INTRODUCTION

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The mortality of patients with acute respiratory distress syndrome (ARDS) remains high (40 to 60%) despite numerous investigations of potential treatment modalities for it (1, 2). Since alterations of the endogenous surfactant system have been shown to contribute to lung dysfunction (3), exogenous surfactant administration has also been tested as a potential therapy for this disease. Several animal studies have shown that exogenous surfactant can improve gas exchange and pulmonary compliance shortly after administration (6). However, clinical studies involving the administration of exogenous surfactant to patients with severe ARDS have shown variable results (9). These different outcomes may be related to the various factors that may influence a host's response to exogenous surfactant. Such factors include: (1) the particular surfactant preparation utilized (12); (2) the specific method used for surfactant delivery (8, 12); (3) the timing and dosing schedules of surfactant administration over the course of the disease (9, 13); and (4) the ventilation strategy utilized after the surfactant was administered (16). One common mechanism by which each of these factors may influence the efficacy of exogenous surfactant therapy is through metabolic differences of the administered surfactant once deposited within the air space.

Alveolar surfactant exists in two major structural forms; surface-active, large-aggregate (LA) forms, and functionally inactive, small-aggregate (SA) forms (19). Exogenous surfactant preparations consist predominantly of LA forms. Once deposited within the lung, however, these exogenous LA forms are subject to conversion into the inactive SA forms. Previous animal studies have demonstrated that superior physiologic responses observed after exogenous surfactant administration were associated with a smaller conversion of the exogenous LA forms into SA forms (12, 13).

In separate studies, it was demonstrated that mechanical ventilation may affect the endogenous alveolar surfactant system (20, 21). Specifically, ventilation strategies using higher tidal volume (VT) values resulted in a greater conversion of endogenous LA forms into SA forms within the air space than did strategies using lower VT values. In injured lungs, this resulted in an increased ratio of SA to LA pool sizes when the higher VT was used, and this was associated with a deterioration in gas exchange. On the basis of these data, we speculated that ventilation strategies may also affect the metabolism of exogenously administered surfactant, which would subsequently influence the efficacy of this treatment modality.

The purpose of the present study was to evaluate the effects of varying VT and positive end-expiratory pressure (PEEP) levels on the efficacy of exogenous surfactant and surfactant metabolism in the repetitive saline lavage model of lung injury in adult rabbits (13, 22, 23).

	METHODS

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Exogenous Surfactant Preparation

Bovine lipid extract surfactant (bLES; bLES Biochemicals, London, Ontario), a preparation used clinically to treat preterm infants with respiratory distress syndrome, was supplied at a concentration of 25 mg phospholipid/ml. Aliquots (2 to 3 µCi per animal) of a stock solution of [³H]dipalmitoylphosphatidylcholine (DPPC)-labeled bLES (25 µCi/ ml) were combined with the appropriate quantity of unlabeled surfactant in order to prepare treatment doses of 100 mg lipid/kg body weight, or 4 ml/kg surfactant for each animal.

Animal Preparation

Adult New Zealand White rabbits weighing 2.4 ± 0.1 kg were given ketamine hydrochloride (100 mg/kg) intramuscularly. A tracheostomy was performed and the carotid artery was cannulated, with 2% lidocaine used for local anesthesia. Animals were mechanically ventilated with a pressure-limited ventilator (Model IV-100B; Sechrist Co., Anaheim, CA) with the following parameters; gas flow of 10 L/min, FI_O2 of 1.0, respiratory rate (RR) of 30 breaths/min with a 50% inspiratory time, and PEEP of 5 cm H₂O. VT values were measured with a pneumotachometer (Hans Rudolph, Kansas City, MO) placed between the ventilator circuit and the proximal end of the endotracheal tube. The VT was maintained at 10 ml/kg body weight immediately after the onset of ventilation, by adjusting peak inspiratory pressure (PIP) values. An initial arterial blood sample was obtained shortly thereafter to measure arterial blood gases. Subsequently, anesthesia and muscle relaxation were maintained with intermittent vascular infusions of 0.8% thiopental sodium (5 to 10 mg/kg/dose) and pancuronium bromide (0.5 mg/ kg/dose) in order to eliminate spontaneous breathing.

Induction of Lung Injury

After baseline physiologic measurements were recorded, lung injury was induced by repetitive whole-lung saline lavage as previously described (13). Briefly, animals were disconnected from the ventilator, and warmed (37° C) 0.15 M NaCl (30 ml/kg body weight) was instilled from a 60-ml syringe directly into the lungs via the endotracheal tube and recovered by gentle suctioning, with reinfusion of this volume two more times. Animals were then reconnected to the ventilator, with the PIP adjusted to maintain a VT of 10 ml/kg measured with the pneumotachometer. This lavage procedure was repeated every 10 min until the Pa_O2/FI_O2 fell below 100 mm Hg. The total number of lavages required per animal was recorded, as was the total time elapsed from first to final lavage. After the final lavage, animals were mechanically ventilated (VT = 10 ml/kg) for an additional 3 h. This procedure has been shown to induce morphologic changes consistent with progressive lung injury, as previously described (13, 22, 23). This model represents a stable and reproducible lung injury characterized by surfactant depletion due to the lavage procedure, as well as significant neutrophil influx and hyaline membrane formation when followed by positive-pressure mechanical ventilation (13, 22, 23).

Experimental Protocol

Three hours after the final lavage, designated as Time 0, animals with Pa_O2/FI_O2 values below 120 mm Hg were given exogenous surfactant as previously described (13). Briefly, 100 mg/kg of radiolabeled exogenous bLES was instilled through a side port adaptor of the endotracheal tube. Instillation occurred during the inspiratory phase of ventilation, over a span of several breaths while the animal was connected to the ventilator. The entire procedure was completed within 1 min. During instillation, animals were held manually with their heads held vertically in order to prevent upward reflux of the instilled surfactant into the endotracheal tube. The distribution of the surfactant to the peripheral areas of the lung was further enhanced by increasing the PIP by 3 cm H₂O immediately after surfactant instillation, for a total period of 10 min, and the PIP was then returned to pretreatment levels. Arterial blood gases were then measured in order to confirm that all animals subsequently studied had an immediate and significant improvement in oxygenation in response to the exogenous surfactant. Only animals with Pa_O2/FI_O2 values above 400 mm Hg at 10 min after surfactant administration were utilized for this study. Immediately after confirmation of this response, animals were randomly divided into four different ventilatory groups based on their specific VT, PEEP, and RR values. FI_O2 values, gas-flow rates, and the ratio of inspiratory time (TI) to expiratory time (TE) were kept constant for all animals. VT values were measured after every blood gas sampling, and if this was necessary, were adjusted by changing the PIP. Animals were monitored for a total of 3 h after the exogenous surfactant was administered, and were then killed with an overdose of 2.5% thiopental sodium given intravascularly.

Experimental Groups

After exogenous surfactant administration, VT values (VT = 5 ml/kg or 10 ml/kg) and/or PEEP values (5 cm H₂O or 9 cm H₂O) were adjusted. One group of animals were ventilated with a VT of 10 ml/kg (normal), an RR of 30 breaths/min, and a PEEP of 5 cm H₂O and were designated as Normal VT (5 cm H₂O). Another group was ventilated with a VT of 10 ml/kg (normal), an RR of 30 breaths/min, and a PEEP of 9 cm H₂O, and was designated as Normal VT (9 cm H₂O). Animals in a third group were ventilated at a VT of 5 ml/kg (low), an RR of 60 breaths/min, and a PEEP of 5 cm H₂O, which was called Low VT (5 cm H₂O) and a fourth group had a VT of 5 mg/kg (low), an RR of 60/min, and a PEEP of 9 cm H₂O, and was called Low VT (9 cm H₂O). A separate group of animals were killed 10 min after exogenous surfactant administration in order to analyze the surfactant recovered in the lung lavage that reflected the status of the surfactant system of all animals just before ventilatory adjustment. This group was designated as Baseline.

Physiologic Measurements

During the final 3-h monitoring period, arterial blood gases were measured at 10 and 20 min after ventilatory adjustment, and every 30 min thereafter. Oxygenation was expressed as Pa_O2 values (FI_O2 = 1.0). The efficiency of ventilation was assessed from the recorded Pa_CO2 values.

Biochemical Measurements

Immediately after being killed, all animals underwent whole-lung lavage as previously described (8, 13). Briefly, approximately 30 ml/kg of 0.15 M NaCl was slowly instilled into the lungs through the endotracheal tube, retrieved by gentle suction, and reinfused and suctioned two more times. This procedure was repeated with fresh saline a total of five times, and the total recovered volume of lavage fluid was recorded. The remaining lung tissue was then homogenized in saline, and the total volume of lung homogenate was adjusted to 50 to 70 ml for each animal to allow for accurate aliquoting. Aliqouts of the crude lung lavage fluid were centrifuged at 150 × g for 10 min to obtain the cell debris fraction (150-g pellet). The 150-g supernatant was then centrifuged at 40,000 × g for 15 min to separate the surfactant into two major subfractions: LA in the pellet and SA in the supernatant. Aliquots from each of the fractions obtained from these animals (crude lung lavage, 150-g pellet, LA, SA, and lung homogenate), as well as an aliquot of the input sample of [³H]DPPC-labeled bLES used to treat each animal, were extracted with chloroform-methanol (24) and dried under nitrogen to isolate lipids. The total phosphate pool size of the crude lung lavage, 150-g pellet, LA, and SA fractions was measured with the modified method of Duck-Chong (25), and was expressed as µg PO₄/kg body weight. The total protein concentration in the crude lung lavage was determined by the method of Lowry and colleagues (26) using bovine serum albumin (BSA) as a standard. The total ³H-recovery in the extracted aliquots of the crude lung lavage, 150-g pellet, LA, SA, lung homogenate, and input samples was determined by liquid scintillation counting as previously described (12, 13). The radioactivity recovery in the crude lung lavage, 150-g pellet, LA, and SA fractions was compared with the total phosphate recovered in the lavage in order to calculate the contribution of the exogenous bLES to the total surfactant pool size. The total recovery of the exogenous surfactant in the lungs at killing was calculated by dividing the radioactivity of the crude lung lavage plus lung homogenate by the total radioactivity of the input dose of exogenous surfactant. The percent recovery of exogenous surfactant in the lung homogenate was determined by dividing the radioactivity recovered in the lung homogenate by the total recovered radioactivity in the crude lung lavage plus lung homogenate. The proportion of the total alveolar surfactant pool recovered in LA forms, based on both phosphorus and radioactivity, was calculated by dividing LA recovery values by the total recovery in both LA and SA fractions.

Statistics

All values are presented as mean ± SEM. Comparisons of repeated measurements of arterial blood gases within each experimental group were assessed by analysis of variance (ANOVA) with Dunnett's post hoc test. Comparisons of values in two different groups were assessed with unpaired t tests. Values of p < 0.05 were considered significant.

	RESULTS

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

Characteristics of Animals Prior to Ventilatory Adjustment

Three hours after the final lavage and before exogenous surfactant administration, a total of 25 rabbits had Pa_O2/FI_O2 values below 120 mm Hg, and were therefore subsequently given exogenous surfactant. Of these 25 animals, 23 had Pa_O2/FI_O2 values above 400 mm Hg at 10 min after exogenous surfactant administration. These 23 animals were then randomized into the five experimental groups; Normal VT (5 cm H₂O) (n = 5), Normal VT (9 cm H₂O) (n = 4), Low VT (5 cm H₂O) (n = 5), Low VT (9 cm H₂O) (n = 5), and Baseline (n = 4). There were no significant differences in mean body weights or mean values of Pa_O2, Pa_CO2, pH, and PIP among all animals subsequently studied in the different ventilator groups, prior to initiating the lung lavage procedure. For all animals, these values averaged 2.4 ± 0.1 kg, 484 ± 6 mm Hg, 36 ± 2 mm Hg, 7.39 ± 0.01, and 14.5 ± 0.4 cm H₂O, respectively. The number of saline lavages required to decrease Pa_O2 values to less than 100 mm Hg (5.2 ± 0.4), and the time from the first to the final lung lavage (43 ± 4 min), were also similar among the experimental groups.

Three hours after the final lung lavage and immediately before exogenous surfactant administration (Time 0), the mean Pa_O2 and Pa_CO2 values were not significantly different among groups (Table 1). There were also no significant differences in any of these parameters among the different experimental groups at Time 10 min. Because of the design of the study, all animals had Pa_O2 values above 400 mm Hg after surfactant administration. Mean Pa_CO2 values decreased significantly immediately after surfactant administration for animals within each group (p < 0.05, Time 10 min versus Time 0).

Physiologic Data After Ventilatory Adjustment

Mean Pa_O2 values over the 3-h ventilatory period after exogenous surfactant administration and ventilatory adjustment are shown in Figure 1. Pa_O2 values in the Normal VT (5 cm H₂O) group did not change over the initial 30 min after ventilator adjustment, but subsequently decreased such that Pa_O2 values from 1.5 h to 3 h after treatment were significantly lower than values recorded at Time 10 min within this group (p < 0.001). All animals in the Normal VT (9 cm H₂O) group died within 1.5 h after surfactant treatment, from tension pneumothoraces. Initial Pa_O2 values within this group did not significantly change after ventilator adjustment, and were very similar to values recorded for animals in the Normal VT (5 cm H₂O) group at all time points prior to death. For animals in the Low VT (5 cm H₂O) group, Pa_O2 values decreased significantly over the initial period after ventilator adjustment (Time 20 min) compared with values at Time 10 min (p < 0.01), but then increased significantly to levels not significantly different from values recorded at Time 10 min within this group. Pa_O2 values in the Low VT (9 cm H₂O) group did not change significantly immediately after ventilator adjustment, and remained stable over the 3-h monitoring period. Comparisons among groups showed that Pa_O2 values in the Normal VT (5 cm H₂O) group were significantly lower than values in the Low VT (5 cm H₂O) and Low VT (9 cm H₂O) groups at the 2.5 h and 3.0 h time points after treatment (p < 0.01). There were no significant differences in Pa_O2 values between the Low VT (5 cm H₂O) and Low VT (9 cm H₂O) groups from 0.5 h to 3 h after surfactant administration.

View larger version (23K): [in this window] [in a new window]   Figure 1.   Mean PaO2 values over 3-h period after surfactant administration at different ventilatory settings. FIO2 was 1.0; values are expressed as mean ± SEM. PaO2 values decreased significantly in the Normal VT (5 cm H2O) group at the 2 to 3 h time interval as compared with values measured at Time 10 min (p < 0.001). PaO2 values in the Low VT (5 cm H2O) group decreased significantly 10 min after ventilatory adjustment (Time 20 min) compared with Time 10 min values (p < 0.01), and increased significantly thereafter to levels no different than Time 10 min values. PaO2 values in the Normal VT (5 cm H2O) group were significantly lower than values in the Low VT (5 cm H2O) and Low VT (9 cm H2O) groups at 2.5 h and 3.0 h after treatment (p < 0.01).

PIP and Pa_CO2 values are shown in Figure 2. PIP values were significantly higher in the Normal VT (9 cm H₂O) group than were PIP values in the Normal VT (5 cm H₂O) and Low VT (9 cm H₂O) groups at 10 min and 20 min after ventilatory adjustment (p < 0.05) (Figure 2A). There were no significant differences in PIP values between the Normal VT (5 cm H₂O) and Low VT (9 cm H₂O) groups over 3 h. PIP values in the Low VT (5 cm H₂O) group were significantly lower than values in the Normal VT (5 cm H₂O) and Low VT (9 cm H₂O) groups over a 3-h period after ventilatory adjustment (p < 0.01): Pa_CO2 values within each group did not change significantly over the entire ventilatory period as compared with their respective values at Time 10 min (Figure 2B). Comparisons between groups revealed that Pa_CO2 values in the Low VT (5 cm H₂O) and Low VT (9 cm H₂O) groups were generally higher than values recorded in the Normal VT (5 cm H₂O) group, although these differences did not reach statistical significance. Again, mean Pa_CO2 values in the Normal VT (9 cm H₂O) group were not significantly different from values recorded in the Normal VT (5 cm H₂O) group at any time point prior to death.

View larger version (25K): [in this window] [in a new window]   Figure 2.   Mean peak inspiratory pressure (PIP) and PaCO2 values over 3-h period after surfactant administration at different ventilatory settings. (A) PIP values in the Normal VT (9 cm H2O) group were significantly higher than values in the Normal VT (5 cm H2O), Low VT (5 cm H2O) and Low VT (9 cm H2O) groups at 10 min and 20 min after ventilatory adjustment (p < 0.05). PIP values in the Low VT (5 cm H2O) group were significantly lower than in the Normal VT (5 cm H2O), Normal VT (9 cm H2O), and Low VT (9 cm H2O) groups over the 3 h of ventilation (p < 0.01). (B) PaCO2 values did not change significantly over 3 h after ventilatory adjustment within each group. There were no significant differences in PaCO2 values among groups at any time point.

Biochemical Data

Results of the surfactant analysis are shown in Table 2. There were no significant differences in the total phospholipid pool sizes measured in the lung lavages recovered from the five different experimental groups, including the Baseline group at the time of killing. The calculated percentage of the surfactant that was present in the LA form, based on phosphorus measurements was significantly lower in the Normal VT, (5 cm H₂O) group than in either the Low VT (5 cm H₂O) or Low VT (9 cm H₂O) groups, respectively (p < 0.05). There were no significant differences among the other groups. There were also no significant differences in the total recovery of radiolabeled exogenous surfactant among the five experimental groups at the time of killing. The percent of the total radiolabel recovered in the lung homogenate fraction relative to recovery in the alveolar lavage was generally lower in the Baseline and Normal VT (9 cm H₂O) groups than in the Normal VT (5 cm H₂O), Low VT (5 cm H₂O), and Low VT (9 cm H₂O) groups, but these differences did not reach statistical significance (Table 2).

The proportion of the exogenous surfactant recovered in LA forms, based on radiolabel recovery measurements, is shown in Figure 3. A significantly lower percentage of the surfactant was in LA forms in the lavage obtained from animals in the Normal VT (5 cm H₂O) and Normal VT (9 cm H₂O) groups than in lavage from either the Low VT (5 cm H₂O) or Low VT (9 cm H₂O) groups or from the Baseline group (p < 0.05). Statistical comparisons between groups revealed no significant differences in the percent LA recovery for the Baseline, Low VT (5 cm H₂O), and Low VT (9 cm H₂O) groups. The total protein recovered in the lung lavage of these animals was not significantly different among the five experimental groups (data not shown).

View larger version (16K): [in this window] [in a new window]   Figure 3.   The effects of ventilation strategies on the percent of the radiolabeled exogenous surfactant recovered in large aggregate (LA) forms at sacrifice. Values are expressed as mean ± SEM. Percent LA values were significantly lower in the Normal VT (5 cm H2O) and Normal VT (9 cm H2O) groups than in the Low VT (5 cm H2O), Low VT (9 cm H2O), and Baseline groups (p < 0.05).

	DISCUSSION

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The administration of exogenous surfactant to patients with severe ARDS has shown promise, although the clinical results reported to date have been variable (9). Animal studies have shown that several factors may have contributed to inconsistent responses when exogenous surfactant was administered. One of these factors was the particular mode of mechanical ventilation used subsequent to surfactant delivery (16).

Mechanical ventilation is an essential supportive treatment modality for patients with severe respiratory failure, and is currently an important component of the management of patients deemed suitable for receiving exogenous surfactant (1, 27). Despite the importance of this intervention, very little attention has been directed to the ventilatory management of patients after exogenous surfactant has been administered. Jeffries and colleagues (28) showed that high-frequency oscillation (HFO) was superior to conventional ventilation in preterm animals treated with surfactant. HFO was also shown to be superior to conventional modes of administration in saline-lavaged adult rabbits given exogenous surfactant (16). Our results were consistent with these observations and showed that ventilation with lower VT values was superior to ventilation with higher VT values. In addition, our results suggest that the increased conversion of exogenous LA forms into SA forms may have been partly responsible for the observed differences in oxygenation.

The specific VT values chosen for the current study were based on recent clinical observations, as well as on the results of previous animal studies, even though none of these studies involved exogenous surfactant administration (20, 21, 29). For example, the concept of utilizing lower VT values to minimize ventilation-induced lung injury has led to the development of ventilation strategies designated "permissive hypercapnia" for patients with ARDS (29). These strategies have been shown to be superior to more conventional modes of ventilation with higher VT values. Animal data have indicated that a potential mechanism for the beneficial effects observed in patients managed with permissive hypercapnia may be related to the surfactant system. Using the adult rabbit model of acute lung injury induced by N-nitroso-N-methylurethane (NNMU), we showed that physiologic responses were superior in animals ventilated with VT values of 5 ml/kg than 10 ml/kg (21). These superior responses were associated with decreased conversion of better-functioning endogenous LA forms into inactive SA forms within the air space. Similar physiologic differences due to ventilation strategies with different VT values were reported in other studies (16, 29). Moreover, in still other studies it was also shown that an increased proportion of endogenous SA relative to LA forms within the air space (or a decrease in the total amount of LA forms) contributed to lung dysfunction (30). It is therefore feasible that the changes in the structural forms of alveolar surfactant induced by the higher VT values contributed directly to the lung dysfunction associated with ventilation-induced lung injury in animals or humans not treated with surfactant.

Interestingly, in a finding resembling the effects of ventilation on the endogenous surfactant system, we showed that lower VT values resulted in less conversion of exogenously administered surfactant into SA forms. Moreover, these lower aggregate conversion rates were associated with superior physiologic responses, a finding similar to that in previous studies of factors influencing the efficacy of exogenous surfactant administration in animal models of lung injury (12, 13). From our data, it was evident that the proportion of instilled exogenous surfactant present in LA forms within the air space at the onset of ventilator adjustment (Time 10 min) was approximately 80% of the total, as shown by the values observed in the Baseline group (Figure 3). Since bLES is known to exist as 98% LA, this finding indicates that approximately 20% of the instilled material had converted to SA forms prior to ventilator adjustment (Time 10 min). Animals subsequently ventilated for 3 h at a VT of 10 ml/kg (Normal VT [5 cm H₂O]) had approximately 65% of their administered surfactant recovered in LA forms, indicating that 35% of the material recovered in the lung lavage had been converted to SA forms over the 3-h ventilation period. On the other hand, when lower VT values were used, almost all of the recovered surfactant had remained in LA forms (Figure 3).

During the 3-h ventilation period, several metabolic processes could have taken place in addition to aggregate conversion, including surfactant uptake into the Type II cell as well as recycling and degradation of the administered material. However, for animals surviving the 3-h ventilatory period (Normal VT [5 cm H₂O], Low VT [5 cm H₂O], and Low VT [9 cm H₂O] groups), there were no significant differences in the total amount of radioactivity recovered in the lungs at killing or in the percentage of radioactivity that was associated with lung tissue. On the basis of these observations, we conclude that the significant differences observed between the groups in the relative proportion of LA recovered at killing mainly reflect differences in aggregate conversion characteristics within the alveolar space in these animals. Specifically, animals ventilated with lower VT values had lower rates of aggregate conversion than did animals ventilated with conventional or higher VT values. All animals ventilated with a VT of 10 ml/kg and PEEP of 9 cm H₂O (Normal VT [9 cm H₂O]) died within 1.5 h of ventilatory adjustment. Because of this shorter period of ventilation in these animals, comparison of surfactant metabolism between this group and the others is not valid. Interestingly, however, the percentage of exogenous surfactant recovered in LA forms from these animals was similar to that in animals in the Normal VT (5 cm H₂O) group. This finding suggests that for a specific VT there may be a "steady state" of surfactant aggregate forms established within the air space shortly after the onset of ventilation. However, the total amount of LA forms present within the air space may change with the passage of time due to complex uptake and recycling processes occurring between the Type II cell and the alveolar space (2). Specific mechanisms responsible for changes in surfactant structural forms require further study.

We speculate that the major influence of mechanical ventilation on lung function after administration of exogenous surfactant in the present study was mediated by the change in surface area associated with the specific VT utilized. As with the effects of ventilation on the endogenous surfactant system in non-surfactant-treated lungs (20, 21), it is likely that the greater phasic change in surface area induced by the higher VT values used in the present study resulted in a greater conversion of exogenous LA into SA forms. Over time, these changes contributed to the observed decreases in Pa_O2 values. On the other hand, increasing PEEP levels had no effect on aggregate conversion, but did improve physiologic responses, as observed in studies involving non-surfactant-treated animals. The higher PEEP levels used in the Low VT (9 cm H₂O) group in the current study prevented the acute deterioration of gas exchange noted immediately after surfactant administration in the Low VT (5 cm H₂O) group. It is therefore possible that the higher level of PEEP was necessary to "push" the exogenous surfactant out into the periphery of the lung and/or prevent alveolar collapse shortly after the surfactant was administered. Interestingly, animals in the lower PEEP group eventually reached the same oxygenation end point as those in the higher PEEP group over the 3-h ventilation period, suggesting that the higher PEEP levels were required only during the initial phase of ventilation after surfactant administration.

The consequences of using higher PEEP levels with higher VT values (Normal VT [9 cm H₂O]) were significantly different. All animals in this group died of barotrauma around the 1.5 h time point. Although gas exchange just prior to death was adequate in these animals, and not significantly different than in the other groups, the higher VT values and/or higher peak airway pressures resulted in tissue rupture and barotrauma. These observations underscore the need for an adequate understanding of lung mechanics in all patients undergoing mechanical ventilation, particularly in the context of exogenous surfactant administration.

In summary, we have shown that the optimal response to exogenous surfactant in lung-injured adult rabbits was obtained when animals were ventilated at lower VT values and a higher level of PEEP. The low VT decreased the conversion of exogenous LA forms into SA forms, and the higher PEEP level optimized lung function immediately after instillation. Higher PEEP levels had no effect on aggregate conversion, and only appeared to be necessary during the initial phase after surfactant treatment. These results emphasize the need for adequate preclinical data before exogenous surfactant is routinely used in patients with severe ARDS.