The measurement of lung water

Imaging and indicator dilution techniques comprise the most common strategies for measuring lung water at the bedside. The most accurate (within 10% of the gravimetric gold standard) and most reproducible (< 5% between-test variation) are also, unfortunately, the most expensive and most difficult to implement for purposes of large-scale clinical trials or for routine clinical practice.The standard chest radiograph remains the best screening test for the detection of pulmonary edema. Indicator-dilution techniques are probably the best available method at present for quantitation in patient groups.Although about 80% of the lung is made up of water, gas-exchanging air spaces are protected by various barriers and drains. In multiple disease states, through injury or pressure (or both), these protective mechanisms fail, resulting in the abnormal accumulation of extravascular lung water (EVLW). The principle paradigm describing fluid flux in the lung is the 'Starling equation', which can be modified to account for the total surface area over which filtration might occur. 'Lymph flow' is a term summarizing those mechanisms responsible for returning extravasated fluid to the vascular compartment:EVLW=(Lp ×S) [(Pc-Pi)- σ (Πc-Πi)]-lymph flow [1]where EVLW=extravascular lung water (ml), Lp = the hydraulic conductivity for water (cm/min/mmHg), S = surface area (cm2), Pc and Pi = the hydrostatic pressure within the capillary and interstitial spaces respectively (mmHg), σ = the reflection coefficient for protein (no units), and Πc and Πi = the oncotic pressure within the capillary and interstitial spaces (mmHg).This equation describes the formation of interstitial edema accommodated by the interstitium. Subsequent movement of fluid into the air spaces develops by a more rapid process, termed alveolar flooding [2,3]. Normally EVLW is < 500 ml [4,5,6,7]. With alveolar flooding, lung water content is usually > 75-100% above normal [8]. It is at this point that physiologic impairment