Total pulmonary blood flow can be measured by various techniques, such as dye dissolution methods, heat dissolution, Fick's equation, etc., which, in essence, are a cardiological subject and go beyond the present. With the Fick method as it is a practice in catheterization, the required measurements are the oxygen content (concentration) of arterial, Pa, and small venous, Pv̄, (pulmonary arterial) blood, as well as the total oxygen consumption, V̇O2. The total blood flow to the lungs is given by the equation
Q̇t = V̇O2 / [CaO2-Cv̄O2] F Fick equation}
where Q̇t is the total blood flow to the lung, CaO2 and Cv̄O2 the oxygen concentration in arterial and mixed blood, respectively.
A similar equation can be written for carbon dioxide and is relevant in Respiratory Control Laboratories, due to the ease of calculating the Pv̄CO2 content and, consequently, CO2 production through re-inhalation techniques.
Q̇t = V̇CO2 / [Cv̄O2-CaCO2]
The Fick equation, suitably modified, is used to calculate the escape rate (Q̇s to total blood flow Q̇T, Q̇s / Q̇T.
Q̇S / Q̇T = (PAO2-PaO2) 0.003 / PAO2-PaO2) 0.003 + 5
Where 0.003 the blood O2 solubility factor and factor 5 represent the difference in O2 content between mixed venous and arterial blood.
Using the method, the estimated escape in healthy is less than 5% of the total blood flow, and is due to the blood flowing through the Thebesian veins, which flow directly from the right to the left ventricle, bypassing the pulmonary circulation. Intrapulmonary leakage is increased in patients with pulmonary edema, lobular pneumonia, pulmonary arteriovenous disorders, and other pathological conditions that form bloody, unventilated pulmonary areas.
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