CPAP improves regional lung strain rate and diaphragm velocity of relaxation in experimental self-inflicted lung injury

Abstract

Background: Strenuous respiratory effort has been proposed as a second hit in severe acute lung injury (ALI), introducing the concept of "patient self-inflicted lung injury" (P-SILI). In an experimental setting, noninvasive continuous positive airway pressure (CPAP) attenuates lung and diaphragmatic injury, but the underlying mechanisms remains elusive. Here we investigate the effects of noninvasive CPAP on global and regional lung strain and diaphragm velocity of contraction and relaxation in an experimental P-SILI model.

Methods: Lung injury was induced in Sprague Dawley rats through surfactant depletion followed by either three hours of standard oxygen therapy (Control group) or CPAP support (CPAP group). Subjects were assessed through inspiratory and expiratory muscle activation. Regional lung and diaphragmatic deformation amplitude (strain) and the rate of change (strain rate) maps were developed using a micro-computed tomography (µCT) scan. Morphometric tissue assessment was carried out to study biological damage.

Results: Compared with the Control group, the CPAP group resulted in: (1) higher SpO₂ and lower respiratory rate, nasal flaring, inspiratory and expiratory muscle activation, and minute ventilation at the end of the study; (2) lower global and regional tidal ventilation at the beginning of the study; (3) lower regional inspiratory and expiratory lung strain rate over time; and (4) higher muscle area in the diaphragm morphometric analysis. Furthermore, intragroup analysis showed that only the CPAP group reduced the inspiratory and expiratory muscle activation, the global and regional expiratory lung strain rate and the regional velocity of relaxation of the diaphragm over time.

Conclusions: Standard oxygen therapy resulted in worse patterns of lung strain rate and diaphragm velocity of relaxation, consistent with P-SILI and load-induced diaphragm injury. CPAP resulted in improved lung function, decreased lung strain rate, and diaphragmatic relaxation velocity throughout the respiratory cycle. We conclude that CPAP promotes biomechanical protection in injured lungs and diaphragm, more noticeably during the expiratory phase.

Keywords: Acute lung injury; Continuous positive airway pressure; Patient self-inflicted lung injury; Respiratory effort; Spontaneous breathing; Strain; Strain rate.

Fig. 1

Regions-of-interest (ROI) array heat maps in the apical–basal (A-B) and ventral–dorsal (V-D) directions at the beginning (T0) and the ending (T3) of the study (A) End-expiratory intensity. (B) Tidal ventilation. (C) Volumetric strain Significant within-group differences are denoted by P < 0.05 *Significant difference comparing CPAP and control groups

Fig. 2

Regional volumetric strain rate in the dorsoventral (D‹—›V) and basoapical (B‹—›A) axes of the lung. (A, C) Group-wise comparisons between control and CPAP at the beginning (T0) and end (T3) of the study, during the inspiratory phase (A) and expiratory phase (C). (B, D) Time-wise comparisons between T0 and T3 for each group during the inspiratory phase (B) and expiratory phase (D). † denotes P < 0.05 for significant differences between T0 and T3. * denotes P < 0.05 for significant differences between CPAP and control groups.

Fig. 3

Regional diaphragmatic biomechanics along the ventro-dorsal axis. Left panels: Diaphragm position at end-expiration (A, C) and end-inspiration (B, D) at the beginning (T0; A, B) and end (T3; C, D) of the study for control and CPAP groups. Right panels: Regional diaphragm velocity of contraction (E, F) and relaxation (G, H) over time for control (E, G) and CPAP (F, H) groups. Ten equidistant points of interest are shown from ventral to dorsal regions. Solid lines indicate group medians; boxplots represent intersubject variability. † denotes P< 0.05 for significant differences between T0 and T3. * denotes P < 0.05 for significant differences between CPAP and control groups.

Fig. 4

Morphometric analysis of lung aeration, muscular and interstitial areas of the diaphragm Data are expressed as median (IQR). Significant within-groups differences are denoted by p < 0.05. Representative images of lung and diaphragm morphometry for each study group (hematoxylin and eosin, 100x). Findings of interest were highlighted in green Data are expressed as median (interquartile range). Significant within-group differences are denoted by P < 0.05. *Significant difference comparing CPAP and control groups