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Published ahead of print on May 1, 2008, doi:10.1164/rccm.200710-1589OC
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American Journal of Respiratory and Critical Care Medicine Vol 178. pp. 346-355, (2008)
© 2008 American Thoracic Society
doi: 10.1164/rccm.200710-1589OC

Original Article

Lung Stress and Strain during Mechanical Ventilation for Acute Respiratory Distress Syndrome

Davide Chiumello1, Eleonora Carlesso2, Paolo Cadringher2, Pietro Caironi1,2, Franco Valenza1,2, Federico Polli2, Federica Tallarini2, Paola Cozzi2, Massimo Cressoni2, Angelo Colombo1, John J. Marini3 and Luciano Gattinoni1,2

1 Dipartimento di Anestesia, Rianimazione (Intensiva e Subintensiva) e Terapia del Dolore, Fondazione IRCCS–"Ospedale Maggiore Policlinico Mangiagalli Regina Elena" di Milano, Milan, Italy; 2 Istituto di Anestesiologia e Rianimazione, Università degli Studi di Milano, Milan, Italy; and 3 Pulmonary and Critical Care, University of Minnesota and Regions Hospital, St. Paul, Minnesota

Correspondence and requests for reprints should be addressed to Prof. Luciano Gattinoni, M.D., F.R.C.P., Istituto di Anestesiologia e Rianimazione, Fondazione IRCCS–"Ospedale Maggiore Policlinico, Mangiagalli, Regina Elena" di Milano, Via Francesco Sforza 35, 20122 Milan, Italy. E-mail: gattinon{at}policlinico.mi.it


   ABSTRACT
TOP
 ABSTRACT
AT A GLANCE COMMENTARY
 METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Rationale: Lung injury caused by a ventilator results from nonphysiologic lung stress (transpulmonary pressure) and strain (inflated volume to functional residual capacity ratio).

Objectives: To determine whether plateau pressure and tidal volume are adequate surrogates for stress and strain, and to quantify the stress to strain relationship in patients and control subjects.

Methods: Nineteen postsurgical healthy patients (group 1), 11 patients with medical diseases (group 2), 26 patients with acute lung injury (group 3), and 24 patients with acute respiratory distress syndrome (group 4) underwent a positive end-expiratory pressure (PEEP) trial (5 and 15 cm H2O) with 6, 8, 10, and 12 ml/kg tidal volume.

Measurements and Main Results: Plateau airway pressure, lung and chest wall elastances, and lung stress and strain significantly increased from groups 1 to 4 and with increasing PEEP and tidal volume. Within each group, a given applied airway pressure produced largely variable stress due to the variability of the lung elastance to respiratory system elastance ratio (range, 0.33–0.95). Analogously, for the same applied tidal volume, the strain variability within subgroups was remarkable, due to the functional residual capacity variability. Therefore, low or high tidal volume, such as 6 and 12 ml/kg, respectively, could produce similar stress and strain in a remarkable fraction of patients in each subgroup. In contrast, the stress to strain ratio—that is, specific lung elastance—was similar throughout the subgroups (13.4 ± 3.4, 12.6 ± 3.0, 14.4 ± 3.6, and 13.5 ± 4.1 cm H2O for groups 1 through 4, respectively; P = 0.58) and did not change with PEEP and tidal volume.

Conclusions: Plateau pressure and tidal volume are inadequate surrogates for lung stress and strain.

Clinical trial registered with www.clinicaltrials.gov (NCT 00143468).

Key Words: acute respiratory distress syndrome • acute lung injury • stress, mechanical • strain • ventilator-induced lung injury



   AT A GLANCE COMMENTARY
TOP
 ABSTRACT
 AT A GLANCE COMMENTARY
METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Scientific Knowledge on the Subject
Lung stress and strain are the primary determinants of ventilator-induced lung injury. Their surrogates are airway pressure and tidal volume normalized for ideal body weight (VT IBW). Prevention of ventilator-induced lung injury is primarily based on recognizing the "harmful" threshold for these surrogates (30 cm H2O airway plateau pressure and 6 ml/kg VT IBW).

What This Study Adds to the Field
In this study, we demonstrate that VT IBW and airway plateau pressure are inadequate surrogates for lung stress and strain.

 
Injury due to mechanical ventilation has been attributed to excessive pressure (barotrauma [1, 2]) or volume (volutrauma [3]) applied to the lung parenchyma, to shear stresses occurring at the interface of open and closed lung regions (atelectrauma [4, 5]), and to cellular inflammatory response (biotrauma [6]). In the lung, the force-bearing structure is a skeleton composed of a fibrous network (elastin and collagen), embedded in the extracellular matrix. One fiber system originates from the hilum, the other from the lung periphery (visceral pleura), and the two systems are connected at the alveolar level (7). The elastin fibers are the determinants of elastic recoil, whereas the inextensible collagen fibers, folded when the lung is in its resting position, act as a "stop-length" when completely unfolded at total lung capacity (8, 9). Lung cells, anchored to the extracellular matrix, do not directly bear the force, but may activate the inflammatory cascade if subjected to excessive shape changes. When a force is applied to the fiber system, the upper limit of expansion is total lung capacity (fully unfolded collagen), after which stress may induce rupture. Before this limit, however, the nonphysiologic distension of lung cells may result in generalized lung inflammation (10–12).

In bioengineering parlance, stress and strain are mechanical phenomena properly referred to microstructures or to small areas of a body. "Stress" is defined as the internal distribution of the counterforce per unit of area that balances and reacts to an external load. The associated deformation of the structure is called "strain," which is defined as the change in size or shape referred to the initial status. Stress and strain are linked by the following formula (13):

Formula 1(1)

We reasoned that the clinical equivalent of stress is transpulmonary pressure (airway pressure minus pleural pressure) and the clinical equivalent of strain is the ratio of volume change ({Delta}V) to the functional residual capacity (FRC), which is the resting lung volume (14). We used FRC as a reference point, because at this volume the fibers of the lung skeleton are in their natural resting position, at atmospheric airway pressure, and the respiratory muscles, which are the "engine" of the strain, are inactive and relaxed. Accordingly, within the range of pressures and volumes for which the stress and strain relationship is linear, we get the following:

Formula 2(2)

FRC must not be confused with end-expiratory lung volume measured with positive end-expiratory pressure (PEEP); in this case, the volume due to PEEP is part of {Delta}V and must be added to the numerator and not to the denominator. This equation shows that the proportionality constant between stress and strain, called specific lung elastance, is the transpulmonary pressure at which FRC doubles. This parameter reflects the intrinsic elasticity of the lung parenchyma open to gases.

Because the determinants of ventilator-induced lung injury (VILI), stress and strain, are not measured in clinical practice, we sought to determine the extent to which they can be described by their clinical surrogates, plateau airway pressure and tidal volume, referenced to ideal body weight (VT IBW). Therefore, in this article we measured the global average end-tidal stress and defined the stress to strain relationship (specific lung elastance) in patients with acute lung injury/acute respiratory distress syndrome (ALI/ARDS) and control subjects. If lung stress and strain were not predictable from plateau airway pressure and VT IBW, their measurement would ideally allow physicians to tailor a safer mechanical ventilation in the individual patient in question.


   METHODS
TOP
 ABSTRACT
 AT A GLANCE COMMENTARY
 METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
For more information on methods used, see the online supplement.

The study (March 2005 to May 2007) was approved by the institutional review board of the Fondazione IRCCS—"Ospedale Maggiore Policlinico Mangiagalli Regina Elena" di Milano. Written, informed consent was obtained before surgery in conscious subjects and delayed in incompetent patients until they had recovered from sedation (following Italian legal regulations). The study population (Table 1) consisted of control subjects and patients with ALI/ARDS, divided in four subgroups: control subjects, 19 patients studied after elective surgery (excluding thoracic and abdominal surgery; group 1) 11 intensive care unit patients with medical diseases (group 2); and patients with ALI/ARDS, 26 with ALI (group 3) and 24 with ARDS (group 4) (15).


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  		TABLE 1. ADMISSION DIAGNOSIS OF THE STUDY POPULATION

 
Study Design
Patients and control subjects were studied while supine during anesthesia and paralysis. The computer-driven protocol sequence (Figure 1) started with the measurement of FRC and end-expiratory lung volume at 5 cm H2O PEEP. Thereafter, the lungs, at FRC, were inflated by an air-filled super-syringe with a volume equal to FRC, for direct measurement of the specific lung elastance. After this maneuver, we began the ventilatory trial consisting of 40 consecutive breaths with VT IBW of 6, 8, 10, and 12 ml/kg sequentially applied at two different PEEP levels, 5 and 15 cm H2O, which were randomly set. After each VT IBW/PEEP combination, lungs were allowed to deflate from end-inspiratory volume down to atmospheric pressure (released volume, {Delta}V).


Figure 1
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  		Figure 1. Schematic representation of the lung volume changes observed during the different steps of the experimental protocol. The timing of recruitment maneuvers, lung volume measurements (both at FRC and at 5 cm H2O PEEP), measurements of specific lung elastance with super-syringe, and release maneuvers is also indicated. The inset shows the volumes necessary to compute the trapped-gas volume. As shown, inflated gas volume (above FRC) equals the sum of trapped-gas volume, PEEP volume, and tidal volume. Accordingly, trapped-gas volume was computed as end-expiratory lung volume minus FRC minus PEEP volume, where PEEP volume equals the released volume minus tidal volume. PEEP denotes positive end-expiratory pressure and ml/kg VT refers to VT IBW; 1st and 2nd PEEP denote either 5 or 15 cm H2O PEEP, according to the random sequence applied in each subject.

 
Measurements
Flow, airway (Paw), and esophageal (Pes) pressures were recorded as previously described (16) and processed on a dedicated data acquisition system (Colligo; Elekton, Milan, Italy). The correct positioning of the esophageal catheter was checked by an inspiratory occlusion and we assumed that the variations of esophageal pressure measured the variations of pleural pressure. From the tracings, by resampling and an interpolation process (17), we obtained a single "average" breath for each patient and each ventilatory setting (Figure 2). Lung volumes were measured by helium dilution technique (18).


Figure 2
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  		Figure 2. Experimental tracings. Six experimental tracings obtained in a single patient at VT IBW 6 ml/kg and 15 cm H2O positive end-expiratory pressure (PEEP) (A, C, and E). The right panels (B, D, and F) show the average breath (see text for details); black dots represent the overlapped samples within the respiratory cycle, while gray lines represent mean ± SD of the average breath.

 
Computations
The delta transpulmonary pressure ({Delta}PL) was computed as: Formula 2

The average global strain was computed as: Formula 2

The volume/pressure curves and the {Delta}volume/{Delta}transpulmonary pressure curves were derived from the PEEP trial. The power equation y = y0 + a x xb was fit to the data points and the dimensionless b parameter was used to describe the shape of the curves (19) upward (b > 1.1), downward (b < 0.9), and linear (0.9 <= b <= 1.1). The effect of inspiratory recruitment on strain computation are described in the online supplement. The specific lung elastance (Equation 2) was measured as {Delta}PL recorded after inflating the lung (with a super-syringe) with a volume equal to FRC as well as the slope of the stress to strain relationship of each patient measured during the PEEP trial.

Statistical Analysis
Data are reported as mean ± SD, unless otherwise specified, and range, as appropriate. Statistical significance was defined as P < 0.05. Comparison of baseline and physiologic variables was performed by one-way analysis of variance (ANOVA) for variables that were normally distributed, by nonparametric one-way ANOVA for variables that did not appear normally distributed on graphic inspection, and by chi-square test for qualitative variables. Power least squares fitting was used to describe volume/pressure curve shape. A Z-test was used to compare proportions of two groups. Mixed-design, three-way ANOVA was used to test the effects of the presence of the disease, the level of PEEP, and VT IBW. A logarithmic (log10) transformation was used for data not normally distributed. Bonferroni's t test was used to correct for multiple comparisons. Analysis was performed using SAS software, version 8.2 (SAS Institute, Cary, NC).


   RESULTS
TOP
 ABSTRACT
 AT A GLANCE COMMENTARY
 METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
See the online supplement for more details on results.

Study Population
The baseline characteristics of the study population are presented in Table 2. Anthropometric characteristics were similar among the four subgroups. Patients with ARDS had worse gas exchange than patients in the other subgroups, and the differences in mechanical ventilator settings accounted for the different extent of lung impairment, increasing from group 1 to group 4. In Table 3, we summarized the most relevant baseline variables of the respiratory system mechanics: from the healthiest control (the surgical subgroup) patients to the patients with full-blown ARDS, there is a progressive impairment of the elastance of the respiratory system (ETOT), FRC, and FRC normalized for expected FRC and expected total lung capacity.


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  		TABLE 2. BASELINE CHARACTERISTICS

 

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  		TABLE 3. BASELINE MECHANICAL CHARACTERISTICS

 
PEEP Trial
Representative volume/pressure curves and {Delta}volume/{Delta}transpulmonary pressure curves are shown in Figure 3. The volume/pressure curves either of the total respiratory system or of the lung, when fitted with an exponential function, presented a downward concavity in 21% of patients with ALI/ARDS versus 13.3% of the control subjects (P = 0.31), were linear in 30 versus 20% (P = 0.23), and showed a slight upward concavity in 49 versus 67% (P = 0.04) in patients with ALI/ARDS versus control subjects, respectively. The linear fitting was also adequate; 75% of patients with ALI/ARDS and 78% of control subjects presented an R2 greater than 0.95 (see the online supplement for details).


Figure 3
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  		Figure 3. Volume/pressure curves. Representative volume/pressure curves and {Delta}volume/{Delta}transpulmonary pressure curves. A, B, and C show volume/pressure curves of the respiratory system. D, E, and Fs show {Delta}volume/{Delta}transpulmonary pressure curves of the lung. Black solid lines represent exponential fittings; gray dashed lines represent linear fittings.

 
The summary of the respiratory system mechanics during the PEEP trial is reported in Table 4. As shown, the plateau airway pressure significantly increased, either with worsening lung conditions from group 1 to 4, or with increasing PEEP or VT IBW. The respiratory system elastance increased significantly from group 1 to 4 and with increasing PEEP, whereas it slightly but significantly decreased with increasing VT IBW. The lung (EL) and chest wall elastance (ECW) significantly increased from group 1 to 4 and with increasing PEEP, whereas both EL and ECW did not significantly change at different VT IBW levels.


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  		TABLE 4. RESPIRATORY MECHANICS

 
Generation of Stress
The relationship between plateau airway pressure and the corresponding {Delta}transpulmonary pressure (quantitatively equal to stress) is described by the following equation:

Formula 3(3)

As shown, the {Delta}PL/{Delta}Paw ratio equals the ratio of the lung elastance to the total elastance of the respiratory system (EL + ECW) and represents the pressure spent to distend the lung ({Delta}PL) relative to the pressure spent to distend the whole respiratory system ({Delta}Paw). In Figure 4, we report the {Delta}PL as a function of {Delta}Paw recorded during the PEEP trial in the individual patients of the four subgroups at 5 and 15 cm H2O PEEP and VT IBW of 6, 8, 10, and 12 ml/kg. The subgroups of patients with ALI/ARDS reached greater {Delta}Paw pressure and {Delta}PL than the two control subgroups (see also Tables 4 and 5). The slopes, {Delta}PL/{Delta}Paw of the individual regressions (i.e., the individual EL/[EL + ECW]), however, were not different in the four subgroups, and were, respectively, as follows: 0.69 ± 0.15 (range, 0.36–0.92) in the surgical control subjects, 0.74 ± 0.16 (range, 0.37–0.95) in the medical control subgroup, 0.64 ± 0.15 (range, 0.39–0.88) in the ALI subgroup, and 0.71 ± 0.16 (range, 0.33–0.92) in the ARDS subgroup (P = 0.26). In contrast, the EL/(EL + ECW) ratio was significantly greater in pulmonary versus extrapulmonary ARDS (P = 0.01) due to significant differences in lung and chest wall elastances (see the online supplement).


Figure 4
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  		Figure 4. {Delta}Transpulmonary and airway pressure relationship in surgical control subjects, medical control subjects, patients with acute lung injury (ALI) and patients with acute respiratory distress syndrome (ARDS). (A) The {Delta}transpulmonary to airway pressure relationship in 30 control subjects, and (B) the {Delta}transpulmonary to airway pressure relationship in 50 patients with ALI/ARDS. In both panels, gray solid lines represent the relationship observed in each individual subjects in the eight experimental conditions (i.e., four different VT [6, 8, 10, 12 ml/kg idea body weight]), at two different positive end-expiratory pressure (PEEP) levels (5 and 15 cm H2O). A linear function (y = ax + y0) was used. Vertical dashed lines at 20, 30, and 40 cm H2O airway pressure were drawn to underline the large variability of the corresponding {Delta}transpulmonary pressure both in control subjects and patients.

 

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  		TABLE 5. LUNG STRESS, STRAIN AND SPECIFIC LUNG ELASTANCE DURING THE POSITIVE END-EXPIRATORY PRESSURE TRIAL

 
Generation of Strain
The average global strain significantly increased from group 1 to 4, and with increasing VT IBW and PEEP (Table 5), and was significantly greater in patients with ALI/ARDS than in control subjects. In fact, the FRC decreased throughout the subgroups, from the healthiest surgical patients to the patients with ARDS (Table 3). Of note, however, within the same subgroup, for a given applied VT IBW and PEEP, and as already observed for lung stress (Figure 5), the lung strain variability was remarkable (Figure 6) due to FRC variability. We could not find any relationship between FRC, age, height, or body weight, either in patients with ALI/ARDS or in control subjects, who where anesthetized and paralyzed during the measurement.


Figure 5
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  		Figure 5. Lung stress at 6 and 12 ml/kg VT IBW in patients with acute lung injury (ALI) or acute respiratory distress syndrome (ARDS) and control subjects. For clarity, surgical and medical control subjects were grouped together as were the patients with ALI and ARDS (see the online supplement for further details). Individual values of lung stress detected in patients with ALI/ARDS (solid circles) and in control subjects (open circles) are reported, both at 6 and 12 ml/kg VT IBW with positive end-expiratory pressure (PEEP) of 5 and 15 cm H2O. Black solid lines represent mean values of each group. Dashed lines were drawn at lung stress of 10 and 15 cm H2O to underline the overlap of lung stress at different VT IBW and PEEP.

 

Figure 6
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  		Figure 6. Lung strain at 6 and 12 ml/kg VT IBW in patients with acute lung injury (ALI) or acute respiratory distress syndrome (ARDS) and control subjects. For clarity, surgical and medical control subjects were grouped together as were the patients with ALI and ARDS (see the online supplement for further details). Individual values of lung strain detected in patients with ALI/ARDS (solid circles) and in control subjects (open circles) are reported, both at 6 and 12 ml/kg VT IBW with positive end-expiratory pressure (PEEP) of 5 and 15 cm H2O. Black solid lines represent mean values of each group. Dashed lines were drawn at lung strain of 1 and 2 to underline the overlap of lung strain at different VT IBW and PEEP.

 
Stress/Strain Relationship
As shown in Table 5, during the PEEP trial, the lung stress significantly increased from groups 1 to 4 and with increasing PEEP and VT IBW. In contrast, the stress to strain ratio—that is, the specific lung elastance—was similar in the four subgroups and did not change throughout the different steps of the PEEP trial. This indicates a similar slope of stress to strain relationship as shown in Figure 7. In the four subgroups, we obtained the following values of the slope, which compared well with the values of specific lung elastance obtained by super-syringe: 13.3 ± 4.9 versus 13.4 ± 3.4 cm H2O in the surgical control subjects (P = 0.91), 12.8 ± 5.4 versus 12.6 ± 3.0 cm H2O in the medical control subjects (P = 0.83), 13.8 ± 4.5 versus 14.4 ± 3.6 cm H2O in patients with ALI (P = 0.26), and 13.7 ± 7.0 versus 13.5 ± 4.1 cm H2O in patients with ARDS (P = 0.83). No association was found between specific lung elastance and the pathogenesis of ALI/ARDS, or the duration of mechanical ventilation before the study.


Figure 7
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  		Figure 7. Lung stress to strain relationship in patients with acute lung injury (ALI) or acute respiratory distress syndrome (ARDS) and control subjects. For clarity, surgical and medical control subjects were grouped together as were the patients with ALI and ARDS (see the online supplement for further details). The relationships between mean values of {Delta}transpulmonary pressure (i.e., lung stress) and lung strain recorded for both patients with ALI/ARDS (solid circles) and control subjects (open circles) are reported. For clarity, data are shown as mean and standard error. The alignment of the average data points of patients with ALI/ARDS and healthy subjects emphasizes the similarity of the stress to strain relationships of the two populations.

 
Lung Strain and Lung Recruitment
Computing the strain as {Delta}V/FRC is correct only if the pulmonary units open at end inspiration are also open at end expiration. If new pulmonary units are recruited at end inspiration, the real strain will be lower, because the straining volume will be distributed on a greater number of pulmonary units (see model in the online supplement Figure E2). This model assumes that the recruited pulmonary units behave as the already-open units. According to this model (see online supplement for computation), we estimated the potential effect of lung recruitment (from 0 to 50% of FRC) on decreasing lung strain in patients with ALI/ARDS. The results are presented in Figure 8. As shown, the effects of lung recruitment are remarkable at high {Delta}PL, and progressively decrease as {Delta}PL decreases.


Figure 8
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  		Figure 8. Lung stress to strain relationship as corrected for lung recruitment in patients with acute lung injury (ALI) or acute respiratory distress syndrome (ARDS). The relationship between lung stress (i.e., {Delta}transpulmonary pressure) and lung strain is reported after the adjustment of lung strain for different percentage of lung recruitment. Adjustment of lung strain for recruitment was computed as follows: the maximal lung recruitment (the recruitment occurring at the highest pressure) was first arbitrarily established as a fraction of FRC (from 0 to 50%). Because the fractional recruitment is a function of the airway pressure applied, the fractional lung recruitment at each inspiratory airway pressure was then computed according to a sigmoid function derived from Crotti and colleagues (45) (computed tomography scan–based data), and the recruited gas volume was finally calculated. The adjusted strain was computed as the volume change divided by the sum of gas volume at FRC and the recruited gas volume. Solid circles denote values for a lung recruitment of 0% FRC, open circles denote values for a lung recruitment of 10% of FRC, inverted solid triangles denote values for a lung recruitment of 20% of FRC, open triangles denote values for a lung recruitment of 30% of FRC, solid squares denote values for a lung recruitment of 40% of FRC, and open squares denote values for a lung recruitment of 50% of FRC.

 

   DISCUSSION
TOP
 ABSTRACT
 AT A GLANCE COMMENTARY
 METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Our study population included a wide spectrum of mechanically ventilated patients, from healthy surgical patients to patients with the most severe forms of ARDS, throughout intermediate states, such as patients ventilated for medical systemic diseases without ALI/ARDS, and patients with ALI. In the whole population, we found that the plateau airway pressure and the tidal volume were not adequate surrogates for lung stress and strain. Moreover, we found that the stress to strain relationship was the same in all the subgroups we studied, that it was linear, or nearly linear, in the ranges of pressures and volumes we explored, and that the specific lung elastance was similar in the subgroups and independent of ventilator setting.

Because most of our results and inferences are based on the accuracy of esophageal manometry, a highly controversial measurement in critically ill patients (20, 21), a brief discussion on its value is appropriate. Many factors may alter the esophageal/pleural pressure relationship, such as the esophageal balloon elastance, the tone of the esophageal wall, as well as heart/lung weight and patient position (21). Moreover, the pleural pressure varies along the lung vertical axis as detected by esophageal manometry and inferred by the computed tomography scan (22). We studied experimentally the esophageal/pleural pressure relationship in supine, oleic acid–injured dogs (23). We found that esophageal pressure nearly equals the surface pleural pressure measured directly by wafers in the middle lung, is greater than the pleural pressure in the upper lung, and is lower than pleural pressure in the lower lung. The variations of pleural pressure, however, were similar to the variations of esophageal pressure at each lung level, as previously observed by other investigators (24–27). Indeed, the bulk of data suggests that the esophageal pressure variations, as used in this study, are the best available surrogate of pleural pressure variations. Of course, if the esophageal pressure variations are not coincident with pleural pressure variations, the error introduced in our stress estimate will be equal to the ratio of {Delta}(esophageal pressure) to {Delta}(average true pleural pressure).

PEEP Trial and Stress/Strain Generation
In most of the patients, during the PEEP trial, we obtained linear or nearly linear volume/pressure curves. Indeed, our derived variables, such as lung and chest wall elastance, the EL/(EL+ECW) ratio, and the specific lung elastance, can be adequately described by a single value. Our results confirm that {Delta}PL cannot be predicted from the airway pressure, as previously shown (28), due to the great variability of the ratio of lung elastance to the total respiratory system elastance (14, 29, 30) (see Equation 3). In fact, the EL/(EL+ECW) ratio in the ALI/ARDS subgroups ranged from 0.33 to 0.92 and, surprisingly, from 0.36 to 0.95 in the subgroups of surgical and medical patients. This implies that, for a given applied airway pressure to the whole respiratory system (e.g., 30 cm H2O), the resulting {Delta}transpulmonary pressure may be as low as approximately 10 cm H2O or as high as approximately 28 cm H2O (see Figure 4). This range of EL/(EL+ECW) ratios may appear unrealistic, particularly in control subjects, in whom normal lung and chest wall elastances are expected. However, it must be considered that EL/(EL+ECW) is a ratio and each "normal value" has its own standard deviation and range. For example, in a normal patient, in whom the chest wall elastance is in its lowest normal value (~2 cm H2O/L) and in whom, during anesthesia and paralysis, the lung elastance rises to 20 cm H2O/L due to the lung collapse, the EL/(EL+ECW) ratio will be 20/22 = 0.91, which is only apparently an unrealistic value. In fact, the mechanical set described above will produce a plateau pressure of 11 cm H2O with a tidal volume of 500 ml, which is a very common observation during normal anesthesia. On the other hand, a remarkably low EL/(EL+ECW) ratio, such as 0.33, may be observed in a patient with ALI from abdominal disease, with a respiratory system elastance of 24 cm H2O/L and an intraabdominal pressure of 30 cm H2O. In this case, the chest wall elastance could rise up to 16 cm H2O/L (31) and this set will produce an EL/(EL+ECW) ratio of 8/(8+16) = 0.33. Interestingly, in this series of patients, we confirmed that the EL/(EL+ECW) ratio was significantly lower and the chest wall elastance significantly higher in extrapulmonary than in pulmonary ARDS (31). Indeed, the variability of the EL/(EL+ECW) ratio accounts for the inadequacy of plateau airway pressure as a surrogate of lung stress and explains why, in a fraction of patients, high or low tidal volume produces a similar stress as shown by the remarkable overlap of individual data in Figure 5.

In this study, we found that tidal volume normalized for ideal body weight is a poor surrogate of lung strain. This is due to the high variability of FRC, well known in ALI/ARDS (32, 33), in which the relationship between FRC and body weight is lost due to the presence of lung pathology. As a consequence, a 70-kg–body weight subject, depending on the kind and extent of his or her lung pathology, may have completely different FRC values (and strain for the same applied tidal volume). The control subjects had lower average strain than patients with ALI/ARDS, but they presented a similar variability of lung strain. This may appear surprising, because in normal, awake, spontaneously breathing subjects, the FRC, height, and IBW are correlated (34), and this should reduce the variability of the lung strain. In fact, in our control subjects, we could not find any correlation between FRC and body weight/height. We have to consider, however, that all our control subjects were studied during anesthesia and paralysis, which, per se, alter the FRC/body weight relationship due to collapse of the dependent lung regions (35). In our healthiest surgical subgroup, the reduction of FRC compared with the expected FRC (46) (see Table 3) is in the order of 17%, and anesthesia and paralysis fully account for this reduction (36, 37). In medical control subjects, however, the FRC reduction of 45% exceeds the usual effects of anesthesia. These patients, however, were ventilated for a systemic disease (Table 1), and the lung may have not been as healthy as in the surgical subgroup. In summary, between the subgroups, as well as within the subgroups, we found that VT IBW is a poor surrogate of lung strain. Not surprisingly, although higher VT IBW produced, on average, greater lung strain than low VT IBW, in individual patients high or low VT IBW could produce the same strain, due to the remarkable data overlap (see Figure 6).

Stress to Strain Relationship in Healthy and ALI/ARDS Lungs
In this study, we found that the strain to stress relationship was similar throughout the entire population, as shown by specific lung elastance, which remained constant at a value of approximately 13.5 cm H2O throughout the subgroups and did not change with the mechanical ventilator settings. A similar specific lung elastance implies that similar {Delta}transpulmonary pressure produces similar fractional changes of FRC either in healthy or in acutely injured lung. Therefore, a {Delta}PL of approximately 13.5 cm H2O may cause an FRC of 3 L to inflate up to 6 L in normal man, and an FRC of 0.5 L to inflate up to 1 L in patients with ARDS. These findings strongly support the "baby lung" concept (38, 39): the greater stiffness of ALI/ARDS lung, as observed by the marked reduction of lung compliance, is due to the large decrease in FRC (baby lung) rather than to deteriorated mechanics of the aerated portion. The ALI/ARDS lung is "small" rather than "stiff." These observations imply that, in ALI/ARDS lungs, at least in the early phase, interstitial lung edema, cellular infiltration, early fibrosis, or surfactant deficit are not so extensive as to alter intrinsic lung mechanics in the lung regions open to ventilation (40), being the specific lung elastance of ALI/ARDS patients similar to that of control subjects.

Recruitment Effect
Our quantification of strain must be considered an approximation. In theory, the maximal strain naturally applied to the lung (i.e., at total lung capacity) should be around 2–2.5. In fact, the inextensible collagen fiber network comprises a "stop-length" system that is folded at FRC and should be fully unfolded at total lung capacity (8), preventing further lung expansion. When computing strain not taking into account lung recruitment, we observed, in a few patients, unadjusted raw values as high as 3 or 4, which are clearly overestimates (Figure 6). It is likely that in these patients the lung recruitability was remarkable, and the real strain was lower than that assessed by using raw data.

Clinical Implications
To date, we cannot define a threshold for "harmful" stress and strain. However, due to the similarity of the stress to strain relationship in acutely injured and healthy lungs, it is conceivable that data derived from healthy animals subjected to "lethal" mechanical ventilation may be used as a first attempt to speculate on a possible harmful threshold. In a study on healthy sheep, in which FRC was reported, "lethal mechanical" ventilation, applied for about 12 (41) and 24 (42) hours, corresponded to an average strain of 2.8 and 2.5, respectively, the range of strain expected with a lung expanded to total lung capacity. Accordingly, we believe that a strain greater than 2 (i.e., corresponding to an end-inspiratory lung volume in the range of total lung capacity) may be lethal for the lung. Because stress and strain are linked by a remarkably constant proportionality factor, the specific lung elastance (i.e., stress approximately 13.5 cm H2O x strain), in clinical practice, measuring stress as {Delta}PL or measuring strain as {Delta}V/FRC is equivalent. If we know, for example, that a harmful threshold of strain is around 2, it follows that the harmful threshold of stress will be approximately 2 x 13.5 cm H2O (i.e., approximately 27 cm H2O {Delta}PL). Therefore, the recommended plateau airway pressure below 30 cm H2O (43) seems reasonable for most of the patients with ALI/ARDS because only few of them may show, at that level of plateau airway pressure, a {Delta}PL of approximately 27 cm H2O. In our practice, we have been measuring stress and strain for almost 2 years in all patients with ALI/ARDS. In a very small fraction of patients whose "baby lung" was extremely small, we recorded strain values higher than 2 even at VT IBW lower than 6 ml/kg. It is likely that, for these patients, "safe" mechanical ventilation does not exist, and for them we consider extracorporeal support (44). On the other hand, measuring stress and strain may rescue some patients condemned to very low VT IBW or PEEP, if physicians only looked at airway plateau pressure. We believe that introducing the measurement of stress and strain into clinical practice will allow to better clarify the safe limits of mechanical ventilation.


   Acknowledgments
 
The authors thank Prof. Rolf D. Hubmayr, Section Head, Pulmonary and Critical Care Medicine, Mayo Clinic College of Medicine (Rochester, MN), for his helpful criticism in reviewing the manuscript. They also thank the physicians and nursing staff of the Dipartimento di Anestesia, Rianimazione (Intensiva e Subintensiva) e Terapia del Dolore, Fondazione IRCCS–"Ospedale Maggiore Policlinico Mangiagalli Regina Elena" di Milano for their valuable cooperation.


   FOOTNOTES
 
Supported by departmental funding only.

This article has an online supplement, which is accessible from this issue's table of contents at www.atsjournals.org

Originally Published in Press as DOI: 10.1164/rccm.200710-1589OC on May 1, 2008

Conflict of Interest Statement: D.C. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. E.C. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. P. Cadringher received fees from GE Medical Systems Information Technologies, Inc., as a consulting Hospital Biomedical Engineer for the study entitled "Assessment of the Engstrom Carestation's FRC Measurement against Current Practices and Determination of Lung Thresholds" (since 2007). P. Caironi does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. F.V. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. F.P. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. F.T. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. P. Cozzi does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. M.C. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. A.C. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. J.J.M. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. L.G. reports receiving fees from GE Medical Systems Information Technologies, Inc., as Principal Investigator for the study entitled "Assessment of the Engstrom Carestation's FRC Measurement against Current Practices and Determination of Lung Thresholds" (since 2007).

Received in original form October 29, 2007; accepted in final form April 21, 2008


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TOP
 ABSTRACT
 AT A GLANCE COMMENTARY
 METHODS
 RESULTS
 DISCUSSION
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