS patient. PEEP, positive end expiratory pressure; ARDS, acute respiratory distress syndrome.

Figure 3

Correlation between increase in measured lung volume (ΔEELV) and ΔEELV calculated from the change in PEEP divided by lung elastance, ΔPAWEE/EL (ΔPAWEE × CL) based on mean data from Pelosi et al. (37), Gattinoni et al. (4) and Garnero et al. (38). PEEP, positive end expiratory pressure.

Figure 4

Correlation between measured increase in EELV and ΔEELV calculated as ΔPEEP/EL following an increase of PEEP, based on pooled data of three different sizes of PEEP steps (≈5, ≈7 and ≈9 cmH2O, indicated by different colors in the figure. Modified from (15) (with permission from Persson et al./Br J Anaesth). PEEP, positive end expiratory pressure.

Figure 5

Change in PEEP (ΔPEEP) compared to transpulmonary driving pressure (ΔPLCONV) 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 of the PEEP-induced change in end-expiratory lung volume and corresponding tidal volume. Bars in diagram represents mean values presented with standard deviation [with permission from Persson et al./Br J Anaesth (15)].

Figure 6

PEEP trial with PEEP steps 0-4-8-12-16 cmH2O in patients with acute lung injury (ALI) (14). Note the close agreement between the lung P/V curve (calculated using esophageal pressure measurements) and the end-expiratory airway P/V curve. PEEP, positive end expiratory pressure.

Figure 7

Schematic airway (red), lung (blue) and chest wall (green) P/V curves at ZEEP and at PEEP 10 cmH2O. Left panel: inspiration. Right panel: expiration. The grey vertical up-arrows in the expiratory panel symbolize rib cage spring out force and down-arrows symbolize lung elastic recoil. The dashed green line indicates the end-expiratory chest wall P/V curve compiled from Rahn and coworkers 1946, West 1985 and Nunn 1993 (50-52). The end-expiratory chest wall elastance is equal to the end-expiratory pleural pressure difference (ΔPPLEE) between FRC (−5 cmH2O) and the chest wall resting volume, where end-expiratory pleural pressure is zero, divided by the volume between FRC and the chest wall resting volume ≈2.8 liter, 5/2.8=1.8 cmH2O/L. The transpulmonary driving pressure (10 cmH2O) of the tidal volume, which is equal to ΔEELV is equal to the change in end-expiratory airway pressure (ΔPEEP =10 cmH2O). The lung P/V slope is identical to the end-expiratory airway P/V slope as demonstrated by the blue dashed line (the lung P/V curve parallel shifted to the left to start from zero). The difference between the end-inspiratory airway pressure from FRC/ZEEP and the end-expiratory airway pressure at 10 cmH2O PEEP, is equal to the pressure needed to displace the chest wall complex, the tidal pleural pressure variation (ΔPPL) (14,16,53). PEEP, positive end expiratory pressure.

Figure 8

PEEP levels of 5 (baseline), ≈10, ≈12, and ≈14 cmH2O in lung healthy patients during anesthesia (15). Red arrows: Tidal airway (respiratory system) P/V curves. Green arrows: Tidal chest wall P/V curves. The difference between the starting point of the chest wall P/V-curves at different PEEP compared to the first one at baseline PEEP is calculated as the difference between change in PEEP and change in end-expiratory transpulmonary pressure (= calculated change in end-expiratory pleural pressure). Red dash line: end-expiratory airway (respiratory system) P/V curve. Blue line: lung P/V curve. Green dash line: end-expiratory chest wall P/V curve. Note that as end-expiratory respiratory system and lung elastance is equal. The end-expiratory chest wall elastance is zero, i.e., the inverse, end-expiratory chest wall compliance is infinite, as an indication that the chest wall complex yields completely to PEEP inflation. PEEP, positive end expiratory pressure.

Figure 9

PEEP step procedure [with permission from Persson et al./Br J Anaesth (15)]: 1. baseline ventilation and PEEP; 2. increased PEEP ≈0.7× airway driving pressure (ΔPAW) at baseline. Determine the increase in end-expiratory lung volume (ΔEELVup) during 60–90 seconds at the new PEEP level; 3. change PEEP back to baseline level after 120 seconds. Determine the decrease in lung volume (ΔEELVdown) during 60–90 seconds; 4. set tidal volume (VT) as mean of ΔEELVup and ΔEELVdown. Calculate lung elastance as change in PEEP divided by mean change in end-expiratory lung volume (EL = ΔPEEP/ΔEELV) and transpulmonary driving pressure as lung elastance multiplied by tidal volume (ΔPL = EL × VT). PEEP, positive end expiratory pressure.

Figure 10

Lung P/V curves (blue arrows) and airway (respiratory system) P/V curves (red arrows) in lung healthy patients during anesthesia (15). Transpulmonary driving pressure of a tidal volume equal to ΔEELV is equal to the change in end-expiratory airway pressure (ΔPEEP) [with permission from Persson et al./Br J Anaesth (15)].

Figure 11

Bland & Altman analysis (51) of the transpulmonary driving pressure derived from esophageal pressure (∆PLPES) measurements and transpulmonary driving pressure derived from a PEEP step procedure (∆PLPSM) based on pooled data of lung healthy patients during anesthesia of three different sizes of PEEP steps (≈5, ≈7 and ≈9 cmH2O, indicated by different colors in the figure (15). [With permission from Persson et al./Br J Anaesth (15)]. PEEP, positive end expiratory pressure.

Figure 12

A PEEP step procedure is used to establish the lung P/V curve from baseline PEEP level, to end-expiration at the high PEEP level. The curve is extrapolated to ZEEP (A,B). On this lung P/V curve (blue line), tidal lung P/V curve moves as PEEP and/or tidal volume is changed as visualized in panel C, D, E, and F. PEEP, end-expiratory airway pressure; VT, tidal volume; ∆PL, transpulmonary driving pressure; PLEI, end-inspiratory transpulmonary pressure. (A) PEEP step procedure lung P/V curve at baseline PEEP of 4 cmH2O and at the high PEEP level of 12 cmH2O; (B) lung P/V curve (blue line) from end-expiration at baseline PEEP to end-inspiration at the high PEEP level established by the PEEP step procedure of panel A. Dotted blue line: extrapolation of curve to FRC/ZEEP and to an end-inspiratory transpulmonary pressure of 28 cmH2O; (C) black arrow indicates tidal lung PV curve at PEEP 12 cmH2O and a tidal volume of 500 mL; (D) effect of decreasing tidal volume from 500 to 375 mL at PEEP 12 cmH2O. Transpulmonary driving pressure decreases by ≈30% and end-inspiratory transpulmonary pressure decreases below upper limit for protective level of 24 cmH2O (58); (E) if inadequate oxygenation requires an increase of PEEP, an increase from 12 to 14 cmH2O, without reducing the tidal volume, is enough to cause a dangerous increase in both ∆PL (>16 cmH2O) and absolute end-inspiratory transpulmonary pressure, PLEI (>30 cmH2O); (F) a reduction of tidal volume from 500 to 375 mL reduces both ∆PL and PLEI to still high levels, but below upper limits. PEEP, positive end expiratory pressure.