Pulmonary Rehabilitation |

An Increase in Percentage Shunt Is Predictable in the Wake of an Increase in the Inspired Oxygen Fraction FREE TO VIEW

Wayne Wallace, MBA; Robert Demers, BS
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Kaiser Permanente SoCal, Pasadena, CA

Chest. 2013;144(4_MeetingAbstracts):840A. doi:10.1378/chest.1668775
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SESSION TITLE: Physiology/PFTs/Rehabilitation Posters

SESSION TYPE: Original Investigation Poster

PRESENTED ON: Wednesday, October 30, 2013 at 01:30 PM - 02:30 PM

PURPOSE: The equation for calculating percentage shunt reads as follows: QS/QT = [ (CcO2 − CaO2 ) / (CcO2 − CvO2 ) ] × 100% where “CcO2” represents the oxygen content of pulmonary capillary blood, “CaO2” is the oxygen content of arterial blood, and “CvO2” signifies the oxygen content of mixed venous blood.

METHODS: By rearranging the denominator of this equation, it reverts to the following: QS/QT = [ (CcO2 − CaO2 ) / { (CcO2 − CaO2 ) + (CaO2 − CvO2 ) } ] × 100% where “CaO2 - CvO2” represents the arteriovenous oxygen content difference (“a-v difference”).

RESULTS: By expressing the Shunt Equation in this form, it becomes obvious that a numerical increase in the numerator will result in an obligatory increase in percentage shunt, provided that the a-v difference does not vary substantially. The value of the numerator will always rise when the inspired oxygen fraction (FIO2) is increased, owing to the obligatory elevation in the content of dissolved oxygen which occurs, even after hemoglobin is fully saturated.

CONCLUSIONS: Shapiro and co-workers1 documented the changes in shunting which they observed in a group of 156 data sets which they obtained from patients whose FIO2 was increased from a mean of 0.50 to 1.0 (32 data sets) and from 0.40 to 1.0 (124 data sets). Figure 1 depicts the magnitude of increase in shunting which was triggered by the enhancement in the inspired oxygen fraction of these patients.

CLINICAL IMPLICATIONS: Clinicians should not necessarily be concerned when an apparent increase in shunting occurs in the wake of oxygen enrichment. Indeed, this is a virtually obligatory change and, although it reflects a reduction in the efficiency of the lung as a gas exchanger, the net effect on whole-body oxygen economy is usually salutary.

DISCLOSURE: The following authors have nothing to disclose: Wayne Wallace, Robert Demers

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