Hence the shunt fraction is usually represented as Qs/Qt. The relationship between the different fractions must therefore remain a ratio, rather than a real oxygen difference in ml. We can only say that if all blood were ideally oxygenated the Ca O2 should be the same as Cc O2, and if all blood were completely shunted the Ca O2 should be the same as Cv O2. Instead an assumption is made that the ventilated areas of the lung are perfused with "ideal" capillaries, and the gas exchange in them is so perfect that their end-capillary oxygen tension is equal to alveolar oxygen tension, and their saturation is 100%.ĭetermining Qs is therefore a matter of subtracting (Cc O2 + Cv O2) from Ca O2. Obviously nobody is ever going to get into those capillaries and measure it directly. Well, the pulmonary end-capillary oxygen content is not known, but it is assumed. Of these quantities, all are known except the Qs, which is the shunt fraction. (Cc O2 - Ca O2) is the difference in oxygen content between "perfect" endcapillary blood and systemic arterial blood this drop in oxygen content is due to the venous admixture.Ca O2 is the oxygen content of systemic arterial blood, which will be lower than the content of "perfect" endcapillary blood because it is mixed with the relatively hypoxic Qs.Qs is the flow through the shunt fraction, which is returning perfectly unchanged mixed venous blood back to the systemic arterial circulation.(Cc O2 - Cv O2) is the difference in oxygen content between mixed venous and "perfect" endcapillary blood.the arterial and alveolar oxygen content is the same). This assumption is based on a two-compartment model, where this compartment is perfectly oxygenated (i.e. Cc O2 is the oxygen content of pulmonary endcapillary blood, and it is assumed that it is equal to Ct O2(A), the alveolar oxygen content.Cv O2, the oxygen content of mixed venous blood, is a known variable (as you can measure it), and it is returning to the lungs at a flow rate equal to the cardiac output, Qt. ![]() The shunt equationĬonfusingly, the shunt equation is used to estimate the venous admixture, relying on a model of the lung which divides it conceptually into regions with V/Q of 1 and regions with V/Q of 0.Īs always, these things are easier to represent as a big confusing diagram. The best free reference is Bigeleisen (2001), which is an excellent resource to "help students master these equations as well as their practical limitations". This technique does not separate "true" shunt from anatomical shunt (contribution from thebesian veins and bronchial veins) or cardiac defects. ![]()
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