© 2003 American Thoracic Society
Are We Really Reducing Tidal Volume—And Should We?Service de Réanimation Médicale Hôpital Louis Mourier (Assistance Publique–Hôpitaux de Paris) Colombes, France After the publication of compelling experimental evidence (1, 2) that excessive end-inspiratory lung volume may induce lung injury, a recent trial (3) has provided resounding clinical relevance to this experimental concept. The investigators showed a significant reduction in mortality of patients with acute respiratory distress syndrome (ARDS) when tidal volume was reduced to 6 ml/kg in comparison with patients ventilated with 12 ml/kg (3). After what was termed the culmination in an era of research (4), several questions arise. How do the results of the ARDS Network trial affect the routine ventilator care of patients with ARDS? Has the trial established a new standard of care for mechanical ventilation of patients with ARDS? If not, can clinicians justify the failure to apply what some would consider as evidence-based medicine? Preliminary reports suggest that clinicians do not readily apply such drastic reductions of tidal volume in their patients. In a preliminary report, Rubenfeld and colleagues (5) noted that a reduction in tidal volume was applied in less than 5% of patients with ARDS or acute lung injury despite their institution being one of the centers participating in the ARDS Network trial. In this issue of AJRCCM (pp. 1304–1309), Weinert and colleagues (6) report a review of medical charts of patients with ARDS or acute lung injury between May 1994 and May 2001, focusing on the first 3 days of mechanical ventilation. Importantly, this study included patients from three teaching hospitals that did not participate in the ARDS Network trial. The authors wished to assess whether there was a trend in tidal volume over 7 years, if there was an inflection in the trend, and at what time it occurred. The authors did indeed identify both a trend toward lower tidal volume and an inflection. It occurred, however, before the results of the ARDS Network trial were first released. Moreover, the reduction in tidal volume was modest and was noticeable only in patients with greater lung injury scores (6). One possible explanation for the inflection in tidal volume, which Weinert and colleagues (6) did not specifically discuss, is that the inflection may reflect clinicians' belief that only the patients with the most severe lung injury require a reduction in tidal volume. Indeed, mortality of their patients increased considerably over the last 3 years, suggesting the inclusion of sicker patients (6). In any case, the data of Weinert and colleagues (6) clearly show that tidal volumes had started to be reduced before the completion of the ARDS Network trial (and probably back in the early 1990s), and that publication of the results of the ARDS Network trial did not impact on the approach taken by clinicians in the ventilatory management of patients with ARDS (6). These observations are in agreement with several other pieces of data. In a review of their experience of ventilator settings over time, Jardin and colleagues (7) noted that tidal volumes used in 1993 were already significantly lower (9 ml/kg) than tidal volumes used in 1981 (13 ml/kg). This reduction came after a radical change in their ventilatory strategy, which focused on limiting of plateau pressure rather than on a precise determination of tidal volume based on body weight. Thompson and colleagues (8) reviewed the baseline characteristics of more than 600 patients enrolled in the ARDS Network trial. Tidal volume at baseline was lower (10.3 ml/kg) than in the patients assigned to the conventional arm of the ARDS Network trial (11.8 ml/kg) (8). This observation suggests that even in the centers participating in the ARDS Network trial, the clinicians had already reduced tidal volume to a moderate extent. Esteban and colleagues (9) prospectively recorded ventilatory settings used outside of a research setting in more than 200 patients with ARDS ventilated worldwide during March 1998. They observed mean tidal volumes of 8.5 ml/kg and resulting plateau pressures of 28 cm H2O. These results are in agreement with the finding of Weinert and colleagues (6) of mean plateau pressures below 30 cm H2O even in the early years of their study. The findings suggest that clinicians were adopting existing guidelines about limiting plateau pressures soon after the guidelines (10) were published. Is there a new standard in the management of patients with ARDS? In the light of the study by Weinert and colleagues, the answer is no. Less than 1% of their patients received ARDS Network volumes of 6.2 ml/kg or less. As stated by the authors, clinicians may simply not be convinced of the beneficial effects of a drastic reduction in tidal volume. Some authors have argued that the studies of both Amato and colleagues (11) and the ARDS Network (3) showed the detrimental effects of a tidal volume of 12 ml/kg rather than the benefits of a tidal volume of 6 ml/kg. The following arguments support that opinion. First, patients who met the ARDS Network entry criteria but declined to participate in the trial (and were thus ventilated at the clinician's discretion, possibly with 10.3 ml/kg tidal volume [8]) had a much lower mortality rate than did the patients allocated to the control arm of the ARDS Network trial (12 ml/kg) and a similar mortality to patients allocated to the low tidal volume (6 ml/kg) arm (12). Second, a meta-analysis of trials testing low tidal volumes suggests an increased risk of mortality in patients ventilated with very low tidal volumes (13). This conclusion is in keeping with the greater incidence of renal failure and the need for dialysis observed in patients receiving a tidal volume of 7.2 ml/kg as compared with those receiving 10.8 ml/kg (14). How do we reconcile conflicting evidence? Although the Bayesian perspective has been formalized mostly in medical decision making, it may also apply to the interpretation of evidence from clinical trials (15). Because our interpretation of the evidence is influenced by our former opinion on the subject (15), one must remain very cautious when analyzing the impact of these trials on daily practice. For sure, Weinert and colleagues tell us that clinicians have not radically changed their attitude toward tidal volume. This may be because the change had already taken place some years ago, when clinicians reduced tidal volume from 13–15 ml/kg (7, 16) to 10 ml/kg, establishing then a new standard of care for the management of ARDS. There is little doubt that tidal volumes exceeding 12 ml/kg are detrimental, but the unresolved question (which may well remain unanswered) is whether or not it is prudent to further reduce tidal volume below 8–10 ml/kg. In this matter as in others, clinicians should continue to apply (as they have apparently done during the last decade) the principle of precaution and maintain plateau pressure below 30–32 cm H2O (4) rather than clinging to determination of tidal volume dictated on the basis of body weight. One must accept the complexity of ARDS and the ensuing uncertainty in its management (17). REFERENCES
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