A 5 year-old male with acute respiratory failure due to bacterial pneumonia has a PaO2 of 50 mm Hg on pressure control ventilation. After changing his ventilatory settings including increasing the FiO2 from 0.4 to 0.8, his PaO2 has increased to 100 mm Hg on his next arterial blood gas. His hemoglobin has remained stable at 10.8 g/dL. Because of this increase in his PaO2, which of the following is a true statement?
A. Oxygen content has increased by 75%
B. Oxygen content increased more than 40%
C. Oxygen content increased more than 10%
D. There was only a small (<1%) increase in oxygen content
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Answer
The answer is C. To solve this problem you need to remember that CaO2 (arterial oxygen content) = (1.34 x Hb x SaO2) + 0.003 x PaO2, so the actual contribution of PaO2 (i.e. dissolved oxygen) to the total arterial oxygen content is actually quite small. However, PaO2 also affects Hgb oxygen saturation.
While you aren't directly provided the oxygen saturation, you can determine the oxygen saturation assuming the patient has a normal oxygen-hemoglobin dissociation curve. The mnemonic of "90-60,80-50"is a good tool for easy calculations. This means that at a PaO2 of 60 mm Hg, oxygen saturation is around 90%, while at a PaO2 of 50 mm Hg, oxygen saturation is around 80%. So, increasing PaO2 in this patient from 50 to 100 mm Hg will increase the saturation from 80% to about 95-100%. While increasing the dissolved oxygen won't make that much of a difference, the increase in saturation of hemoglobin will make a significant change. The increase in this patient will be around 10-20%.
Notes
S. A. McLellan,T. S. Walsh. Oxygen delivery and haemoglobin. Contin Educ Anaesth Crit Care Pain (2004) 4 (4): 123-126.
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