Relationship between extravascular lung water and oxygenation indices in patients with acute respiratory failure

Introduction

The main strategy to restore organ perfusion in critically ill patients is fluid administration, which increases the likelihood of acquiring pulmonary edema with detrimental effects on gas exchange. The thermal-dye dilution technique has been used to measure extravascular lung water (EVLW). Recently, this technique has been simplified using only the thermal indicator [1]. The relationship between EVLW and the PaO₂/FiO₂ ratio has been sparsely studied and described as weak [2]. Oxygenation index (OI) relates PaO₂, FiO₂, and mean airway pressure (Paw) by the following formula: OI = Paw × FiO₂ × 100 / PaO₂ [3]. We are not aware of publications correlating EVLW with OI.

Methods

Simultaneous measurements of EVLW, arterial blood gases and Paw were performed in mechanically ventilated patients at different moments of their illness course. EVLW was measured by a single transpulmonary thermodilution technique using a PiCCO^® monitor. The EVLW value was indexed (EVLWi) by the predicted body weight [4]. PaO₂/FiO₂ and OI were calculated and correlated with EVLWi using linear regression analysis with a 95% confidence interval (CI).

Results

We studied 21 patients, 20 with ALI/ARDS (age 64 ± 19 years, APACHE II score 24.4 ± 6.8, SOFA 10.3 ± 3.2, LIS 2.55 ± 0.57). Mean admission EVLWi, Paw, PaO₂/FiO₂ and OI values were 14 ± 6.5 ml/kg, 17.8 ± 3.9 cmH₂O, 186 ± 80 and 12.1 ± 7.5, respectively. We obtained 152 EVLWi measurements with simultaneous PaO₂/FiO₂ and OI values. A significant negative correlation was found between EVLWi and PaO₂/FiO₂ (r = -0.53, CI -0.63 to -0.40, P < 0.01) (Fig. 1). A significant positive correlation was found between EVLWi and OI (r = 0.70, CI 0.60–0.77, P < 0.01) (Fig. 2).

Figure 1

(abstract P47)

Figure 2

(abstract P88)

Conclusions

There is a stronger correlation between EVLWi and OI than with PaO₂/FiO₂. This finding emphasizes the importance of incorporating Paw into the evaluation of patient oxygenation during mechanical ventilation.