Oxygen is the drug most commonly administered to patients having blood gas measurements. The degree of hypoxemia can be estimated for the patient receiving oxygen therapy because increasing the inspired oxygen concentration by 10 per cent (FiO 2 = 0.1) increases the inspired oxygen tension by approximately 75 mmHg (10 kPa), i.e. 10 per cent of 760 mmHg (101 kPa). In the normal lung, this represents an increase in the ideal alveolar oxygen tension of approximately 50 mmHg (6.7 kPa). Since PaO2 increases by approximately 50 mmHg for every 10 per cent increment of inspired oxygen concentration, multiplying the percentage inspired oxygen by 5 (or multiplying FiO2 by 500) will result in the minimal predicted PaO2 (in mmHg) for that level of oxygen therapy. If the measured PaO2 value is less than the predicted value, it may be assumed that the patient will be hypoxemic when breathing room air (SMpIro,§La( 1994).
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