Measurement technique

The normal value of 4-7ml/kg for extravascular lung water has been derived by gravimetric techniques performed post-mortem. A double indicator technique may be used in living patients. Two indicators are injected via a central vein; one distributes within the vascular space and the other throughout the intra- and extravascular space. The volume of distribution of the indicators is derived from the dilution curves detected at the femoral artery. Cooled 5% glucose is used as a thermal indicator for intra- and extravascular volume and indocyanine green bound to albumin as a colorimetric indicator for intravascular volume. Detection at the femoral artery is by a fibreoptic catheter with a thermistor tip. The cardiac output is measured by thermodilution at the femoral artery. The rate of exponential decay of the thermodilution curve allows the derivation of the volume of distribution between the injection and detection sites (the heart and lungs).

Pulmonary thermal volume = thermodilution CO x rate of exponential decay of thermodilution curve (intra- and extravascular volume)

Similar principles may be applied to the dye dilution curve produced on injection of indocyanine green which is assumed to distribute within the vascular space only.

Pulmonary blood volume = dye dilution CO x rate of exponential decay of dye dilution curve (intravascular volume) EVLW may be calculated by subtracting pulmonary blood volume from pulmonary thermal volume.

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