Stress and strain within the lung

BackgroundStress and strain are parameters to describe respiratory mechanics during mechanical ventilation. Calculations of stress require invasive and difficult to perform esophageal pressure measurements. The hypothesis of the present study was: Can lung stress be reliably calculated based on non‐invasive lung volume measurements, during a decremental Positive end‐expiratory pressure (PEEP) trial in mechanically ventilated patients with different diseases?MethodsData of 26 pressure‐controlled ventilated patients admitted to the ICU with different lung conditions were retrospectively analyzed: 11 coronary artery bypass graft (CABG), 9 neurology, and 6 lung disorders. During a decremental PEEP trial (from 15 to 0 cmH2O in three steps) end‐expiratory lung volume (EELV) measurements were performed at each PEEP step, without interruption of mechanical ventilation. Strain, specific elastance, and stress were calculated for each PEEP level. Elastance was calculated as delta PEEP divided by delta PEEP volume, whereas specific elastance is elastance times the FRC. Stress was calculated as specific elastance times the strain. Global strain was divided into dynamic (tidal volume) and static (PEEP) strain.ResultsStrain calculations based on FRC showed mainly changes in static component, whereas calculations based on EELV showed changes in both the static and dynamic component of strain. Stress calculated from EELV measurements was 24.0 ± 2.7 and 13.1 ± 3.8 cmH2O in the lung disorder group at 15 and 5 cmH2O PEEP. For the normal lungs, the stress values were 19.2 ± 3.2 and 10.9 ± 3.3 cmH2O, respectively. These values are comparable to earlier publications. Specific elastance calculations were comparable in patients with neurologic and lung disorders, and lower in the CABG group due to recruitment in this latter group.ConclusionStress and strain can reliably be calculated at the bedside based on non‐invasive EELV measurements during a decremental PEEP trial in patients with different diseases.

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“…Strain describes the relation between end‐inspiratory volume (i.e., tidal volume + PEEP volume) and FRC, and is calculated using formula (1), 8:

{Strain}_{normalglobal} = \frac{V_{normalT} + V_{normalPEEP}}{FRC}

…”

Section: Methodsmentioning

confidence: 99%

Lung stress and strain calculations in mechanically ventilated patients in the intensive care unit

Blankman

Hasan

Bikker

et al. 2015

Acta Anaesthesiol Scand

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“…Strain is the physical deformation, or change in shape, of an alveolus, caused by stress. Stress and strain can be described by the relationship, 13 P L (stress) ϭ specific lung elastance ϫ ⌬V/functional residual capacity (strain), where P L is the trans-alveolar pressure, and ⌬V is the change in lung volume above resting functional residual capacity with the addition of PEEP and V T . Specific lung elastance (compliance per lung volume) is constant at 13.5 cm H 2 O.…”

Section: Stress and Strainmentioning

confidence: 99%

“…13 Consider 2 adjacent alveoli fully expanded at a P L of 30 cm H 2 O. If one of the 2 regions collapses, the applied force concentrates in the other, thereby increasing its strain and stress.…”

Section: Stress and Strainmentioning

confidence: 99%

Recruitment Maneuvers and PEEP Titration

2015

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The injurious effects of alveolar overdistention are well accepted, and there is little debate regarding the importance of pressure and volume limitation during mechanical ventilation. The role of recruitment maneuvers is more controversial. Alveolar recruitment is desirable if it can be achieved, but the potential for recruitment is variable among patients with ARDS. A stepwise recruitment maneuver, similar to an incremental PEEP titration, is favored over sustained inflation recruitment maneuvers. Many approaches to PEEP titration have been proposed, and the best method to choose the most appropriate level for an individual patient is unclear. A PEEP level should be selected that balances alveolar recruitment against overdistention. The easiest approach to select PEEP might be according to the severity of the disease: 5-10 cm H 2 O PEEP in mild ARDS, 10 -15 cm H 2 O PEEP in moderate ARDS, and 15-20 cm H 2 O PEEP in severe ARDS. Recruitment maneuvers and PEEP should be used within the context of lung protection and not just as a means of improving oxygenation.

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“…In pre-injured lungs, persistently small tidal volumes have been shown to be associated with lower oxygenation and reduced compliance [13,14], presumably due to advancing atelectasis. If so, the number of 'stress risers' may actually increase [6]. As pointed out by Lellouche and Lipes [1], providing enough PEEP may forestall atelectasis, and perhaps the discomfort associated with unremitting inflations with low tidal volumes-but this remains to be shown.…”

mentioning

confidence: 96%

“…For example, marathon athletes may repeatedly take breaths exceeding 2 l ([20 ml/kg) for hours on end without apparent problems afterward. Patients at risk of ARDS may be predisposed to injurious responses to considerably lower tidal volumes by reduced aerated reservoir (functional residual capacity, FRC), stress focusing at points of mechanical heterogeneity [6], and by tissues primed for inflammation by the first hit of their underlying disease [7,8]. Yet, until the airspace begins to flood or consolidate, the aeratable space is seldom reduced by more than half, and when precautions are taken to avoid fully horizontal positioning and to apply modest PEEP, tidal re-opening and closure risk is reduced considerably.…”

mentioning

confidence: 99%