Critical Care Medicine-Neurologic Disorders>>>>>Calculated Cardiovascular Parameters
Question 5#

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:

• Hb = 11.7 g/dL
• SaO2 = 97%
• SvO2 = 72%
• CO (thermodilution): 5.1 L/min

Assuming minimal contribution from dissolved oxygen, what is the patient’s calculated oxygen consumption?

A. 0.2 L/min
B. 0.76 L/min
C. 1.3 L/min
D. D. 20 L/min

Comment:

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/vO + 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).

References:

1. Bizouarn P, Blanloeil Y, Pinaud M. Comparison between oxygen consumption calculated by Fick’s principle using a continuous thermodilution technique and measured by indirect calorimetry. Br J Anaesth. 1995;75:719-723 .
2. Miller RD. Miller’s Anesthesia. 8th ed. Philadelphia, PA: Elsevier Churchill Livingstone; 2015.