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Pulmonary Rehabilitation |

The Normal Range for A-a Gradient During Oxygen Breathing Varies With the Arteriovenous Oxygen Content Difference

Bryan Chen, BS; Robert Demers, BS
Author and Funding Information

Kaiser Permanente SoCal, Pasadena, CA


Chest. 2013;144(4_MeetingAbstracts):843A. doi:10.1378/chest.1668735
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Abstract

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: Two to five percent of the cardiac output (C.O.) is shunted past the lungs, even in normal subjects1. This elicits a discrete gradient between the oxygen tension in alveolar gas (pAO2) and the oxygen tension in arterial blood (paO2). This arithmetic difference, the so-called “A-a Gradient” (A-aΔO2), is frequently used to quantify the presence and magnitude of disordered gas exchange in critically ill patients. It has been observed that the A-aΔO2 widens as the inspired oxygen fraction (FIO2) is increased from air breathing (FIO2 = 0.21) to pure oxygen breathing (FIO2 = 1.0). This variability has been addressed in certain texts, such as the excellent monograph edited by Kacmarek, Mack, and Dimas, in which it is stated that “....at an FIO2 of 1.0, [the A-aΔO2] increases to 31 to 56 mm Hg”1. Most clinicians, however, are not aware that the range of normal for A-aΔO2 will also be subject to variation secondary to changes in the arteriovenous oxygen content difference (a-vΔO2), which varies inversely with C.O., even when the shunt fraction remains at a stable two to five percent of that C.O.

METHODS: The interdependence between A-aΔO2 and a-vΔO2 is made obvious by inspecting the approximation equation for percentage shunt1: QS/QT = [ ( 0.0031 × A-aΔO2 ) / ( { 0.0031 × A-aΔO2 } + a-vΔO2 ) ] × 100%

RESULTS: Using the equation displayed above, we generated a Table (below) which displays the ranges of normal for A-aΔO2 at various levels of a-vΔO2.

CONCLUSIONS: It is important to determine the normal range for A-aΔO2 during oxygen breathing for normal subjects at various a-vΔO2 levels, in order to be able to identify the abnormal ranges for that variable which are associated with pathologic states.

CLINICAL IMPLICATIONS: The wide variability observed in A-aΔO2 secondary to alterations in cardiac output and a-vΔO2 render it important that clinicians account for such variations before characterizing a given A-aΔO2 figure as pathologic.

DISCLOSURE: The following authors have nothing to disclose: Bryan Chen, Robert Demers

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