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Clinically or experimentally, it is often useful to calculate the pulmonary shunt fraction (QS/QT). The pulmonary shunt calculation estimates the magnitude of the anatomic and physiologic shunts or the fraction (percentage) of pulmonary blood flow that passes from the right to left heart without undergoing complete oxygenation by the lung. The relationships and assumptions important in the calculation of the shunt fraction are depicted schematically in the figure. The total O2 in blood presented for oxygenation by the pulmonary artery is the mixed venous O2 content (CvO2) multiplied by total cardiac output (QT), or CvO2 x QT. However, a portion of this blood flow may be shunted from the left to right heart without undergoing gas exchange. This flow is designated as shunt flow (QS). Thereby, the total O2 passing through the shunt is the venous O2 content multiplied by shunt flow, or QS x CvO2. The remaining portion of the blood flow is delivered to the alveoli for gas exchange and equals total flow minus shunt flow, or QT - QS. If this blood completely equilibrates with the alveolar O2, then the total amount of O2 leaving the lung capillaries to enter the pulmonary vein is total flow minus shunt flow (QT - QS) multiplied by capillary O2 content (CcO2), or (QT - QS) x CcO2. In the systemic circulation, the total O2 delivered to the tissues is the systemic arterial O2 content (CaO2) multiplied by cardiac output (QT), or CaO2 x QT. If it is assumed that total right heart cardiac input or venous return (QTinflow) is equal to left heart cardiac output (QToutflow), then O2 delivered to the tissues from the left heart (QT x CaO2 ) is the sum of O2 content in shunt flow (QS x CvO2) and O2 content in alveolar blood flow, or (QT - QS) x CcO2 as expressed in the equation:
If it is assumed no diffusion limitation is present and complete O2 equilibration occurs between the alveoli and capillary blood, then the capillary PO2 is equal to alveolar PO2. Since it is difficult to sample capillary blood to measure the PO2 or CcO2, the CcO2 is usually derived from a measurement of the alveolar PO2 and expressed as CAO2. By substituting CAO2 for CcO2, and rearranging the equation, the shunt fraction (QS/QT) is calculated as:
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Intrapulmonary shunting is defined as the portion of the cardiac output that perfuses through the lung without coming in contact with ventilated alveoli. This portion of the cardiac output, therefore, passes through the lungs and into the left side of the heart without being oxygenated. In the normal, healthy person, intrapulmonary shunting occurs. This results from blood flow through the bronchial, pleural, and thebesian veins. These veins return blood to the left atrium, therefore bypassing the oxygenation process in the lungs (called an anatomic shunt). Normally, intrapulmonary shunting is about 2% to 5% of the cardiac output and is primarily caused by anatomic shunting. Physiologic shunting represents only a small portion of the normal anatomic shunt. Increased physiologic shunting results in a worsening cardiopulmonary status. Conditions that increase physiologic shunting:
The amount of shunt may be determined using the clinical shunt formula:
This formula requires a 100% Hb saturation of O2 in arterial blood. If measurement of PVO2 is not available, (via pulmonary artery catheter), the modified shunt equation may be used:
Calculate a patients percentage of shunt given the following data:
PaO2 = (PB – 47)(FiO2) – (PaCO2 x 1.25) (747 – 47)(.4) – (45 x 1.25) 280 - 56 = 224 torr
= .098
QS/QT = .098 x 100 = 9.8%
This means that almost 10% of the patients cardiac output is not being oxygenated in the lungs.
A simplified method of calculating shunt is: A-a gradient / 20 + 4% Interpreting calculated shunt values:
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