Fluid Measurement and Monitoring
Measurement of Pulmonary Edema
Accurate measurement and quantification of pulmonary edema are important for diagnostic and therapeutic interventions. Clinically pulmonary edema can be detected on physical examination by the presence of rales and may be confirmed by chest radiography. However, it has been shown to be difficult to quantify the amount of pulmonary edema based on chest radiography alone . Other indirect radiological estimates of lung water, such as CT scanning and MRI, while more accurate, are impractical for real-time measurement in critically ill patients.
Extravascular Lung Water
Limitations of indirect measures of lung water led to the development of thermal dye techniques to allow estimation of lung water at the bedside. Currently, the single transpulmonary thermodilution technique is considered the clinical gold standard of lung water estimation . Using this technique, the extravascular lung water (EVLW), which is the amount of fluid that is accumulated in the interstitial and alveolar spaces, can be calculated. EVLW obtained via the single transpulmonary thermodilution technique has been shown to compare favorably with other methods of lung water assessment including the double-indicator dilution technique and the ex vivo gravimetric method . Compared to non-indexed EVLW, EVLW adjusted for body weight (EVLWI, mL/kg) is better correlated with the severity of lung injury and the oxygenation in patients with ARDS . An increased value of EVLWI is a pathophysiological hallmark of hydrostatic as well as inflammatory lung edema, and this technique can detect small (10-20%) increases in lung water . The normal value for EVLWI is reported to be approximately 7 mL/kg and results from the equilibrium between fluid leakage and lymphatic drainage . Values of the EVLWI exceeding 30 mL/kg have been reported during severe pulmonary edema .