Multiple inert gas elimination technique

The multiple inert gas elimination technique (or MIGET) is a technique used mainly in pulmonology that involves measuring the concentrations of various infused, inert gases in mixed venous blood, arterial blood, and expired gas of a subject. The technique quantifies true shunt, physiological dead space ventilation, ventilation versus blood flow (V_A/Q) ratios, and diffusion limitation.

Background

Hypoxemia is generally attributed to one of four processes: hypoventilation, shunt (right to left), diffusion limitation, and ventilation/perfusion (V_A/Q) inequality. Moreover, there are also "extrapulmonary" factors that can contribute to fluctuations in arterial PO₂.

There are several measures of hypoxemia that can be assessed, but there are various limitations associated with each. It was for this reason that the MIGET was developed, to overcome the shortcomings of previous methods.

Theoretical Basis

Steady-state gas exchange in the lungs obeys the principles of conservation of mass. This leads to the ventilation/perfusion equation for oxygen:

$VA/Q=8.63\times {\frac {Cc'O2-CvO2}{PIO2-PAO2}}$

and for carbon dioxide:

$VA/Q=8.63\times {\frac {CvCO2-Cc'CO2}{PACO2}}$

where:

• Cc' denotes the end-capillary concentration of the gas (mL/dL),
• Cv denotes the mixed venous concentration of the gas (mL/dL),
• PI denotes the inspired partial pressure of the gas (mmHg), and
• PA denotes the alveolar partial pressure of the gas (mmHg)

For the purposes of utilizing the MIGET, the equations have been generalized for an inert gas (IG):

$VA/Q=8.63\times solubility\times {\frac {PVIG-PC'IG}{PAIG}}$

where:

• solubility is the ratio of concentration to partial pressure expressed in mL of gas dissolved per dL of blood per mmHg of the gas in blood

Assuming diffusion equilibration is complete for the inert gas, dropping the subscript IG, and substituting the blood-gas partition coefficient (λ) renders:

$VA/Q={\lambda }\times {\frac {Pv-PA}{PA}}$

Rearranging:

$PA/Pv={\frac {\lambda }{{\lambda }+VA/Q}}=Pc'/Pv$

From this equation, the fraction of inert gas not eliminated from the blood via the lung is a function of the partition coefficient and the V_A/Q ratio.