As part of a clinical study, you are measuring oxygen consumption using indirect calorimetry in a ventilated patient in the ICU and want to compare your measurements to a calculated value. Radial artery and pulmonary artery catheters are placed, and the following measurements are obtained:
Assuming minimal contribution from dissolved oxygen, what is the patient’s calculated oxygen consumption?
A. 0.2 L/minCorrect Answer: A
The Fick equation relates CO, oxygen consumption, and the arteriovenous oxygen content difference as follows:
CO = VO2 / (CaO2 - CvO2)
Rearrange to solve for oxygen consumption yields:
VO2 = CO x (CaO2 - CvO2)
Oxygen content of arterial or venous blood is given by the equation:
Ca/vO2 = 1.34 x Hb x Sa/vO2 + 0.003 x Pa/vO2
Excluding the dissolved oxygen term, CaO2 and CvO2 can be calculated from the information given:
CaO2 = 1.34 x 11.7 x 0.97 = 15.2 ml O2 / 100 mL blood
CvO2 = 1.34 x 11.7 x 0.72 = 11.3 ml O2 / 100 mL blood
Therefore:
VO2 = 5100 mL blood / min x (15.2 - 11.3 ml O2 / 100 mL blood) = 199 mL O2 / min , or approximately 0.2 L/min (A).
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