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Monitoring patient oxygen status – a review article
Summarized from Toffaletti J, Rackley C. Chapter 3 – Monitoring oxygen status. Advances in Clinical Chemistry 2016; 77: 104-23
The first two, pH and pCO2 along with other derived (calculated) parameters (bicarbonate, base excess) generated during blood gas analysis allow monitoring of patient acid-base status, and the third, pO2 is used to monitor patient oxygen status.
As the authors of this review article make clear, pO2 is essential but not sufficient for full assessment of patient oxygen status.
Modern gas analyzers have an incorporated CO-oximeter that allows measurement of all four types of hemoglobin, and derived calculation of two further parameters of importance for assessment of oxygenation status: oxygen saturation (sO2) and percent oxyhemoglobin (%O2Hb).
The authors begin by defining these three oxygen-monitoring parameters (pO2, sO2 and %O2Hb) and explaining the difference between them. There follows an extended discussion of the structure and oxygen-carrying function of hemoglobin, with brief reference to the two dyshemoglobin species (carboxyhemoglobin and methemoglobin) present in blood that cannot bind oxygen.
A further section is devoted to discussion of the technical detail of the pO2-measuring electrode, and principles of CO-oximetry. This is followed by consideration of the potential preanalytical errors in pO2, sO2 and %O2Hb measurement that can occur if blood is not collected in the correct way, and samples are not transported and analyzed in a timely manner.
The results of blood gas/CO-oximetry (i.e. pO2, sO2, %O2Hb, along with pCO2) can be used to calculate a number of other parameters that are sometimes clinically useful in determining cause and severity of oxygen deficit.
These are: the Alveolar-Arterial pO2 gradient, the ratio of pO2 to fraction of inspired oxygen (FIO2), the oxygenation index (OI), the total blood oxygen content, and oxygen delivery. The authors briefly discuss the clinical significance and utility of these five supplementary parameters, and describe just how they are calculated.
The causes of hypoxemia (reduced pO2) are discussed under the usual five headings: reduced inspired oxygen (low FIO2), hypoventilation, mismatch between ventilation and perfusion, intrapulmonary shunting and diffusion impairment.
The article concludes with discussion of two case studies that explain how some of the oxygen parameters are applied in clinical practice; the first concerns a case of pulmonary embolism and the second, one of acute respiratory distress syndrome (ARDS).
In all, a quite comprehensive and readily understandable overview of monitoring oxygen status, a major utility of blood gas analysis.
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