Combined effects of ventilation mode and positive end-expiratory pressure on mechanics, gas exchange and the epithelium in mice with acute lung injury

Abstract

The accepted protocol to ventilate patients with acute lung injury is to use low tidal volume (V(T)) in combination with recruitment maneuvers or positive end-expiratory pressure (PEEP). However, an important aspect of mechanical ventilation has not been considered: the combined effects of PEEP and ventilation modes on the integrity of the epithelium. Additionally, it is implicitly assumed that the best PEEP-V(T) combination also protects the epithelium. We aimed to investigate the effects of ventilation mode and PEEP on respiratory mechanics, peak airway pressures and gas exchange as well as on lung surfactant and epithelial cell integrity in mice with acute lung injury. HCl-injured mice were ventilated at PEEPs of 3 and 6 cmH(2)O with conventional ventilation (CV), CV with intermittent large breaths (CV(LB)) to promote recruitment, and a new mode, variable ventilation, optimized for mice (VV(N)). Mechanics and gas exchange were measured during ventilation and surfactant protein (SP)-B, proSP-B and E-cadherin levels were determined from lavage and lung homogenate. PEEP had a significant effect on mechanics, gas exchange and the epithelium. The higher PEEP reduced lung collapse and improved mechanics and gas exchange but it also down regulated surfactant release and production and increased epithelial cell injury. While CV(LB) was better than CV, VV(N) outperformed CV(LB) in recruitment, reduced epithelial injury and, via a dynamic mechanotransduction, it also triggered increased release and production of surfactant. For long-term outcome, selection of optimal PEEP and ventilation mode may be based on balancing lung physiology with epithelial injury.

Figure 1. Time course of the protocol.

Once the animal was connected to the ventilator, a recruitment maneuver (RM) was followed by impedance measurement (Z) and HCl treatment. After a stabilization period and several RM and Z, a 60 min ventilation period (thick arrow) was started. During the ventilation period, Z and peak airway pressure (PAP) were recorded at 5 min intervals. At the end of the protocol, blood gases and a lavage sample were obtained and the lung was isolated for further processing.

Figure 2. Mechanical parameters from the single compartment model.

The graphs compare the time courses of Newtonian resistance (R, panels A and B), tissue elastance (H, panels C and D) and the change in H (ΔH, panels E and F) during 60 min of ventilation using conventional ventilation (CV), conventional ventilation with large breaths (CV_LB), or variable ventilation (VV_N) in HCl-injured mice at PEEPs of 3 (left panels) and 6 cmH₂O (right panels). * denotes significant difference between CV and CV_LB as well as CV and VV_N at 60 min; # denote significant difference between CV and VV_N at 60 min. Additional significance levels are given in the text.

Figure 3. Mechanical parameters from the distributed model.

The graphs compare the time courses of minimum (H_min, panels A and B), maximum (H_max, panels C and D) and standard deviation (SD, panels E and F) of the distribution of elastance in the heterogeneous tissue model during 60 min of ventilation using CV, CV_LB or VV_N in HCl-injured mice at PEEPs of 3 (left panels) and 6 cmH₂O (right panels). * denotes significant difference (p<0.001) between CV and CV_LB as well as CV and VV_N at 60 min.

Figure 4. Peak airway pressures as a function of ventilation mode.

The graphs compare the time courses of the relative percentage change in mean peak airway pressure during 60 min of ventilation using CV, CV_LB or VV_N in HCl-injured mice at PEEPs of 3 (panel A) and 6 cmH₂O (panel B). Each point is calculated from the average of the peak airway pressure in a 5–minute ventilation period compared to its value at time 0. *denotes significant difference (p<0.001) between CV and CV_LB as well as CV and VV_N at 60 min; # denote significant difference (p<0.02) between CV_LB and VV_N at 60 min.

Figure 5. Gas exchange as a function of ventilation mode.

The graphs show the partial pressures of oxygen and carbon dioxide (PaO₂ and PaCO₂, respectively, in panels A and B), percent oxygen saturation (SO₂, panel C) and Alveolar-arterial gradient (A–a gradient, panel D) obtained at the end of 60 min ventilation using CV, CV_LB or VV_N in HCl-injured mice at PEEPs of 3 and 6 cmH₂O. * denotes significant difference compared to CV (p<0.05).

Figure 6. Example Western blots.

Representative blots are shown for surfactant protein (SP)-B and its pro form (proSP-B) as well as E-cadherin obtained from lung homogenates and lavage fluid at the conclusion of 60 min of ventilation using CV, CV_LB or VV_N at PEEPs of 3 (left) and 6 cmH₂O (right). UV denotes blots from HCl-injured but unventilated group of animals. Note the small gaps between several images for a given protein. These blots are from the same film, but not in the same order as the rest of the blots. The original image was first cut into pieces, without changing the image, and then reassembled in the desired order.

Figure 7. Analysis of Western blots.

Graphs show the relative amounts of SP-B, proSP-B and soluble E-cadherin in HCl-injured mice at the conclusion of 60 min of ventilation using CV, CV_LB or VV_N at PEEPs of 3 and 6 cmH₂O. The data are normalized with the corresponding mean values of the unventilated group with baseline injury. The two bars in the unventilated group at the two PEEP levels correspond to samples from the same lungs on two separate Western blots. * denotes significant difference compared to CV (p<0.05).

Figure 8. Comparison of ventilation modes.

The graph compares the average improvements of VV_N and CV_LB over CV. The SD bar represents different experimental conditions including 2 PEEP levels and normal lung from our previous study . Only those parameters are included for which either CV_LB, VV_N or both significantly improved over CV. See text for more explanation.

Figure 9. Schematic representation of ventilation along the normalized pressure-volume curve during CV_LB and VV_N.

The vertical dashed black line at 0.4 represents PEEP. The intersections of PEEP and the pressure-volume curves mark the end-expiratory lung volumes (EELV) during the two ventilation modes upon which V_T is superimposed. For CV_LB (red), we also show the large breaths (LB) and the corresponding peak airway pressure (PAP). For VV_N (blue), there is a range of V_Ts superimposed on EELV. The corresponding end-inspiratory volumes have a distribution shown by the shaded area. The probability of a given tidal volume is proportional to the gray scale. Also notice that the mean V_T in VV_N is the same as in CV_LB, but the distribution of V_Ts goes below the V_T of CV_LB and stretches up to the LBs in CV_LB.