Quantifying pulmonary oxygen transfer deficits in critically ill patients

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Abstract
The clinical picture describing oxygen transfer deficits in literature is complicated by inconsistent terminology, and a weak perception of the influence total errors of measured and estimated values have on clinical decision-making. Clinical and analytical terminology: Terms like hypoxia, hypoxaemia and tissue hypoxia in clinical literature are often used synonymously. In present terminology, arterial hypoxia (pO2(a)) is considered to be based on measurements of oxygen tension in arterial blood. On the other hand, arterial hypoxaemia (ctO2(a)) is considered to be based on measurements of both pO2, total haemoglobin (ctHb), saturation (sO2), carboxyhaemoglobin (FCOHb), and methaemoglobin (FMetHb). Arterial hypoxia is simply a low oxygen tension in arterial blood. Arterial hypoxaemia is thus simply a low oxygen concentration in arterial blood. Pulmonary indices: The tension-based indices. At the bedside, assessment of the oxygen uptake in the lungs has been evaluated by calculating indices like pO2(a)/FO2(I),pO2(A-a),pO2(a/A) and the respiratory index (RI = pO2(A-a)/pO2(a)). The different oxygen tension-based indices all require the calculation of the alveolar oxygen tension from the alveolar equation. These calculations involve many assumptions (exact analytical measurements of the respiratory quotient (RQ), FO2(I), etc.) to be fulfilled, and might include clinically unacceptable errors. The concentration-based index (FShunt). Considering a fixed arterio-mixed venous oxygen difference (3-5 mL/dL), this index is by some researchers indicated to be superior to the oxygen tension-based (the correlation coefficient to the true measured shunt being 0.94 for the FShunt compared to 0.72 for the best tension-based (RI = pO2(A-a)/pO2(a))). However, the scatter around the line is considerable and this index seems to fail, as well as the tension-based in the many cases where the assumed difference is not equal to the assumed (3-5 mL/dL). The intrapulmonary shunt: The best available means of outlining the extent to which the pulmonary system contributes to hypoxic hypoxaemia, is to calculate the intrapulmonary shunt. It reflects the degree to which the lung deviates from ideal as an oxygenator of pulmonary blood. Exact calculation of the intrapulmonary shunt requires measurements of oxygen concentration in both arterial and mixed-venous blood samples. Calculation of the intrapulmonary shunt at 100% inspired oxygen represents the term (Qs/Qt). Venous admixture or the physiologic shunt (Qsp/Qt) represents measurements of the intrapulmonary shunt at less than 100% inspired oxygen. Interpretative guidelines for (Qsp/Qt) in critically ill patients having a pulmonary catheter are: A calculated shunt less than 10% is clinically compatible with normal. A shunt of 10-19% seldom would require significant support. A calculated shunt of 20-29% may be life threatening in a patient with limited cardiovascular function. A calculated shunt greater than 30% usually requires significant cardiopulmonary support. The necessity of sampling mixed-venous blood seems to be the most limiting factor for a widespread clinical use of shunt calculations.