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

Abstract

Background: Stress 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?

Methods: Data 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 cmH2 O 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.

Results: Strain 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 cmH2 O in the lung disorder group at 15 and 5 cmH2 O PEEP. For the normal lungs, the stress values were 19.2 ± 3.2 and 10.9 ± 3.3 cmH2 O, 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.

Conclusion: Stress 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.

Figure 1

Strain calculated for patients with different lung conditions during a decremental PEEP trial. Data are shown as mean ± SE. Open triangles: global strain; open circles: static strain (PEEP); closed circles: dynamic strain (tidal volume); dashed lines: interpolation lines. Differences are considered to be significant if P < 0.05. *Significant changes in global strain; #Significant changes in static strain; $Significant changes in dynamic strain.

Figure 2

Calculated specific elastance per PEEP level for each group. Data are shown as mean ± SE. Solid squares: CABG group; Solid arrow: neurology group; Solid diamond: lung disorders group. Data are considered to be significantly different if P < 0.05.

Figure 3

Changes in global stress during the decremental PEEP trial in the three patient groups. Data are shown as mean ± SE. In both the CABG and lung disorders groups, global stress significantly decreased with each PEEP step, as indicated by * and $, respectively. In the neurology group, global stress only significantly decreased at 5 cmH2O PEEP as compared to 15 cmH2O PEEP, as indicated by #. At all PEEP levels, the global stress was significantly lower in the CABG group as compared to the neurology and lung disorders groups (indicated by ** and ##, respectively). Solid squares: CABG group; Solid arrow: neurology group; Solid diamond: lung disorders group. Data are considered to be significantly different if P < 0.05.

Figure 4

Changes in strain during a decremental PEEP trial for patients with normal oxygenation or impaired oxygenation within the CABG group. Data are shown as mean ± SE. Strain is calculated for coronary artery bypass graft (CABG) patients with a PaO₂/FiO₂ ratio smaller or larger than 40 kPa. Open triangles: global stress or strain; open circles: static stress or strain (PEEP); closed circles: dynamic stress or strain (tidal volume); dashed lines: interpolation lines. All differences are considered to be significant if P < 0.05. *Significant differences in global strain; #Significant differences in static strain; $Significant differences in dynamic strain.

Figure 5

Strain calculated, based on EELV, for patients with different lung conditions, during a decremental PEEP trial. Data are shown as mean ± SE. The horizontal wide‐dashed line represents a threshold strain of 0.27 according the suggestion of Gonzalez‐Lopez et al.¹². Open triangles: global strain; open circles: static strain (PEEP); closed circles: dynamic strain (tidal volume); dashed lines: interpolation lines. Differences are considered to be significant if P < 0.05. *Significant changes in global strain; #Significant changes in static strain; $Significant changes in dynamic strain.