What is the role of arterial blood gases in the workup of smoke inhalation injury?

Updated: Oct 15, 2021
  • Author: Keith A Lafferty, MD; Chief Editor: Joe Alcock, MD, MS  more...
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Answer

Arterial oxygen tension (partial pressure of arterial oxygen [PaO2]) does not accurately reflect the degree of CO poisoning or cellular hypoxia. The PaO2 level reflects the oxygen dissolved in blood that is not altered by the hemoglobin-bound CO. Because dissolved oxygen makes up only a small fraction of arterial oxygen content, a PaO2 level within the reference range may lead to serious underestimation of the decrement in tissue oxygen delivery and the degree of hypoxia at the cellular level that occurs when CO blocks the delivery of oxygen to the tissues.

With most blood gas machines, the oxygen saturation is calculated on the basis of the PaO2 level. Thus, such a reading does not give an accurate determination of oxygen saturation, which must come from CO-oximetry.

ABG measurements are nonetheless useful to assess the adequacy of pulmonary gas exchange. Although the presence of a PaO2 level that is within the reference range may not exclude significant tissue hypoxia due to the effects of CO, the presence of a low PaO2 (< 60 mm Hg in room air) or hypercarbia (alveolar [arterial] carbon dioxide pressure [PaCO2] level of 55 mm Hg) indicate significant respiratory insufficiency. Metabolic acidosis suggests inadequate oxygen delivery to the tissues.

The difference between the partial pressure of oxygen in the alveolus and that measured on an ABG is the alveolar-arterial (A-a) gradient. This value, usually less than 5-10 mm Hg, may be several hundred mm Hg in the setting of significant pulmonary injury and can be used to assess improvement or deterioration in lung function when measured at a stable fraction of inspired oxygen (FiO2).

The alveolar gas equation can be used to estimate the efficiency of pulmonary oxygen delivery to the arterial circulation in the presence of supplemental oxygen administration. This formula determines the alveolar oxygen pressure.

The formula is as follows: PaO2 = (FiO2)(PB - PH2 O) - (PaCO2/RQ). PB represents barometric pressure, PH2 0 represents the partial pressure of water vapor (47 mm Hg at body temperature, ambient pressure), and RQ represents the respiratory quotient (estimated at 0.8).


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