Lung ventilation injures areas with discrete alveolar flooding, in a surface tension-dependent fashion

Abstract

With proteinaceous-liquid flooding of discrete alveoli, a model of the edema pattern in the acute respiratory distress syndrome, lung inflation over expands aerated alveoli adjacent to flooded alveoli. Theoretical considerations suggest that the overexpansion may be proportional to surface tension, T. Yet recent evidence indicates proteinaceous edema liquid may not elevate T. Thus whether the overexpansion is injurious is not known. Here, working in the isolated, perfused rat lung, we quantify fluorescence movement from the vasculature to the alveolar liquid phase as a measure of overdistension injury to the alveolar-capillary barrier. We label the perfusate with fluorescence; micropuncture a surface alveolus and instill a controlled volume of nonfluorescent liquid to obtain a micropunctured-but-aerated region (control group) or a region with discrete alveolar flooding; image the region at a constant transpulmonary pressure of 5 cmH2O; apply five ventilation cycles with a positive end-expiratory pressure of 0-20 cmH2O and tidal volume of 6 or 12 ml/kg; return the lung to a constant transpulmonary pressure of 5 cmH2O; and image for an additional 10 min. In aerated areas, ventilation is not injurious. With discrete alveolar flooding, all ventilation protocols cause sustained injury. Greater positive end-expiratory pressure or tidal volume increases injury. Furthermore, we determine T and find injury increases with T. Inclusion of either plasma proteins or Survanta in the flooding liquid does not alter T or injury. Inclusion of 2.7-10% albumin and 1% Survanta together, however, lowers T and injury. Contrary to expectation, albumin inclusion in our model facilitates exogenous surfactant activity.

Keywords: acute respiratory distress syndrome; plasma proteins; surface tension; surfactant; ventilation injury.

Fig. 1.

Model of pressures acting in adjacent aerated (left) and flooded (right) alveoli. Alveolar wall represented by black line; alveolar liquid by gray shading. Air pressure (P_ALV) and surface tension (T) are the same in both alveoli. Liquid pressure in the flooded alveolus is P_LIQ. The air-liquid interface of the flooded alveolus forms a meniscus of radius R_M. Across the meniscus there is a pressure drop ΔP_M. The tissue of the septum between the two alveoli supports tension T_T. Across the liquid lining layer of the aerated alveolus and the septum between the two alveoli, both of which have radius R_S, there is a pressure drop ΔP_S.

Fig. 2.

Dependence of peak inspiratory transpulmonary pressure, P_ALV·MAX, on positive end-expiratory pressure (PEEP); tidal volume, V_T; and body weight. Each data point is the mean of n = 4 replicates obtained in the same isolated lung. The relation between PEEP and end-expiratory transpulmonary pressure, P_ALV·MIN, is described in methods. Within the boxed region, P_ALV·MAX is independent of body weight.

Fig. 3.

Ventilation is injurious to areas of discrete alveolar flooding. A: ventilation causes fluorescence movement from vasculature to liquid phase of discretely flooded alveoli but not to the liquid lining layer of control, aerated areas. Images in first three rows, unprocessed, are of areas with discrete alveolar flooding (row 1 and row 2) and of a capillary in a control, aerated area (row 3; larger field view, inset). Areas are imaged for 5 min prior to and 10 min following five ventilation cycles at 0.33 Hz with PEEP of 10 cmH₂O and V_T as specified. Solution instilled in alveoli is 3% albumin in normal saline. Perfusate is labeled with fluorescein. At baseline in flooded areas, L marks alveoli filled with nonfluorescent liquid; following ventilation, fluorescence entry into the alveoli enables visualization of the liquid. Because the liquid lining layer is not apparent at any time point in raw images of the control, aerated area, control images are replicated, with uniformly enhanced brightness at all time points (row 4). Red circles indicate representative areas of alveolar liquid phase fluorescence intensity measurement. B: calcein red-orange AM labeling of epithelium demonstrates that the low-intensity green band lining capillaries of the control area, as seen in row 4 of (A), is the aerated alveolar liquid lining layer. C: normalized fluorescence data from ventilation of areas with discrete alveolar flooding or control, aerated areas, with frequency of 0.33 Hz, PEEP of 10 cmH₂O, and V_T of 6 or 12 ml/kg. Flooding liquid is 3% albumin in normal saline. Statistics: n = 4/group; in control, data from V_T of 6 and 12 ml/kg are combined (total n = 8). *P < 0.01 vs. baseline; #P < 0.01 vs. both other groups at same time point; °P < 0.01 vs. control group at same time point.

Fig. 4.

Injury score due to ventilation at 0.33 Hz with specified PEEP and V_T. Instilled solution is 3% albumin in normal saline without or with 1% Survanta. Statistics: n = 4/group; in control groups, data from the 10 tested ventilation conditions are combined (total n = 40/control group). All discrete flooding groups differ from control, aerated groups (P < 0.01, statistics not shown on graph). *P < 0.01 vs. group with same ventilation conditions but lacking Survanta. Among all groups with discrete flooding, a group with a letter at its base differs (P < 0.05) from all other groups except those with same letter above their error bars.

Fig. 5.

Effects of flooding solutions on injury score and surface tension. A: injury score due to ventilation at 0.33 Hz with PEEP of 15 cmH₂O and V_T of 6 ml/kg. Control, aerated groups include solutions with surface tensions spanning the full range tested: 0–5% albumin, 5% dextran, 10 μM NaOH, or 5% dextran plus 10 μM NaOH without (n = 24) or with (n = 24) 1% Survanta. Flooding liquid is normal saline with additives as specified (n = 4/group). All discrete flooding groups differ from control, aerated groups (P < 0.01, statistics not shown on graph). *P < 0.01 vs. no Survanta for same flooding liquid. #P < 0.01 vs. 3% albumin plus Survanta and P < 0.05 vs. 5% fibrinogen plus Survanta. B: surface tension in alveoli flooded with normal saline plus additives, as specified (n ≥ 4/group). Survanta concentration is 0.9%. *P < 0.05 vs. no Survanta for same flooding liquid; #P < 0.01 vs. no Survanta for same flooding liquid. C: injury score data from (A) plotted vs. surface tension data from (B). R² = 0.65.

Fig. 6.

Determination of maximum instilled albumin concentration with which Survanta lowers injury and surface tension. A: injury scores for areas flooded with solutions containing albumin, as specified, without or with 1% Survanta (n = 4/group). Control group combines data for two solutions, 5% and 30% albumin, in the absence of Survanta, whose albumin concentrations bracket those of the solutions tested with Survanta and between which there is no difference in injury score. Ventilation at 0.33 Hz with PEEP of 15 cmH₂O and V_T of 6 ml/kg. *P < 0.05 vs. 5%/30% albumin without Survanta. B: surface tension in areas flooded with solutions containing albumin, as specified, without or with 0.9% Survanta (n ≥ 4/group). Control group combines data for two solutions, 4.6% and 28% albumin, in the absence of Survanta, between which there is no difference in surface tension. *P < 0.05 vs. 4.6%/28% albumin without Survanta; #P < 0.01 vs. 4.6%/28% albumin without Survanta.