The determination of extravascular thermal volume from the mean circulation time and downslope time is illustrated in Fig, 2.
Fig. 2 (a) Calculation of extravascular lung water (extravascular thermal volume) from mean transit times; (b) calculation of extravascular lung water from downslope times. (Reproduced with permission from McLuckje. ...(.1.996.).)
The Edwards lung water computer was the first system that allowed the bedside computation of extravascular lung water. Unfortunately, the measurements were not reliable and showed reciprocal dependence on cardiac output. More recently, the COLD computer ( circulation, oxygenation, lung water, and liver function diagnosis) (Pulsion Medical Systems, Munich) has been developed. This system utilizes a thermistor-tipped fiber-optic catheter to detect the thermal and optical signals in the femoral artery. In addition to the calculation of extravascular lung water, this system can be used for the accurate determination of a number of blood volumes (total blood volume, intrathoracic blood volume, and global heart volume).
A number of potential flaws in the use of a thermal indicator have been identified. Thermal volumes will overestimate lung water volume owing to the loss of the heat indicator into non-aqueous structures such as the heart and blood vessel walls as well as the dry tissue mass of the lung. Lewiseta/ (1.9.82.) estimate that this loss is about 9 per cent of the injected indicator. However, given that the dry tissue mass usually remains unchanged, thermal indicator loss will remain constant and can be allowed for in the calculation of lung water.
The establishment of thermal equilibrium between blood and thermistor may artificially prolong the thermal mean transit time. The response time of the fiber-optic system to changing dye concentration is much faster. This inequality in response time may lead to an overestimation of the extravascular thermal volume. This potential source of error could be avoided by using deuterium oxide rather than heat as the diffusible indicator; the advantage is that deuterium oxide can also be detected by a fiber-optic system.
Whatever the diffusible indicator used, accurate lung water measurement must depend logically on diffusion of the indicator throughout the whole extravascular lung space, which in turn depends upon pulmonary perfusion. Interestingly, a study that investigated the effects of pulmonary emboli on the double-indicator dilution measurement of extravascular lung water showed that both over- and underestimation of lung water could occur, depending on the size of the emboli. Despite these theoretical limitations there is excellent correlation between the thermal dye dilution measurement of lung water and gravimetric determination.
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