INTRODUCTION

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

Intrapulmonary shunting of blood past poorly or nonventilating lung units is the principal cause of hypoxemia during both clinical and experimental noncardiogenic pulmonary edema. However, the degree of shunting is not just a function of how much pulmonary edema accumulates, but also of how much of the pulmonary blood flow is diverted away from the nonfunctional units toward more normally functioning lung regions a process referred to as "perfusion redistribution" (1).

The mechanisms controlling perfusion redistribution are likely to be multifactorial. Hypoxic vasoconstriction, changes in vascular tone mediated by other vasoactive agents, and mechanical influences, among others, are all possible.

The role of "hypoxic pulmonary vasoconstriction" (HPV) per se is difficult to discern because the mechanism of this well-known phenomenon is still unknown. Multiple attempts to identify the specific mediator of HPV have not been successful. However, it has been known for many years that a small dose of endotoxin, one largely devoid of systemic hemodynamic effects, could prevent the HPV effect (4, 5). Because this effect of low-dose endotoxin could itself be prevented by treatment with cyclooxygenase inhibitors, it seemed likely that the endotoxin effect was mediated by the increased production of a vasodilator eicosanoid such as prostacyclin.

Accordingly, we hypothesized that if perfusion redistribution during experimental acute lung injury is due to HPV, prior treatment with low-dose endotoxin should prevent it. The following studies not only provide evidence that this is in fact the case, but also, that endotoxin and lung injury interact synergistically to produce hemodynamic effects greatly in excess of that caused by either factor alone.

	METHODS

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

Animal Preparation

These studies were approved by the Washington University School of Medicine's Animal Studies Committee. All dogs weighed 18 to 22 kg and were anesthetized with pentobarbital sodium (40 mg/kg), intubated with a cuffed endotracheal tube, and ventilated (fraction of inspired oxygen [FI_O2] = 1.0) with a Harvard pump respirator (South Natick, MA) at a tidal volume of 15 ml/kg in the supine position, with the respiratory rate adjusted to achieve a normal arterial Pa_CO2. Positive end-expiratory pressure was not used. Additional barbiturate was administered, as necessary, to eliminate spontaneous breathing.

All surgical procedures were performed with animals in the supine position. Through bilateral femoral incisions, a 7.5-Fr balloon-tipped pulmonary artery catheter and a 110-cm 7-Fr pig-tailed catheter were positioned in the pulmonary artery under fluoroscopic visualization. A 12-cm-long piece of standard pressure tubing with interior diameter (ID) of 3 mm was inserted into the femoral artery for blood sampling; a 5-cm premature infant feeding tube catheter was placed into the external jugular vein for drug and radionuclide administration. Catheter patency was maintained by periodic infusion of heparinized saline (1 U/ml).

Cardiac output was measured by the thermodilution technique with an Edwards Laboratories cardiac output computer (Santa Ana, CA). Transducers (Baxter, Irvine, CA) were calibrated to the center of the lateral chest and connected to a Mennen model 742 monitor (Clarence, NY) for pulmonary arterial, pulmonary wedge, and systemic arterial pressure recordings. Blood gases were analyzed using an Instrumentation Laboratories model 1306 (Milano, Italy) blood gas analyzer.

PET Techniques

Regional pulmonary blood flow (PBF) and regional lung water concentration (LWC) were measured with positron emission tomographic (PET) imaging. These measurements were obtained with an in-house built "Super-PETT" 3000 system. Design features, methods for calibration, corrections for activity decay, and corrections for photon attenuation have been described previously (6).

The animals were placed in the scanner with the most caudal tomographic slice about 1 to 2 cm below the level of the dome of the diaphragm. Data were recorded simultaneously from seven slices with a center-to-center separation of 1.05 cm and an in-plane full-width half-maximum spatial resolution of 0.85 cm. The image reconstruction resolution was set at 12 mm.

The methods used to measure PBF and LWC, including supporting validation studies, have also been described previously in detail (7). In general, PET is used to measure the tissue concentration and distribution of a positron-emitting radionuclide, which in the present study was simply H₂¹⁵O. The activity data measured with PET, when combined with blood activity (used as a reference) and analyzed with an appropriate compartmental mathematical model, yield tomographic images representative of PBF and LWC.

Experimental Protocols

We studied 23 dogs, divided into four groups (Figure 1). In group "E only" (n = 5), the only intervention was 15 µg/kg of Escherichia coli endotoxin (Fisher Scientific, Pittsburg, PA) given intravenously. In group "OA only" (n = 6), the only intervention was 0.08 ml/kg oleic acid (OA) given into the central venous catheter. In group "E + OA" (n = 7), the same dose of endotoxin preceded the same dose of OA by 30 min. In group "M + E + OA" (n = 5), 2 mg/kg of meclofenamate preceded the same dose of endotoxin by 20 min and the same dose of OA by 50 min. Otherwise, all animals were treated the same.

View larger version (7K): [in this window] [in a new window]   Figure 1.   Experimental timeline. Asterisks indicate timing of blood sample collections and hemodynamic measurements. First upright arrow (M ) indicates timing at which either meclofenamate or placebo was given. Second upright arrow (E ) indicates timing at which either endotoxin or placebo was given. Third upright arrow (OA) indicates timing at which either oleic acid or placebo was given.

After instrumentation was completed, each experiment began with a background ("blank") PET scan. Then, the following data were obtained at the time of each "dataset" collection: (1) a transmission scan used to correct for photon attenuation during emission scans and for the placement of regions of interest for later image analysis (see below); (2) a 15- to 18-s scan (used for the PBF measurement) obtained during a continuous infusion of about 60 mCi O-15-labeled water; (3) a 300-s scan obtained after equilibration of the O-15 water (for measurement of LWC and for the "apparent" blood-tissue partition coefficient for water used in the PBF calculation); and (4) pulmonary artery, pulmonary wedge, and systemic arterial pressures, cardiac output, blood gas analysis, and blood withdrawal for eicosanoid analysis (see below).

After the first dataset was collected, either meclofenamate or placebo was administered (Figure 1). Twenty minutes later, either endotoxin or placebo was given. After another 30 min, either OA or placebo was administered, followed 90 to 120 min later by a second dataset. After this, the dog was euthanized with additional pentobarbital followed by 15 to 20 ml saturated KCl.

Biochemical Analysis

Blood was drawn at baseline, just prior to giving OA (or placebo), and 90 min after OA (or placebo) for measurement of the stable metabolites of thromboxane (TxB₂) and prostacyclin (6-keto prostaglandin F₁ alpha [PGF₁]) by an enzyme-immunoassay technique (Figure 1). At each time, a sample (for plasma) was obtained by drawing blood into a tube with EDTA (1 mg/ml) and indomethacin (5 µg/ml) added, and then spun immediately at 5° C at 1,800 g for 20 min. The plasma was removed and stored frozen at 30° C until assay.

Enzyme immunoassay of 6-keto PGF₁ and TxB₂ was performed in 96-well microtiter plates precoated with 2 µg/well goat anti-rabbit immunoglobulin G (11). Before use, the plates were washed with 10² M phosphate buffer (pH 7.4) containing 0.05% Tween 20 (wash buffer). The assay was performed in a total volume of 150 µl. In brief, 50 µl of acetylcholinesterase-conjugated eicosanoid tracer (Caymen, Ann Arbor, MI), 50 µl of antiserum directed against 6-keto PGF₁ or TxB₂ (Advanced Magnetics, Framingham, MA), and 50 µl of a standard or sample in assay buffer were combined and incubated at 25° C for 18 to 20 h. After the plates were washed three times with wash buffer, Ellman's reagent (200 µl) was dispensed into each well. Absorbance was recorded at 412 nm in a microtiter plate spectrophotometer (BioTech, Winooski, VT) when the absorbance for the well containing the "0" standard (B₀) exceeded 0.200 absorbance unit. Each sample was assayed in duplicate. A standard curve was generated for each assay. Sample eicosanoid concentrations were determined by comparison to a log-logit transformation of the standard curve. Eicosanoid concentrations were expressed as pg/ml blood.

Image Analysis

From each dog, the four contiguous tomographic slices with the most lung were analyzed from the seven slices reconstructed as part of each PET scan, encompassing most of the caudal lobes. Regions of interest from the right and left lungs were defined on each transmission scan, as previously described (1, 14).

The position of each region was kept in computer memory, and mean values for each region were obtained for all PET measurements performed. PBF was measured as ml/min/100 ml lung, and LWC as ml of water/100 ml lung. To normalize the regional PBF data for differences in cardiac output, PBF in each picture element (pixel) was expressed as a fraction of the total blood flow to the region.

To evaluate the relationship of PBF to anatomic position within a region, the x- and y-coordinates for each pixel, along with the respective fractional PBF values for each pixel, were recorded. The pixel data were then sorted, first by their y-coordinate. Next, within each value for y, the data were sorted again by their x-coordinate. The result was a listing of the pixels by location, beginning in the most ventral-medial portion of the region and ending with the most dorsal-lateral portion of the region. Each region contained ~ 400 to 500 pixels. Arbitrarily, the data were divided into 20 "bins" stacked vertically in the ventral-dorsal direction, so each bin contained ~ 20 to 25 pixels, which could then be averaged. By keeping the number of bins per region and the number of tomographic slices per dog constant, bin values could be averaged across dogs, allowing comparisons between experimental groups.

To quantify perfusion redistribution, we determined the difference in fractional PBF between the baseline and subsequent PET data sets, and summed the difference in those bins in which PBF decreased between the two times (1, 14).

Statistical Analysis

Data are presented as means ± SD. Statistical significance was determined by a repeated measures analysis-of-variance using the General Linear Models Procedure of the Statistical Analysis System (SAS). Post hoc testing was limited to comparisons of baseline data, and to changes from baseline data among the four experimental groups. We accepted p < 0.05 as indicating statistical significance.

	RESULTS

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

Baseline Data

In general, the four groups were similar at baseline. In the "OA only" group, the mean pulmonary artery pressure was slightly but statistically significantly less at baseline than in the other three groups (Table 1). Similarly, the "M + E + OA" group showed a small but statistically significantly lower cardiac output at baseline compared with the other three groups (Table 1).

Effect of Endotoxin Alone

Endotoxin alone caused the circulating levels of TxB₂ and 6-keto PGF₁ to increase after 30 min (Figures 2 and 3). After an additional 90 min, 6-keto PGF₁ concentrations continued to increase significantly compared with baseline, whereas the TxB₂ concentrations actually decreased. However, in the dose used in this study, endotoxin had no statistically significant effect on any of the pulmonary hemodynamic, blood gas, lung water, or cardiac output variables. There was no effect on systemic blood pressure 30 min after endotoxin, although a modest fall in blood pressure did occur after 2 h (Figure 4). There was also a minor amount of perfusion redistribution after 2 h (Table 2).

View larger version (19K): [in this window] [in a new window]   Figure 2.   Plasma thromboxane B2 (TxB2) (stable metabolite of thromboxane A2) levels in the four experimental groups. The "OA only" and the "M + E + OA" group showed no change, even after injury. There was a tendency for TxB2 levels to increase 30 min after endotoxin; compared with baseline values, this achieved statistical significance in the "E + OA" group (+) but not in the "E only" group. After injury, TxB2 levels decreased in the "E only" group but stayed elevated in the "E + OA" group compared with the other three experimental groups (*).

View larger version (17K): [in this window] [in a new window]   Figure 3.   Plasma 6-keto PGF1 (stable metabolite of prostacyclin) levels in the four experimental groups. Data arranged as in Figure 2. Endotoxin alone (as indicated by the "pre-OA" time point) caused a 3-fold increase in 6-keto PGF1 concentrations. However, in neither group that received endotoxin did this increase achieve statistical significance. With additional time, however, in the "E only" group the 6-keto PGF1 level did increase significantly nearly 9-fold compared with baseline (+). The additional increase in 6-keto PGF1 that occurred after injury in the group that also received OA was even more dramatic, being nearly 30-fold higher than baseline and statistically significant compared with baseline (+) and with the other three experimental groups (*).

View larger version (19K): [in this window] [in a new window]   Figure 4.   Mean arterial pressure in the four experimental groups. Baseline data were obtained just prior to giving meclofenamate or placebo. "Pre-OA" data were obtained just prior to giving oleic acid or placebo. "After injury" data were obtained 90 to 120 min after oleic acid or placebo (i.e., no injury in group "E only") (see Figure 1). In all experimental groups, except in the group given meclofenamate, the "after injury" blood pressure was significantly less than either of the previous blood pressures in the same group, although the relative decrease was smallest in the "E only" group. The mean blood pressure in the "E + OA" group at the "after injury" time point was statistically significantly lower (*) than at the same time in any of the other experimental groups.

Effect of OA Alone

In the "OA only" group, there was no significant change in either circulating 6-keto PGF₁ or TxB₂ either before or after OA infusion (Figures 2 and 3). In addition, there was no change in any other measured variable before OA infusion, indicating a stable experimental preparation. These results are similar to previously reported data from our laboratory (1, 14).

However, 90 min after oleic acid infusion, the pulmonary arterial pressure had increased significantly by 46 ± 32%, oxygenation had deteriorated, cardiac output had fallen (from 2.8 ± 0.6 at baseline to 2.0 ± 0.5 L/min), lung water concentrations had increased, and a significant amount of perfusion redistribution was detected compared with baseline values (Table 2, Figures 5 and 6). Systemic blood pressure had also fallen, but only to a level comparable to that seen with endotoxin alone (Figure 4). All of these changes are the expected result of noncardiogenic pulmonary edema and injury induced in this model (15, 16).

View larger version (13K): [in this window] [in a new window]   Figure 5.   Average ventral-dorsal distribution of fractional PBF in the "OA only" group (A) and in the "E + OA" group (B). Bin numbers represent equal collections of picture elements on multiple positron emission tomography images of PBF, with lower bin numbers situated in ventral regions and higher bin numbers situated in the dorsal regions of the lungs. Each symbol represents the mean value for all dogs in that group. In the "OA only" group, there is a significant reduction of relative PBF ("perfusion redistribution") in the dorsal lung regions (the same regions which develop the greatest increase in lung water; see Figures 6 and 7). In the "E + OA" group, the relative pattern of pulmonary blood flow is essentially unchanged by injury. Likewise, there was little change in the pattern of PBF in the "E only" group (data not shown).

View larger version (59K): [in this window] [in a new window]   Figure 6.   Single tomographic images of PBF (upper images) and LWC (lower images) before (left-hand images) and after (right-hand images) oleic acid injury in one dog from the "OA only" group. PBF images are color-coded so that the lung region with the peak flow is white, and each color segment on the scale represents progressive 5% decrements from this peak value. Thus, the images provide a pictorial demonstration of fractional PBF. The LWC images are scaled to the same absolute value (100 ml H2O/100 ml lung). Despite an 86% increase in LWC, oxygenation remains very well preserved because of the dramatic reduction in PBF to the dorsal lung regions ("perfusion redistribution"), where the edema collection is the greatest.

View larger version (69K): [in this window] [in a new window]   Figure 7.   Single tomographic images of PBF (upper images) and LWC (lower images) before (left-hand images) and after (right-hand images) oleic acid injury in one dog from the "E + OA" group. PBF images are color-coded so that the lung region with the peak flow is white, and each color segment on the scale represents progressive 5% decrements from this peak value, as in Figure 6. The LWC images are scaled to the same absolute value (100 ml H2O/100 ml lung). Despite an increase in LWC that is nearly the same as the animal shown in Figure 6, the change in oxygenation is much worse. The reason is that there is no perfusion redistribution; thus, the relative intrapulmonary shunt is greater.

Effect of Endotoxin + OA

Thirty minutes after endotoxin, the changes in the circulating levels of both eicosanoids were similar to those observed in group "E only," except in the "E + OA" group the increase in TxB₂ concentrations was significant compared with baseline (Figure 2). All changes in the remaining measured variables 30 min after endotoxin were the same as in the "E only" group, except for cardiac output which showed a modest but statistically significant decrease (from 2.7 ± 0.4 at baseline to 2.2 ± 0.7 L/min).

Ninety minutes after OA infusion, TxB₂ concentrations remained elevated compared with the other three groups, unlike the pattern in the "E only" group in which the TxB₂ levels decreased (Figure 2). Even more dramatic were the changes in 6-keto PGF₁ concentrations, which increased more than 9 times the 30 min level, nearly 30 times the baseline value, and nearly 2 ¹/₂ times the level seen with endotoxin alone (Figure 3).

Equally dramatic were the changes in hemodynamic and oxygenation variables after OA: there was no significant increase in pulmonary arterial pressure, the deterioration in oxygenation was much greater, systemic blood pressure was significantly lower (Table 2 and Figure 4), and perfusion redistribution was largely eliminated (Figures 5-7) compared with either the "E only" or "OA only" groups. However, there was no further decrease in cardiac output compared with the time point before OA infusion, and the percent increase in LWC was nearly the same as in the OA only group (Table 2).

Effect of Meclofenamate + Endotoxin + OA

Given the apparent association between the dramatic differences in intrapulmonary and systemic hemodynamics with the equally dramatic increase in prostacyclin production, we tested an additional group of animals with the cyclooxygenase inhibitor meclofenamate before administering either endotoxin or OA. With this prior treatment with meclofenamate, the increase in TxB₂ and 6-keto PGF₁ after endotoxin or OA was virtually eliminated (Figures 2 and 3), and intrapulmonary perfusion redistribution was restored to the same level as in the "OA only" group (Table 2). However, with the inhibition of prostacyclin production, an increase in pulmonary arterial pressure still occurred, in fact greater than that observed in the "OA only" group (Table 2). This increase in pulmonary arterial pressure was associated with a marked reduction in cardiac output from acute right heart failure. Despite perfusion redistribution as in the "OA only" group, the deterioration in arterial oxygenation was comparable to that in the "E + OA" group, probably because of the severe decrease in cardiac output.

	DISCUSSION

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

The main finding of this study is that a low dose of endotoxin, which itself is too low to cause significant hemodynamic effects, can nevertheless dramatically alter the hemodynamic consequences of experimental acute lung injury. Furthermore, some of the effects of endotoxin can be neutralized by cyclooxygenase inhibitors like meclofenamate. If a similar phenomenon is possible in humans, the implications would be important for our understanding of the pathogenesis, and potentially for the treatment of acute lung injury syndromes such as the acute respiratory distress syndrome (ARDS).

Pulmonary Effects

The oleic acid model is a well-characterized and predictable model of acute lung injury (16). Within 90 to 120 min of administration, animals develop significant pulmonary edema. Many animals show a significant reduction of relative pulmonary blood flow to these edematous lung regions ("perfusion redistribution") associated with mild-moderate pulmonary hypertension. Unlike some other models of lung injury in other species (17, 18), these hemodynamic changes continue to develop during the first 8 h after injury is initiated, and are sustained for at least 28 h (15). The combination of varying amounts of pulmonary edema and varying degrees of perfusion redistribution is the primary reason that the development of hypoxemia after OA-induced injury is also so variable.

In a series of previous reports (1, 14, 15, 19), we developed the hypothesis that pulmonary hypertension and perfusion redistribution in this model is driven primarily by increased intrapulmonary thromboxane synthesis, since lung tissue thromboxane content is increased while circulating plasma levels are not (at least, as indicated by the concentrations of the stable metabolite TxB₂), and because neither lung content nor circulating plasma levels of prostacyclin are changed by OA-induced injury (also as indicated by the concentrations of its stable metabolite, 6-keto PGF₁). Data from the present study are consistent with these previous observations.

Because it has been known for many years that low doses of endotoxin, doses which are themselves largely devoid of significant hemodynamic effects, can prevent the development of pulmonary hypertension and/or perfusion redistribution in response to alveolar hypoxia (4, 20), we decided to test whether a similar low dose of endotoxin could affect the development of perfusion redistribution in the OA lung injury model.

The current study clearly demonstrates that low-dose endotoxin can virtually eliminate perfusion redistribution and the development of pulmonary hypertension as a consequence of OA-induced lung injury in dogs (Table 2 and Figures 5-7). The elimination of perfusion redistribution and of pulmonary hypertension indicates that vasoconstriction, by one or more mediators, and not intravascular obstruction or mechanical compression, must be responsible for virtually the entire amount of perfusion redistribution and pulmonary hypertension in this lung injury model.

The effects of low-dose endotoxin on the pulmonary and systemic blood pressure response to OA-induced acute lung injury were in turn neutralized as a result of pretreatment with the cyclooxygenase inhibitor meclofenamate (Table 2, Figure 2-4), further supporting the role of prostacyclin in these effects. However, pulmonary arterial pressure increased despite inhibition of thromboxane production (Figure 2 and 3). Therefore, it seems likely that vasoconstrictors other than thromboxane (including but not limited to hypoxic pulmonary vasoconstriction) must also be present after E + OA, which then remain unopposed after cyclooxygenase inhibition. Endotoxin effects must be important here, because the combination of meclofenamate and OA alone does not produce these effects, as reported elsewhere (14, 15).

The elimination (by low-dose endotoxin) of the normally expected perfusion redistribution that develops after lung injury resulted in hypoxemia that was much more profound than that which occurred when perfusion redistribution was present. There are obvious potentially important clinical implications if a similar phenomenon occurs during human syndromes of acute lung injury. Given the current concern about secondary forms of lung injury induced by certain modes and patterns of mechanical ventilation (21), any condition which would impose additional requirements for airway pressure support to bring oxygenation to acceptable levels might potentially affect outcome.

The results of the current study are seemingly at odds with a previous study by our group (22) that failed to demonstrate any effect of low-dose endotoxin on perfusion pattern when lung injury was confined to one lobe. However, the lobar and diffuse injury models (the latter model being the one used in the current study) are physiologically quite distinct (23), and therefore, presumably, the mechanisms underlying perfusion redistribution must also be different. The diffuse injury model is similar to ARDS in many ways (16), and so the results of the current study may be more relevant to the human syndrome.

Synergistic Effects

Figure 3 shows that although endotoxin alone caused a progressive increase in prostacyclin production, lung injury greatly amplified the response to endotoxin. Thus, it seems quite clear that low-dose endotoxin and lung injury had a synergistic effect that was greater by far than either effect alone. These observations are consistent with a "priming" effect of low-dose endotoxin on the pulmonary endothelium.

The concept of endotoxin "priming" is not new; others have reported that low-dose endotoxin can upregulate prostaglandin H synthase-2 expression (manifested by an increase in thromboxane production) in alveolar macrophages, and can alter the response to a subsequent challenge with higher doses of endotoxin, or a new exposure to exotoxin, platelet activating factor, or arachidonic acid (24). What is unique about the current study is that after the priming stimulus, the subsequent challenge was not a second dose of a proinflammatory agent, but the development of a neutrophil-independent injury to the lungs (16).

The mechanism of this synergistic effect cannot be discerned completely from the results of the current studies, although some likely inferences are possible. In this model, organ injury is limited to the lungs because OA is introduced into the central venous circulation and is rapidly bound to albumin (rendering it harmless) after it exits the lungs (16). Since the synergistic effect was seen only after the onset of lung injury, it seems that the pulmonary endothelium, and not the vascular systemic endothelium, was the likely source of the increased prostacyclin production. Because endotoxin was necessary to "prime" the endothelium prior to lung injury, it is reasonable to suspect that endotoxin upregulated production of either or both cyclooxygenase I and II (29). The time course of the experiments (2 or more hours after endotoxin administration) is consistent with this possibility. It is also possible that other enzyme systems responsible for vasodilator production, such as inducible nitric oxide synthase, may have been upregulated simultaneously. The increasing availability of specific inhibitors of these enzymes makes it possible to test these possibilities in future experiments.

Systemic Effects

In recent years, a widely accepted notion about the pathogenesis of ARDS is that it represents only one of several organ failures which develop as a result of dysregulated and overwhelming inflammatory responses to infection or similar stimuli (30). While this paradigm may certainly be true, the results of the current study suggest that the paradigm may need to be modified. The massive increase in prostacyclin production (30-fold baseline values) as a result of the synergism between low-dose endotoxin and lung injury resulted in dramatically lower systemic arterial blood pressure than that which occurred after either intervention alone. If the lung was indeed the source of the increased prostacyclin production, as seems likely, this result represents a kind of "lung shock," in which the lung is actually the "engine" responsible for significant changes in systemic hemodynamics, and not just an "innocent bystander." It is easy to imagine a "feed forward" system in which infection leads to low levels of endotoxemia, which then greatly amplify both pulmonary and systemic responses to any additional lung injury, finally culminating in a syndrome completely consistent with "septic shock" and ARDS. Such a scenario would place the lungs back at the center of attention in ARDS, instead of simply another organ affected by systemic events. Additional studies, of course, will be required to support these speculations.