Induction of severe hypoxemia and low lung recruitability for the evaluation of therapeutic ventilation strategies: a translational model of combined surfactant-depletion and ventilator-induced lung injury

Abstract

Background: Models of hypoxemic lung injury caused by lavage-induced pulmonary surfactant depletion are prone to prompt recovery of blood oxygenation following recruitment maneuvers and have limited translational validity. We hypothesized that addition of injurious ventilation following surfactant-depletion creates a model of the acute respiratory distress syndrome (ARDS) with persistently low recruitability and higher levels of titrated "best" positive end-expiratory pressure (PEEP) during protective ventilation.

Methods: Two types of porcine lung injury were induced by lung lavage and 3 h of either protective or injurious ventilation, followed by 3 h of protective ventilation (N = 6 per group). Recruitment maneuvers (RM) and decremental PEEP trials comparing oxygenation versus dynamic compliance were performed after lavage and at 3 h intervals of ventilation. Pulmonary gas exchange function, respiratory mechanics, and ventilator-derived parameters were assessed after each RM to map the course of injury severity and recruitability.

Results: Lung lavage impaired respiratory system compliance (C_rs) and produced arterial oxygen tensions (P_aO₂) of 84±13 and 80±15 (F_IO₂ = 1.0) with prompt increase after RM to 270-395 mmHg in both groups. After subsequent 3 h of either protective or injurious ventilation, P_aO₂/F_IO₂ was 104±26 vs. 154±123 and increased to 369±132 vs. 167±87 mmHg in response to RM, respectively. After additional 3 h of protective ventilation, P_aO₂/F_IO₂ was 120±15 vs. 128±37 and increased to 470±68 vs. 185±129 mmHg in response to RM, respectively. Subsequently, decremental PEEP titration revealed that C_rs peaked at 36 ± 10 vs. 25 ± 5 ml/cm H₂O with PEEP of 12 vs. 16 cmH₂O, and P_aO₂/F_IO₂ peaked at 563 ± 83 vs. 334 ± 148 mm Hg with PEEP of 16 vs. 22 cmH₂O in the protective vs. injurious ventilation groups, respectively. The large disparity of recruitability between groups was not reflected in the C_rs nor the magnitude of mechanical power present after injurious ventilation, once protective ventilation was resumed.

Conclusion: Addition of transitory injurious ventilation after lung lavage causes prolonged acute lung injury with diffuse alveolar damage and low recruitability yielding high titrated PEEP levels. Mimicking lung mechanical and functional characteristics of ARDS, this porcine model rectifies the constraints of single-hit lavage models and may enhance the translation of experimental research on mechanical ventilation strategies.

Keywords: Acute lung injury; Acute respiratory distress syndrome; Closed-loop ventilation; Mechanical power; Recruitment maneuver; Surfactant depletion; Ventilator-induced lung injury.

Fig. 1

Experimental protocol. Anesthetized pigs underwent lavage-induced surfactant depletion followed by either low (LV_T; N = 6) or high (HV_T; N = 6) tidal volume ventilation during ventilation phase 1 (3 h). LV_T ventilation was resumed in ventilation phase 2 (3 h) in both groups. Recruitment maneuvers (RM) and PEEP trials were performed at three instances throughout the protocol to assess injury, recruitability, and “best PEEP”. For more details see “Methods” Section in the main text

Fig. 2

Respiratory mechanics and function. Dynamic compliance of the respiratory system (C_rs) and arterial partial pressure of oxygenation (P_aO₂) were assessed in anesthetized pigs at baseline, after lavage-induced surfactant depletion, and after consecutive recruitment maneuvers (RM) at 3 h intervals. Group A, N = 6: continuous automated protective low tidal volume ventilation (LV_T). Group B, N = 6: 3 h of injurious high tidal volume ventilation (HV_T) prior to RM 2, resumption of protective low tidal volume ventilation for 3 h prior to RM 3. Two animals died after injurious HV_T and RM 2, i.e., Group B, N = 4 at RM 3. For experimental protocol, see Fig. 1. Scatter plots with horizontal (means) and error bars (SD). **p < 0.01, Mann–Whitney U test

Fig. 3

Ventilation-induced deviation from lavage-induced impairment of respiratory mechanics and function. Dynamic compliance of the respiratory system (C_rs) and arterial partial pressure of oxygenation (P_aO₂) were assessed in anesthetized pigs after lavage-induced surfactant depletion, and after consecutive recruitment maneuvers (RM) at 3 h intervals. Figure depicts the relative deviation of C_rs and P_aO₂ (obtained after RM) from the respective values after initial lavage. Group A, N = 6: continuous automated protective low tidal volume ventilation (LV_T). Group B, N = 6: 3 h of injurious high tidal volume ventilation (HV_T) prior to RM 2, resumption of protective low tidal volume ventilation for 3 h prior to RM 3. Two animals died after injurious HV_T and RM 2, i.e., Group B, N = 4 at RM 3. For experimental protocol, see Fig. 1. Scatter/bar plots with horizontal (means) and error bars (SD). **p < 0.01, Mann–Whitney U test

Fig. 4

Decremental PEEP trials: identification of “best PEEP”. PEEP trials with decrements of 2 from 24 to 6 cmH₂O were performed at three instances following a recruitment maneuver (RM) in anesthetized pigs which had undergone combined lavage-induced surfactant depletion and therapeutic ventilation. Group A, N = 6: continuous automated protective low tidal volume ventilation (LV_T). Group B, N = 6: 3 h of injurious high tidal volume ventilation (HV_T) prior to PEEP trial 2, resumption of protective low tidal volume ventilation for 3 h prior to PEEP trial 3. Two animals died after injurious HV_T and RM 2, i.e., Group B, N = 4 at PEEP trial 2 and 3. For experimental protocol, see Fig. 1 and “Methods” Section. Line graphs depict the group means of C_rs and P_aO₂ obtained at each PEEP level. PEEP values at the maximum mean C_rs and maximum mean P_aO₂ (red vertical dotted lines) are consistently disparate within each group and differ between groups. Adjacent scatter plots depict the PEEP values at maximum C_rs and P_aO₂ of each individual animal and the means ± SD thereof. *p < 0.05, **p < 0.01, Mann–Whitney U test

Fig. 5

Tidal volume and mechanical power in combined lavage and ventilator-induced lung injury. Tidal volume (V_T) and mechanical power (M_P) of ventilation were assessed in anesthetized and surfactant-depleted pigs which underwent either an automated closed-loop protective low tidal volume (6 ml/kg BW LV_T) and tabular PEEP ventilation strategy (group A) or non-automated injurious high tidal volume ventilation (HV_T) with V_T of 17 ml/kg BW and PEEP 2 cmH₂O (group B) during ventilation phase 1. Ventilation phase 2 consisted of automated LV_T ventilation in both groups (N = 6 each). Two animals died after injurious HV_T and RM 2, i.e., Group B, N = 4 during ventilation phase 2. For experimental protocol, see Fig. 1 and “Methods” Section. For calculation of the M_P, the simplified equation proposed by Becher et al. for pressure-controlled ventilation is used (see “Methods” Section in the main text). Means ± SD; p-values indicate significant group effects and were calculated using two-way ANOVA-type nparLD package (see Table 2, associated relative treatment effects (RTE) see Additional file 1: Table S2a). For more statistical details refer to main text

Fig. 6

Histopathology. Representative tissue sections stained with hematoxylin/eosin of the right lower lung lobe of pigs after lavage-induced surfactant depletion and subsequent protective A, B or injurious C, D mechanical ventilation. High magnification fields (B, D, × 400) correspond to red box on low magnification field (A, C, × 100). Note the condensed histoarchitecture and signs of diffuse alveolar damage present after lavage and injurious ventilation (C, D), including septal thickening (*), massive interstitial ( →) and intra-alveolar ( >) infiltration of neutrophils, intra-alveolar erythrocytes (∆), protein strands (#), and disruption of the alveolar integrity ( +). In contrast, more aerated surface area with preserved alveolar architecture ( +), identification of type I pneumocytes (»), and less alveolar damage is present after lavage and protective ventilation (A, B). Septal thickening, neutrophil infiltration, and intra-alveolar protein strands occurred to a much lower extent

Fig. 7

Computed tomography. Representative computed tomography (CT) images of the basal lungs segments of a pig following lavage-induced surfactant depletion and 3 h of injurious ventilation. Images were taken during ventilation with PEEP of 15 vs. 6 mbar and a V_T of 6 mL/kg body weight. Ground glass opacities ( >), interlobar and intralobular septal thickening (→) are representative of the severity of lung injury. Note the significant increase of atelectatic regions (*) as a sign of derecruitment in the dependent lung areas upon PEEP reduction