Effects of positive end-expiratory pressure and recruitment maneuvers in a ventilator-induced injury mouse model

Abstract

Background: Positive-pressure mechanical ventilation is an essential therapeutic intervention, yet it causes the clinical syndrome known as ventilator-induced lung injury. Various lung protective mechanical ventilation strategies have attempted to reduce or prevent ventilator-induced lung injury but few modalities have proven effective. A model that isolates the contribution of mechanical ventilation on the development of acute lung injury is needed to better understand biologic mechanisms that lead to ventilator-induced lung injury.

Objectives: To evaluate the effects of positive end-expiratory pressure and recruitment maneuvers in reducing lung injury in a ventilator-induced lung injury murine model in short- and longer-term ventilation.

Methods: 5-12 week-old female BALB/c mice (n = 85) were anesthetized, placed on mechanical ventilation for either 2 hrs or 4 hrs with either low tidal volume (8 ml/kg) or high tidal volume (15 ml/kg) with or without positive end-expiratory pressure and recruitment maneuvers.

Results: Alteration of the alveolar-capillary barrier was noted at 2 hrs of high tidal volume ventilation. Standardized histology scores, influx of bronchoalveolar lavage albumin, proinflammatory cytokines, and absolute neutrophils were significantly higher in the high-tidal volume ventilation group at 4 hours of ventilation. Application of positive end-expiratory pressure resulted in significantly decreased standardized histology scores and bronchoalveolar absolute neutrophil counts at low- and high-tidal volume ventilation, respectively. Recruitment maneuvers were essential to maintain pulmonary compliance at both 2 and 4 hrs of ventilation.

Conclusions: Signs of ventilator-induced lung injury are evident soon after high tidal volume ventilation (as early as 2 hours) and lung injury worsens with longer-term ventilation (4 hrs). Application of positive end-expiratory pressure and recruitment maneuvers are protective against worsening VILI across all time points. Dynamic compliance can be used guide the frequency of recruitment maneuvers to help ameloriate ventilator-induced lung injury.

Fig 1. Positive-pressure ventilation (2-hour).

1a No significant differences in histology lung injury scores. 1b No significant differences were noted in bronchoalveolar absolute neutrophil counts. 1c No significant differences were noted in bronchoalveolar lavage proinflammatory cytokines between all groups. 1d Bronchoalveolar lavage total protein was significantly elevated in the HTV RM group compared to the LTV RM group (p = 0.0086). 1e Bronchoalveolar lavage albumin was significantly elevated in the high tidal volume groups compared to the air control group, but not when compared to the low-tidal volume ventilation groups (p = 0.0077) 1f Pulmonary compliance was significantly higher in the low-tidal volume ventilation group (TV 8, PEEP 4, RM) compared to all other groups (p<0.0001) and in both LTV and HTV groups with RM versus no RM (p<0.0001, p = 0.0155). Data in boxplots is presented in median ± minimum and maximum values.

Fig 2

Positive-pressure ventilation (2-hour vs 4-hour) 2a Standardized histology scores were significantly higher in the HTV group at 4 hours compared to the air control, LTV group at 2 hours, and HTV group at 2 hours (p = 0.0002) 2b Absolute neutrophils were significantly elevated in the TV 15, PEEP 0, RM group at 4 hours, but not at 2 hours of positive-pressure ventilation (p = 0.0046) and were significantly elevated in the HTV group compared to the LTV group (p = 0.0079) 2c Bronchoalveolar lavage albumin was significantly increased in the HTV groups at 2 and 4 hours compared to the air control (p = 0.0046) 2d Proinflammatory cytokines were significantly elevated in the HTV group at 4 hours, but not at 2 hours (see text for p values) 2e Lung histology image stained with H&E. Top Left: LTV ventilation (TV 8, PEEP 4, RM (4 hour). Top Right: HTV ventilation (TV 15, PEEP 0, RM (2 hour)). Bottom: HTV ventilation (TV 15, PEEP 0, RM (4 hour)) 2f Compliance was significantly higher in the LTV group at 4 hours compared to 2 hours of ventilation (p = 0.0027) and was significantly higher in the HTV group at 2 hours compared to 4 hours of ventilation (p = 0.0007). Data in boxplots is presented in median ± minimum and maximum values. Neutrophils are denoted with an arrow, macrophages with a dotted line arrow, and proteinaceous debris with a curved line arrow.

Fig 3. Positive-pressure ventilation (4-hour with RM +/- PEEP).

3a Standardized histology scores were higher in all groups compared to the air control group, but not the TV 8, PEEP 4, RM group. The addition of PEEP in the LTV group resulted in a significant decrease in the histology score. 3b Absolute neutrophils were significantly elevated in the TV 15, PEEP 0, RM group compared to the air control, TV 8, PEEP 4, RM, the TV 15, PEEP 4, RM group. The addition of PEEP to the HTV group resulted in a significant decrease in absolute neutrophils. 3c BAL albumin was significantly increased in both LTV and HTV groups without PEEP compared to the air control group 3d BAL cytokines were elevated in the HTV ventilation group. 3e Pulmonary compliance in the LTV ventilation group was significantly higher compared to all other groups. 3f Left: Lung histology image stained with H&E of low tidal volume (TV 8, PEEP 4, RM) showing influx of macrophages within the airway. Right: Lung histology image stained with H&E of high tidal (TV 15, PEEP 0, RM) showing neutrophils, proteinaceous debris, and hemorrhage within the airway. Neutrophils are denoted with an arrow, macrophages with a dotted line arrow, and proteinaceous debris with a curved line arrow.

Fig 4. Positive-pressure ventilation (4-hour +/- PEEP and +/- RM).

4a Standardized histology scores were increased in all groups except the low-tidal volume ventilation group (TV 8, PEEP 4, RM) compared to the air control group (not shown) and the LTV RM group. 4b Bronchoalveolar lavage absolute neutrophil counts were significantly increased in the LTV group with no RM and both HTV groups compared to the air control group. 4c Bronchoalveolar lavage albumin was significantly increased in both HTV groups compared to the air control group. 4d Proinflammatory cytokines were significantly increased in the HTV group with no RM. 4e Pulmonary compliance was significantly elevated in the LTV ventilation group compared to all other groups. Lung histology sections are shown from all groups. 4f Left: Lung histology section stained with H&E of LTV with no RM showing macrophages within the airway. Right: Lung histology section stained with H&E of HTV with no RM showing proteinaceous debris within the airway. Neutrophils are denoted with an arrow, macrophages with a dotted line arrow, and proteinaceous debris with a curved line arrow.