Question

Which part of the classic shunt equation the numerator or denominator is most affected by atelectasis? Which is affected most decreased cardiac output?

Answer 1

• Calculations of the pulmonary shunt equation* are used in frequently ill patients to monitor the effectiveness of pulmonary gas exchange. The numerator of the shunt equation is represented by differences in C_cO₂ and C_aO₂ whereas the dinominator  of the equation is represented by the differences in C_cO₂ and C_vO_{2 .}
• Pulmonary shunt is a pathological conditions which results when the alveoli of the blood is perfused with blood as normal but supply of air fails to supply the perfused region and it is very common in atelectasis. Shunting causes the blood supply leaving a shunted area of the lung to have lower level of oxygen and high level of carbon dioxide .
• The shunt which is V/Q = 0 , for that particular part of the lung field under considerations results in deoxygenated blood going to the heart from the lungs through pulmonary veins. Due to the number of non ventilated alveoli atelectasis will affect numerator.
• With low cardiac output, dinominator will get affected most because of very low venous side. Venous congestion is main cause of decreased cardiac output.

* ( Fic's law based; but now modified as Breggler's law)