Shunt Equation

Physiological shunt can be calculated as a fraction of total pulmonary blood flow (cardiac output). Figure RR.21 shows the flows and oxygen contents of systemic, pulmonary capillary and shunt circulations.

CvOj = mixed venous 03 contcnt

Cci5, = pulmonary capillary Oj content

The total O2 content of blood leaving the lungs is: = (cardiac output X arterial Oa content)

But the total oxygen content is also:

= (sl)tllll fluw X miii'd vellum; O , CLHHdll) + (pulmonary capillary flow x pulmonary

This can be rearranged to give the 'shunt' equation as follows:

Summarizing for physiological shunt

Shunt effectively reduces oxygen content of pulmonary capillary blood by admixture of venous blood bypassing the lungs.

The 'shunt' equation assesses the shunt as a fraction of the total flow, and can be stated as:

[reduction in oxygen content due to *liunt) (total oxygeti content added by lung*)

This fraction is usually quoted as a percentage and is a useful clinical index of oxygenation efficiency in critically ill patients. The effect of physiological shunt on arterial blood gases is shown in Figure RR.22, which illustrates an 'iso-shunt' diagram. Shunt fractions up to 10% are not considered clinically significant, however when in > 30%, they are associated with poor survival. The iso-shunt diagram is helpful in following clinical progress and in adjusting oxygen therapy.

Virtual ikunt linflj.

Virtual ikunt linflj.

20 30 40 50 60 70 SO 90 100 Inspired oxygen (%)

20 30 40 50 60 70 SO 90 100 Inspired oxygen (%)

Conditions Hb 10-14 g/dl PbCO: 3.3-5.3 kRa o-y oxygen eonient different» 5 ml/100ml

Figure RR.22 Iso-shunt diagram

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