The original technique was described using inert gas measurements determined from mixed venous blood ( Pv), arterial blood (Pa), and mixed expired gas (PE). An alternative approach without mixed venous sampling can be used with similar accuracy. In this instance, cardiac output needs to be obtained directly. However, the mixed venous variables can be computed from the mass balance equation:
where VE is minute ventilation and is cardiac output.
Finally, a third modality of the multiple inert gas elimination technique requiring only mixed expiratory and peripheral venous sampling is available, based on the assumption that inert gases are not metabolized in the peripheral tissues. Virtual equilibration between blood and tissues is achieved and the peripheral venous blood reflects the inert gas concentrations of the inflowing arterial blood. Using this approach, it has been shown that partial pressures of the inert gases in a peripheral vein are 95 per cent of Pa and sufficiently reproducible to permit substitution of peripheral venous for arterial sampling. However, this approach cannot be used to estimate
the first moments of the distributions, the amount of intrapulmonary shunt and dead-space, or the percentage perfusion or ventilation for a given range of VpJQ ratios (Wagner,, etal 1985).
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