Shunt fraction and deadspace measurements

Intrapulmonary shunt fraction as a percentage of cardiac output can be calculated from the shunt equation:

where Q s is the flow-through shunt, Qt is the total cardiac output, Cco2 is the oxygen content of pulmonary capillary blood, Cvo2 is the oxygen content of mixed venous blood, and Cao2 is the oxygen content of arterial blood. The normal range is 1 to 4 per cent. Mechanical ventilation is usually deemed necessary at shunt levels of 15 to 20 per cent. However, the complexity of this calculation, the need for mixed venous blood samples, and the difficulty in accurately measuring saturation in a given patient have made this approach less popular.

The same can be said for the calculation of dead-space as a percentage of the tidal volume ( Vd/Vt ratio):

where Vd is the dead-space, Vt is the tidal volume, Paco2 is the arterial CO2 tension, and Peco2 is the mixed expired CO2 tension. This is normally 30 per cent, and ventilation is recommended at ratios above 60 per cent. Although the calculation is straightforward, collecting mixed expired air renders the process laborious for everyday use.

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