Affiliations: Bicêtre Hospital Paris Sud Medical School Le Kremlin Bicêtre, France,
Correspondence to: Frédéric Michard, MD, PhD, Department of Anesthesia & Critical Care, Massachusetts General Hospital-Harvard Medical School, 55 Fruit St, Boston, MA 02114-2696; e-mail: email@example.com
We read with interest the study by Sakka et al (December 2002)1–
demonstrating the prognostic value of extravascular lung water (EVLW) in critically ill patients, and we would like to emphasize another potential clinical value of this parameter. In patients with pulmonary edema (defined as EVLW > 7 mL/kg), fluid restriction/depletion has been shown to improve outcome.2–
However, in patients with acute lung injury (ALI)/ARDS, the effectiveness of such a fluid-conservative approach remains a subject of ongoing controversy.3
The radiographic criterion used in the American-European definition of ALI/ARDS showed high interobserver variability,4
and arterial hypoxemia can be due to other disease processes than pulmonary edema. Therefore, we postulated that ALI/ARDS criteria could be inaccurate to identify patients with pulmonary edema, ie, patients who may benefit from fluid restriction/depletion.
Seventy-five chest radiographs, blood gas measurements, and EVLW measurements done simultaneously in 37 patients receiving mechanical ventilation without evidence for left heart failure have been analyzed. The EVLW was evaluated by transpulmonary thermodilution, a technique validated against the double-indicator (thermo-dye) and the gravimetric methods.5
Chest radiographs were analyzed independently by each author. When discrepancies were observed (21 of 75 [28%]) between individual analysis, radiographs were reanalyzed for a consensual decision.
Bilateral pulmonary infiltrates were observed in 51 instances. The Pao2/fraction of inspired oxygen (Fio2) ratio was < 300 or 200 mm Hg in 60 instances and 44 instances, respectively. A negative and weak (r2 = 0.27, p < 0.001) relationship was observed between Pao2/Fio2 ratio and EVLW. The ALI/ARDS criteria were fulfilled in 46 of 75 instances (61%). The EVLW was higher (11.3 ± 5.4 mL/kg vs 8.6 ± 5.2 mL/kg, p < 0.05) in the cases of ALI/ARDS (mean ± SD). However, ALI/ARDS criteria (ALI, n = 7; ARDS, n = 9) were associated with an EVLW value ≤ 7 mL/kg in 16 of 46 instances (35%) [Fig 1
These findings suggest that a substantial number (approximately one third) of patients with ALI/ARDS criteria have no significant pulmonary edema. In this regard, EVLW measurement should be helpful to identify patients with ALI/ARDS who might benefit from fluid restriction/depletion. Clinical studies are required to confirm the value of EVLW in the decision-making process concerning fluid restriction/depletion in patients with ALI/ARDS.
We appreciate the correspondence by Michard and colleagues with reference to our article on the prognostic value of extravascular lung water (EVLW) in critically ill patients.1
They address an important issue as to whether EVLW should be a criterion in the definition of acute lung injury (ALI) or ARDS.
Currently, oxygenation index (Pao2/fraction of inspired oxygen [Fio2] ratio) is part of the ALI/ARDS criteria. Although relatively easy to obtain in critically ill patients, Pao2/Fio2 ratio as marker of pulmonary edema has been questioned. In their letter, Michard et al report data from 37 critically ill patients on the reliability of Pao2/Fio2 ratio as evaluated by EVLW. In their analysis, only 65% of patients fulfilling the ALI/ARDS criteria had pulmonary edema (as defined by EVLW > 7 mL/kg). Since thus approximately one third of patients with ALI/ARDS were ascribed to have no significant pulmonary edema, the authors suggest that EVLW may be more helpful to identify patients with ALI/ARDS.
We also feel that EVLW might be a valuable criterion for ALI/ARDS. However, EVLW > 7 mL/kg was uniformly considered as marker of pulmonary edema in their analysis. Nevertheless, appropriate cut-off values for ALI and ARDS have to be established for both entities in prospective trials before EVLW can be added to ALI/ARDS criteria.
Currently, data on EVLW-based fluid management in critically ill patients are still limited. So far, using EVLW to guide the management of patients with both cardiac and noncardiac pulmonary edema (ARDS) has been shown to reduce the duration of mechanical ventilation, length of stay in the ICU, and potential intensive care costs.2–
Moreover, EVLW-guided therapy also reduced mortality in patients with congestive heart failure and ARDS.3
Further studies on the value of EVLW in critically ill patients are highly warranted, ie, for potential implementation in scoring systems or as part of ARDS/ALI criteria. We re-emphasize the need to perform appropriate prospective trials in the future, especially since determination of EVLW has become possible by a simplified approach based on the single transpulmonary thermodilution technique.4
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