Lung Stress, Strain, Hysteresis, and Recruitment

A ventilator applies pressure and delivers volume, but the lung experiences neither uniformly. The same breath can open one region while stretching another. Stress, strain, and recruitment connect the measurements at the airway to these different tissue responses.

24.1 Lung Stress Is the Force Distending the Lung

Stress describes force distributed over an area. In lung physiology, transpulmonary pressure provides a useful whole-lung estimate of the distending stress applied to the parenchyma. It is the pressure inside the alveoli minus the pressure surrounding the lung.

Use the same units for both pressures, commonly cm H₂O. Increasing alveolar pressure or lowering pleural pressure increases the distending pressure. This explains how positive-pressure ventilation and inspiratory muscle effort can both inflate the lung through different changes in pressure.

At an end-inspiratory hold in a passive patient, airway plateau pressure can approximate alveolar pressure when flow has stopped and pressures have equilibrated. Peak airway pressure during flow includes the pressure needed to overcome airway resistance. It therefore cannot be substituted directly for alveolar distending pressure.

Consider an illustrative plateau pressure of 28 cm H₂O and pleural pressure of 12 cm H₂O at the same moment. The transpulmonary pressure is 28 − 12 = 16 cm H₂O. If pleural pressure were instead 20 cm H₂O, the same airway pressure would produce 28 − 20 = 8 cm H₂O. The airway reading alone cannot identify how the applied pressure is divided between lung and chest wall.

These examples assume a valid static measurement and a meaningful estimate of pleural pressure. Esophageal pressure can provide information about pleural pressure, but position, balloon filling, calibration, and regional pressure gradients affect interpretation. One esophageal value does not measure the stress in every part of the lung.

Transpulmonary pressure is also a macroscopic distending pressure rather than the actual tensile stress in each alveolar wall. Local geometry and the tissue supporting neighboring regions determine how forces are concentrated. A reassuring global estimate can therefore coexist with high local deformation.

Chiumello and colleagues measured lung stress and strain in ventilated patients with and without lung injury. Their physiological study showed why airway pressure and body-size-adjusted tidal volume do not uniquely determine lung stress across patients. The chest wall and the size of the ventilated lung influence that relationship. 1

24.2 Strain Describes Deformation Relative to a Starting Volume

Strain describes deformation relative to a reference size. For whole-lung volume measurements, it is usually expressed as the increase in lung volume divided by a specified reference volume. Because both quantities use the same volume units, strain has no units.

The denominator is essential. A 400-mL breath entering a lung with a reference gas volume of 2,000 mL produces a volume ratio of 400 / 2,000 = 0.20. The same breath relative to a 1,000-mL reference produces 400 / 1,000 = 0.40. The smaller reference volume undergoes twice the fractional volume increase.

In injured lungs, nonaerated regions reduce the gas-containing lung available to receive the tidal breath. This is one reason a modest tidal volume based on predicted body weight can still produce substantial deformation in the remaining aerated lung. Predicted body weight estimates expected healthy lung size; it does not measure the current functional lung volume.

Different studies use different reference volumes. Functional residual capacity, or FRC, conventionally refers to the resting end-expiratory volume at the equilibrium of lung and chest-wall recoil. Research strain calculations may use a measured or estimated zero-PEEP volume as their reference. End-expiratory lung volume during PEEP is a different starting point. State which denominator is used before comparing strain values.

Using a fixed reference, total inflation above that reference can be separated conceptually into the volume maintained by PEEP and the additional tidal volume. These components are often described as static and dynamic strain when both use that same reference denominator. Dividing tidal volume by the PEEP-supported end-expiratory volume instead describes a different tidal expansion ratio. It should not be silently equated with total strain referenced to zero PEEP.

These calculations are global volume descriptions. Recruitment introduces gas into previously unaerated regions, so not all added gas represents stretching of already open tissue. Regional strain can also differ sharply from the global ratio. A change in the denominator may reflect a change in what is aerated as well as a change in the inflation of the same tissue.

24.3 Specific Elastance Links Stress to Strain Within a Model

Elastance describes how much pressure changes for a change in volume. Specific lung elastance relates the change in distending pressure to the fractional volume change. Normalizing volume to a reference size helps explain why a small functional lung can have a high conventional elastance without every part of its tissue being intrinsically stiffer.

The pressure change and strain must use compatible reference conditions. If strain is defined as volume above a chosen resting reference, the pressure change should be measured from that reference as well. Dividing an absolute end-inspiratory pressure by a tidal-only strain would combine different parts of the inflation sequence.

In an illustrative model, suppose the transpulmonary pressure rises by 12 cm H₂O above the reference and the corresponding volume strain is 0.8. Specific elastance is 12 / 0.8 = 15 cm H₂O. Rearranging gives a modeled pressure change equal to specific elastance multiplied by strain. The ratio retains pressure units because strain is dimensionless.

This relationship is useful for understanding size normalization. It is not an exact material constant that can be assigned to every patient's lung. Measured reference volume, recruitment, chest-wall estimation, tissue nonlinearity, and regional heterogeneity affect the calculation. Extrapolating a ratio beyond the pressure and volume range in which it was measured can be misleading.

The relationship also does not supply a universal injury threshold. A repeated deformation can cause different effects depending on the lung's prior injury, local concentration of forces, breathing frequency, exposure duration, and vascular conditions. One peak value omits much of that history.

Protti and colleagues ventilated healthy pigs for prolonged periods using different volume strains and examined lung injury. The experiment demonstrated that sufficiently large deformation can produce injury even in initially healthy lungs. Its experimental thresholds cannot be transferred directly to heterogeneous human ARDS or used to define a universally safe strain. 2

24.4 Hysteresis Means the Inflation History Changes the Response

A lung does not necessarily have the same volume at a given pressure during inflation and deflation. On a broad inflation–deflation curve, volume is often greater during deflation at the same pressure. This difference between the paths is hysteresis.

Several processes contribute. The surface film lining the airspaces changes as surface area changes. Recruitment and closure change which regions contain gas. Tissue viscoelasticity makes the response depend on time and previous deformation. During ordinary ventilator breaths, airway resistance also adds a flow-dependent difference between inspiratory and expiratory pressure.

Surfactant helps reduce surface tension at the air–liquid interface. Its behavior changes during compression and expansion of the surface film. Lower surface tension during deflation helps stabilize airspaces at lower volumes. Inflation after substantial deflation can require more pressure to reestablish aeration and expand the surface than is needed to maintain the same volume during deflation.

Classic experiments comparing air-filled and liquid-filled excised lungs helped separate the contribution of the air–liquid interface from other mechanical properties. Bachofen and colleagues studied pressure–volume curves in these preparations and analyzed surface tension in situ. This experimental setting establishes important mechanisms without reproducing all the conditions of a perfused, injured human lung. 3

The area inside a pressure–volume loop represents mechanical energy not recovered on expiration under the measured conditions. A dynamic loop can include airway resistance, tissue dissipation, and recruitment-related effects. Its area does not identify how much energy was dissipated in each location or prove that all of that energy produced injury.

Hysteresis also explains why identical settings can yield different lung volumes after different preceding maneuvers. An end-expiratory volume measured just after inflation to a high pressure may differ from one measured after disconnection or prolonged low pressure. Measurement timing and inflation history therefore matter when comparing compliance or recruitment.

A static or low-flow pressure–volume curve reduces some flow-related effects, but it does not make the lung a single compartment. Its shape still combines regional opening, tissue expansion, and chest-wall behavior. A bend in the whole-system curve is not an exact opening pressure shared by every alveolus.

24.5 Recruitment Opens Tissue That Was Not Participating in Aeration

Recruitment means that previously unaerated lung becomes aerated. It can occur when a sufficient pressure is applied for sufficient time to overcome the forces keeping a region closed. These forces differ across the lung, so recruitment is a distributed process rather than a single pressure threshold.

Some regions are compressed by the weight of edematous tissue. Others contain liquid, have impaired surface-film function, or lie behind closed small airways. A dense region on imaging is not necessarily an easily collapsible airspace that will reopen with pressure. Consolidation, flooding, and structural injury can limit recruitability.

Once a region opens, it may remain open at a pressure lower than was required to open it. This difference between opening and closing conditions is part of the rationale for maintaining end-expiratory pressure. PEEP can reduce derecruitment between breaths, although adequate PEEP may also recruit some regions over time. A separate high-pressure maneuver is not a prerequisite for every recruitment response.

Recruitment differs from distension. When pressure rises, gas can enter newly aerated regions and also enlarge regions that were already open. A larger total lung volume therefore cannot be labeled entirely as recruited volume. Overdistension refers to excessive expansion of already aerated tissue, which may occur at the same time as recruitment elsewhere.

Gattinoni and colleagues used whole-lung CT at different airway pressures in patients with acute lung injury or ARDS. The amount of potentially recruitable tissue varied greatly between individuals. Their study demonstrated that a similar diagnosis or oxygenation impairment does not imply a uniform anatomical response to pressure. The high research inflation pressure used to assess potential recruitability was not a therapeutic prescription. 4

Recruitment also depends on the pressure range, duration, and timing of measurement. A lung may recruit substantially over one range and little over another. Tissue that aerates briefly at high pressure may close again when pressure falls. The relevant clinical question includes whether recruitment can be maintained without unacceptable stretch or circulatory compromise.

24.6 Regional Heterogeneity Concentrates Forces

ARDS produces a patchwork of normally aerated, poorly aerated, and nonaerated regions. The delivered breath preferentially enters regions that remain connected to the airway and can expand. The total tidal volume is therefore not distributed in proportion to the anatomical size of the lung.

Neighboring regions also pull on one another through the shared tissue network. If one region expands while an adjacent region remains collapsed or stiff, the interface can concentrate forces. The local mechanical load may be greater than the average suggested by a whole-lung pressure measurement. Such interfaces are often described as stress raisers.

This is not equivalent to multiplying every measured transpulmonary pressure by one fixed factor. The amplification depends on geometry, the size of the affected region, surrounding tissue mechanics, and the direction of deformation. A CT appearance of inhomogeneity is a structural clue; it is not a direct microscopic measurement of septal tensile stress.

Cressoni and colleagues quantified regional inhomogeneity on CT in ARDS. Greater inhomogeneity was associated with more severe disease and worse outcomes. The study supports attention to regional structure, but its associations do not prove that a particular intervention that changes the image will improve survival. 5

Recruitment could reduce some interfaces by making aeration more uniform. The same pressure increase could overstretch regions that were already open. A global improvement in compliance can therefore coexist with persistent areas of overdistension. Likewise, unchanged global compliance can conceal recruitment in one region offset by increased stiffness in another.

Position changes can alter the distribution of tissue weight, pleural pressure, and ventilation. Their mechanical consequences depend on more than the change in oxygenation. The general principle is that modifying the spatial distribution of aeration can change regional deformation even when tidal volume is unchanged.

For clinical interpretation, combine whole-system mechanics with information about distribution when available. The goal is to understand what tissue is receiving the breath and where the forces may be concentrated. No single global pressure or volume measurement resolves that entire problem.

24.7 PEEP Can Stabilize the Lung While Increasing Its Inflation

PEEP raises the pressure maintained at the end of expiration. In recruitable regions, it can prevent closure and preserve a larger aerated lung for the next breath. If the same tidal volume is then distributed across more participating tissue, tidal deformation in some regions may decrease.

PEEP also increases resting inflation in already aerated regions. The lung may therefore experience less cyclic collapse but more sustained distension. These are competing effects rather than mutually exclusive descriptions of the intervention.

Caironi and colleagues analyzed CT-based opening and closing in patients with acute lung injury or ARDS. Increasing PEEP reduced opening and closing most clearly in patients with greater potential recruitability, while increasing strain in both higher- and lower-recruitability groups. This physiological analysis illustrates the competing mechanisms; it was not a randomized demonstration that a particular PEEP choice improves survival. 6

The circulation introduces additional tradeoffs. Higher intrathoracic pressure can reduce the pressure difference driving venous return. Expansion of aerated lung can compress alveolar vessels and increase right ventricular load. Recruitment may improve some pulmonary vascular conditions, but the net effect depends on lung volume, vascular recruitment, cardiac function, and the patient's starting state.

Improved oxygenation does not settle the balance. Opening perfused units can reduce shunt. Changes in cardiac output and redistribution of pulmonary blood flow can also change arterial oxygenation. A better oxygen value may therefore coexist with reduced systemic oxygen delivery if output falls sufficiently.

Sustained inflation and tidal expansion should also be considered separately. A lower driving pressure at higher PEEP may suggest that tidal ventilation is using the respiratory system more efficiently under the measured conditions. It does not establish that end-inspiratory lung stress or the total inflation above the reference has decreased.

Evaluate the observed response across mechanics, gas exchange, and circulation. The relevant benefit is maintained aeration with an acceptable degree of distension and cardiovascular tolerance. The highest PEEP producing an oxygenation gain is not automatically the most protective setting.

24.8 Compliance and Driving Pressure Describe a Response, Not Its Only Cause

During passive ventilation, respiratory-system driving pressure is plateau pressure minus total end-expiratory pressure. Total PEEP includes intrinsic PEEP when it is present. Dividing tidal volume by driving pressure estimates respiratory-system compliance under the measurement conditions.

Consider an illustrative passive patient receiving 420 mL per breath. At a plateau pressure of 28 cm H₂O and total PEEP of 10 cm H₂O, driving pressure is 28 − 10 = 18 cm H₂O. Compliance is 420 / 18 ≈ 23 mL/cm H₂O. After a PEEP change, plateau pressure is 30 and total PEEP is 14. Driving pressure is now 30 − 14 = 16 cm H₂O. Compliance is 420 / 16 ≈ 26 mL/cm H₂O.

The tidal volume is unchanged, but less additional pressure is needed above the new end-expiratory level. Recruitment is one possible explanation. Changes in chest-wall behavior, measurement validity, or other mechanical conditions can also contribute. The end-inspiratory airway pressure has risen even though driving pressure has fallen.

Amato and colleagues analyzed individual-patient data from prior randomized ventilation trials. Driving pressure was strongly associated with survival, and the analysis helped connect delivered volume to functional lung size. The analysis was not itself a randomized test of a standalone driving-pressure target, and it does not establish one universally safe value. 7

The measurement becomes less interpretable when the patient is actively generating pressure during the hold. Inspiratory effort can lower pleural pressure and increase transpulmonary pressure while the airway reading appears acceptable. Incomplete equilibration, leaks, and unmeasured intrinsic PEEP can also distort the calculation.

Thus, a compliance response should prompt a physiological explanation rather than be treated as anatomical proof. A favorable change is more convincing when it agrees with sustained aeration, acceptable end-inspiratory loading, and cardiovascular tolerance. A small numerical change close to measurement variability should not carry more certainty than the measurement supports.

24.9 Recruitability Measurements Answer Different Questions

CT can identify changes in the amount of nonaerated and aerated tissue between defined pressure conditions. Its strength is anatomical detail. Its limits include radiation, transport, the time sampled, and the fact that pressure-dependent aeration does not reveal the entire microscopic injury process.

Electrical impedance tomography, or EIT, estimates regional changes in gas content within the imaged thoracic region. Costa and colleagues described an approach using regional compliance changes during decremental PEEP to estimate collapse and hyperdistension. It provides useful spatial information, but its estimates are model-dependent and do not equal direct whole-lung anatomical measurements. 8

The recruitment-to-inflation ratio offers a bedside mechanical estimate. Chen and colleagues measured the additional expired volume during a PEEP reduction and compared it with the volume expected from inflation of the previously open lung. The excess was used to estimate recruited volume, with adjustment for airway opening pressure when complete airway closure was present. 9

Conceptually, the method separates the increase in gas volume into expected inflation and additional recruited volume. The recruited volume divided by the effective pressure change gives a recruited compliance. Dividing that by the low-PEEP compliance gives the recruitment-to-inflation ratio. It is a comparison of two compliances, rather than the fraction of the whole lung that is recruitable.

The assumptions matter. Leaks, spontaneous effort, inaccurate volume measurement, incomplete expiration, and airway closure can distort the result. Recruitability over one PEEP interval does not describe every other interval. The assessment also does not measure whether the right ventricle tolerates the pressure needed to maintain the recruited volume.

These methods provide complementary observations. CT describes aeration anatomically, EIT estimates regional ventilation behavior, and a mechanical ratio estimates a pressure-dependent volume response. None independently identifies a survival-improving PEEP or justifies escalating pressure until every potentially recruitable region opens.

24.10 Recruitment Maneuvers Require Outcome Evidence

A recruitment maneuver temporarily raises pressure in an attempt to open closed regions. It differs from the subsequent PEEP used to maintain aeration. The maneuver's effects depend on pressure, duration, underlying recruitability, and circulatory reserve.

High pressure can open some regions while overdistending others. It can also reduce venous return, raise pulmonary vascular load, and precipitate hemodynamic deterioration. Even when oxygenation or compliance improves afterward, the intervention may have imposed harmful stress during the maneuver or left excessive inflation at the selected PEEP.

ART directly tested an intensive strategy rather than merely describing a mechanical response. Adults with moderate to severe ARDS were randomized to a recruitment maneuver followed by PEEP titration according to best respiratory-system compliance, or to a conventional low-PEEP strategy. Among 1,010 patients, 28-day mortality was higher with the recruitment/titration strategy: 55.3% versus 49.3%. Barotrauma was also more frequent. 10

This is evidence against routine use of the strategy tested. Because the intervention combined a maneuver and a titration protocol, the trial cannot assign the entire effect to one component or answer every question about every PEEP adjustment. It does show why better physiological measurements cannot substitute for patient outcomes.

A monitoring method likewise requires evaluation as a strategy. EPVent-2 compared esophageal-pressure-guided PEEP with an empirical high-PEEP/oxygen strategy in 200 patients with moderate to severe ARDS. It found no significant difference in its primary ranked outcome incorporating death and ventilator-free days. The result does not invalidate the pressure measurement's physiological information; it limits claims that the tested guidance strategy improved outcomes. 11

Keep those distinctions explicit. A region can be recruitable, a monitor can detect a response, and a protocol can still fail to improve outcomes. Mechanistic understanding is needed to interpret the response, while clinical trials determine what can be claimed about the treatment strategy.

24.11 Integrating Stress, Strain, and Recruitment at the Bedside

Consider an illustrative patient with ARDS whose oxygenation improves after PEEP is increased. Tidal volume is unchanged. Driving pressure decreases modestly, but plateau pressure rises and arterial pressure falls.

The oxygenation and driving-pressure changes are compatible with recruitment, but neither proves it. Review whether the patient was passive, total PEEP was measured, and plateau pressure was obtained after equilibration. A change in effort or intrinsic PEEP could change the apparent response without representing the assumed tissue mechanics.

Next consider both the tidal and sustained inflation. The lower driving pressure describes the smaller pressure increment needed for the breath above the new end-expiratory level. The higher plateau pressure shows that the respiratory system reaches a higher end-inspiratory pressure. Without pleural-pressure information and regional assessment, it is not possible to conclude that lung stress decreased everywhere.

The arterial-pressure fall requires a parallel explanation. Reduced venous return, increased right ventricular load, or another simultaneous change may be responsible. A better oxygenation value does not compensate automatically for a clinically important fall in cardiac output. Examine perfusion and cardiac findings rather than treating the gas-exchange response as the only endpoint.

If regional or recruitment measurements are available, use them to refine this account. Evidence of new dependent aeration alongside increasing nondependent overdistension describes a mixed response. A high estimated recruitability with poor cardiovascular tolerance still presents a limitation. A low recruitability estimate weakens the assumption that more pressure will distribute the breath across substantially more tissue.

As illness, position, fluid balance, and spontaneous effort change, the response can change again. Recruitment achieved earlier may not persist under the same settings, and an improving lung may need a different amount of end-expiratory support. Reassessment should therefore follow the trajectory rather than preserve a setting because it once produced a favorable number.

The integrated aim is to support gas exchange while limiting harmful deformation and preserving circulation. Stress, strain, hysteresis, and recruitment provide the mechanisms needed to understand why that balance differs between patients and over time.

References

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2. Protti A, Cressoni M, Santini A, et al. Lung stress and strain during mechanical ventilation: any safe threshold? Am J Respir Crit Care Med. 2011;183(10):1354–1362. doi:10.1164/rccm.201010-1757OC

3. Bachofen H, Hildebrandt J, Bachofen M. Pressure-volume curves of air- and liquid-filled excised lungs-surface tension in situ. J Appl Physiol. 1970;29(4):422–431. doi:10.1152/jappl.1970.29.4.422

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7. Amato MBP, Meade MO, Slutsky AS, et al. Driving pressure and survival in the acute respiratory distress syndrome. N Engl J Med. 2015;372(8):747–755. doi:10.1056/NEJMsa1410639

8. Costa ELV, Borges JB, Melo A, et al. Bedside estimation of recruitable alveolar collapse and hyperdistension by electrical impedance tomography. Intensive Care Med. 2009;35(6):1132–1137. doi:10.1007/s00134-009-1447-y

9. Chen L, Del Sorbo L, Grieco DL, et al. Potential for lung recruitment estimated by the recruitment-to-inflation ratio in acute respiratory distress syndrome. A clinical trial. Am J Respir Crit Care Med. 2020;201(2):178–187. doi:10.1164/rccm.201902-0334OC

10. Writing Group for the Alveolar Recruitment for Acute Respiratory Distress Syndrome Trial (ART) Investigators; Cavalcanti AB, Suzumura EA, et al. Effect of lung recruitment and titrated positive end-expiratory pressure (PEEP) vs low PEEP on mortality in patients with acute respiratory distress syndrome: a randomized clinical trial. JAMA. 2017;318(14):1335–1345. doi:10.1001/jama.2017.14171

11. Beitler JR, Sarge T, Banner-Goodspeed VM, et al. Effect of titrating positive end-expiratory pressure (PEEP) with an esophageal pressure-guided strategy vs an empirical high PEEP-FIO₂ strategy on death and days free from mechanical ventilation among patients with acute respiratory distress syndrome: a randomized clinical trial. JAMA. 2019;321(9):846–857. doi:10.1001/jama.2019.0555