Predictive validity of a novel non-invasive estimation of effective shunt fraction in critically ill patients
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(2019) 7:49
Intensive Care Medicine Experimental
RESEARCH
Open Access
Predictive validity of a novel non-invasive estimation of effective shunt fraction in critically ill patients Emma M. Chang1† , Andrew Bretherick1,2† , Gordon B. Drummond1 and J Kenneth Baillie1,3* *Correspondence: [email protected] † Emma M. Chang and Andrew Bretherick contributed equally to this work. 1 Anaesthesia, Critical Care and Pain Medicine, Royal Infirmary of Edinburgh, Edinburgh EH16 4SA, UK 3 The Roslin Institute and Royal (Dick) School of Veterinary Studies, University of Edinburgh, Easter Bush, Edinburgh EH25 9RG, UK Full list of author information is available at the end of the article
Abstract Background: Accurate measurement of pulmonary oxygenation is important for classification of disease severity and quantification of outcomes in clinical studies. Currently, tension-based methods such as P/F ratio are in widespread use, but are known to be less accurate than content-based methods. However, content-based methods require invasive measurements or sophisticated equipment that are rarely used in clinical practice. We devised two new methods to infer shunt fraction from a single arterial blood gas sample: (1) a non-invasive effective shunt (ES) fraction calculated using a rearrangement of the indirect Fick equation, standard constants, and a procedural inversion of the relationship between content and tension and (2) inferred values from a database of outputs from an integrated mathematical model of gas exchange (DB). We compared the predictive validity—the accuracy of predictions of Pa O2 following changes in FI O2 —of each measure in a retrospective database of 78,159 arterial blood gas (ABG) results from critically ill patients. Results: In a formal test set comprising 9,635 pairs of ABGs, the median absolute error (MAE) values for the four measures were as follows: alveolar-arterial difference, 7.30 kPa; Pa O2 /FI O2 ratio, 2.41 kPa; DB, 2.13 kPa; and ES, 1.88 kPa. ES performed significantly better than other measures (p < 10-10 in all comparisons). Further exploration of the DB method demonstrated that obtaining two blood gas measurements at different FI O2 provides a more precise description of pulmonary oxygenation. Conclusions: Effective shunt can be calculated using a computationally efficient procedure using routinely collected arterial blood gas data and has better predictive validity than other analytic methods. For practical assessment of oxygenation in clinical research, ES should be used in preference to other indices. ES can be calculated at http://baillielab.net/es. Keywords: Oxygen, Shunt fraction, Arterial blood gas, Respiratory failure
Background Hypoxia is the defining feature of respiratory failure. Accurate quantification of pulmonary oxygenation defect is essential to determine inclusion in clinical trials, to measure outcomes in research studies, and to observe changes in lung function in a clinical setting. In severely hypoxic patients, direct measurement of intrapulmonary shunt provides the most
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