This technique relies upon the central injection of two indicators and their detection in the femoral artery. One (the non-diffusible indicator) remains confined to the intravascular space, while the other (the diffusible indicator) distributes between the intravascular and extravascular space. The only significant extravascular space between the point of injection and the point of detection is within the lungs. Approximately 20 per cent of this space consists of non-aqueous material and the rest is water. Using appropriate detection equipment, dilution curves can be generated for each indicator within the femoral artery ( Fig 1(a)). The volume of distribution of each indicator can be calculated from these curves, and the extravascular lung volume and hence the lung water can be determined by simple subtraction.
Fig. 1 Schematic representation of the dilution curves and their analysis for transit times; At, appearance time; MTt, mean transit time; DSt, exponential downslope time.
Lewis.. eta/ (19,7.9.) used negative heat (a cold crystalloid bolus (2-10 °C)) as the diffusible indicator and indocyanine green dye as the non-diffusible indicator.
Indocyanine green is suitable as the non-diffusible indicator since it is bound to plasma proteins and therefore remains within the intravascular space. Although indocyanine green is widely accepted as a suitable intravascular marker, the choice of the optimal diffusible indicator remains controversial.
The volume of distribution of an indicator is calculated from the product of the flow and the mean circulation or transit time of that indicator (MTt). Hence the volume of distribution of the thermal indicator ( VDT) is given by
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