This is, to our knowledge, the first study investigating in an in-vivo healthy animal model, the effects of a prophylactic exposure to an inhaled volatile anesthetic on the development of ventilator induced lung injury.
The choice of volatile anesthetics in this study features two substances with similar application mode (i.e. delivered by a vapor in relatively low concentration in inspired gas) and broad use in clinical anesthesia, but with different properties regarding pharmacokinetics, adverse effects and costs (31).
Regarding ventilator settings, VT was used to trigger detrimental pulmonary effects and the VT levels selected were intended to distinguish two differently harmful ventilation strategies. Accordingly, while a systemic inflammatory response was associated with MV regardless of the VT applied, MV with high VT additionally attenuated lung function and provoked lung injury. These effects are consistent with data from the literature (2).
The main result of the study however, as assessed by an array of markers of pulmonary and systemic injury and inflammation, is the absence of a clear beneficial effect of both isoflurane and sevoflurane on the detrimental effects exerted by mechanical ventilation.
These findings are in contrast with other studies in this field reporting protective or beneficial effects of isoflurane or sevoflurane on lung injury.
One reason for the disparate findings may be the diversity of settings and models used to investigate the effect of VA on lung function and integrity (32).
Beneficial or protective effects of VA have repeatedly been shown in injured lungs (e.g. endotoxin induced injury) which were pre-treated, post-treated or treated simultaneously with VA (33, 34). In some studies, the effect of VA was explored in two hit models combining e.g. endotoxin induced lung injury and MV (35, 36). In general, these models reflect the effects of VA on lungs already exhibiting a (pro-) inflammatory response.
In contrast, animals with healthy lungs were used in our experiments and MV was the single detrimental stimulus to the lung.
There is limited data on the interaction between VA and MV applied simultaneously in healthy lungs. In three studies in mice, isoflurane administered during MV attenuated inflammatory changes induced by MV of 8 ml/kg (23) and 12 ml/kg body weight (21, 22), whereas another study found evidence of a proinflammatory pattern at the transcriptional level in rats during MV with VA (20)
Different from these studies and hence representing still another distinct setting, in our study VA was applied prophylactically and a delay was established between administration of VA and the onset of MV.
We speculate that, depending on the kinetics and on the duration of the mechanisms (such as effects on gene expression or on pathways of inflammation) possibly induced by VA and MV, the interaction of effects could be different depending on if both interventions are applied simultaneously or consecutively. It is possible that there are time-dependent windows of preconditioning with different pathways involved in the effects of VA (37).
In addition, duration and dose of VA pretreatment could be confounding factors. Timing and dosing of the two volatiles we applied is in accordance with other publications (33, 34, 38, 39). In these studies, VA applied before a noxious stimulus to the lung, have shown protective effects. However, none of these studies used MV as a noxious stimulus to the lungs.
Another reason for the absence of protective or beneficial effects of isoflurane or sevoflurane in our study could be a misbalance between the relative potencies of the interventions applied. In the setting of MV with low VT producing only minor (if any) changes in most of the variables assessed, detection of a possible beneficial effect of VA seems difficult. On the other hand, in the setting of a more harmful stimulus such as MV with high VT provoking pronounced changes, effects of VA may not be discernable if they are only mild.
Of note, although in our study, clear beneficial effects of prophylactic VA administration were absent, this was not equivalent to absence of any effect of VA application: Decreased blood lymphocyte counts at the beginning of the ventilation period as well as lower macrophage counts in lung interstitial tissue in animals pre-treated with VA may reflect effects of VA on the inflammatory milieu. Further, increases in blood IL 6 levels during MV with high VT were even more pronounced in animals pre-treated with VA. Although from our study we are unable to confirm any biological relevance of such effects, in our opinion these findings deserve some attention.
Firstly, inducing general anesthesia by application of VA may per se affect homeostasis and biological integrity. In this regard, associated changes in white blood cell populations (i.e. such as lymphocyte counts in our study), irrespective of being caused by VA directly or by general anesthesia indirectly, are suggestive of some modulation of the immune status or the inflammatory milieu, respectively. This notion is supported by in vitro experiments demonstrating increased apoptosis in lymphocytes exposed to sevoflurane and isoflurane (40).
Such effects in the context of increasing IL 6 levels during MV with high VT after VA pre-treatment may result in a different inflammatory milieu as compared to pre-treatment without VA. However, interpretation of the biological significance of changes in cytokine concentrations is challenged by the fact that huge variations in these values are common (41) expression is time dependent (42) and function is pleiotropic.
Some limitations of our study warrant discussion.
Firstly, we did not investigate potential effects of VA on the level of biochemical or molecular pathways, interactions or cascades. Instead, it was our aim to detect effects translating to more global markers of injury and inflammation. In this context, the markers used in the present study are well established and appropriate to reflect the downstream effects of many cascades involved in ventilator induced lung injury (4, 32).
However, we are unable to distinguish, if below our radar a response to VA occurred, which may not have translated to quantitative effects on cells, proteins and functional markers (e.g. oxygenation) as assessed in the present study.
Secondly, we cannot exclude that other anesthetics used in the experiment counterbalanced the effects of VA pretreatment. Effects on the immune system and on inflammation have been shown for ketamine and thiopentone (both were used in this study) and might have interfered with effects of VA (17, 43). In theory however, this argument takes effect for any choice of anesthetic applied during MV. On the other hand, abandoning anesthetics during instrumentation and MV seems inappropriate for practical and ethical reasons.
Thirdly, different tidal volumes applied might cause different arterial carbondioxide concentrations between groups, which consecutively may have an independent modulatory effect on mechanical properties and inflammatory response of the lung (44, 45). We corrected for this potential confounder by adding CO2 to the inspiratory gas in the groups treated with high VT resulting in comparable CO2 concentrations in all groups.
In conclusion, this study failed to demonstrate in healthy rats significant beneficial or protective effects of pre-treatment with isoflurane or sevoflurane on the detrimental pulmonary effects caused by high tidal volume mechanical ventilation. This is in contrast with other studies showing beneficial effects of VA on lung injury albeit in different experimental settings. The findings suggest, that the experimental settings, in which VA are applied as well as the model of lung injury (e.g. endotoxin vs. ventilator-induced) play a role for their effects.
In the present study there were, nevertheless, some effects of VA pre-treatment reflecting changes in immune function and pulmonary inflammatory response. As long as the relevance of such changes remains unclear, based on our data, prophylactic application of VA to ameliorate ventilator induced lung injury seems not appropriate.
To more precisely identify interactions between VA, anesthesia and MV, modulations of the model, e.g. regarding dose, duration or timing of the pre-treatment with VA should be subject to further research.