Definition
FShunt is calculated as the ratio between the alveoli-arterial difference and the arteriovenous difference in total oxygen concentration. If no mixed venous sample is measured, FShunt is estimated by assuming the arteriovenous difference to be 2.3 mmol/L.
The total oxygen concentration of alveolar blood is calculated from the alveolar oxygen tension, obtained from the alveolar air equation. The systematic symbol for the relative physiological shunt is FShunt. The analyzer symbol may be Shunt or FShunt.
What does FShunt tell you
FShunt is an estimate of the fraction of the venous return that bypasses the pulmonary circulation and/or is not oxygenated during passage through the pulmonary capillaries (pc), i.e. the ratio between the shunted cardiac output and the total cardiac output:
FShunt has two pathological components:
- True shunt, where passage from the right to the left side of the heart is without gas exchange, e.g., heart-septum defects, arteriovenous malformations, pulmonary hemangiomas, pulmonary atelectases.
- Mismatch of regional ventilation and perfusion, where oxygenation is incomplete, e.g., lung diseases with inflammation or edema.
Reference ranges
FShunt reference range (adult) [16]: 4-10 % (0.04-0.10)
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Clinical interpretation
In the absence of extrapulmonery shunting, FShunt provides information about the intrapulmonary component of hypoxemia.
A high FShunt indicates pathological pulmonary mismatch between ventilation and perfusion, e.g., perfusion of non-ventilated areas (atelectases) and/or increased ventilation-perfusion scatter.
Considerations
FShunt may be calculated most exactly with the input of simultaneously drawn arterial and mixed venous samples, or it may be estimated from a single arterial sample (i.e., by assuming a normal arteriovenous oxygen content difference).
Even when estimated from just an arterial sample, FShunt presents the most comprehensive information on the lung function available from arterial blood gas analysis.
