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Question 8#

All of the following are true regarding the fractional saturation of hemoglobin in mixed venous blood (SVO₂) EXCEPT:

A. It will decrease with worsening heart failure
B. It will decrease with increased sedation
C. It will decrease with worsening anemia
D. It will decrease with fever

Correct Answer is B

Comment:

The Fick equation for cardiac output can be rearranged as follows: 

CVO₂ = Cao₂ - VO₂/Q_T

If the small contribution of dissolved oxygen to CVO₂ and Cao₂ is ignored, the rearranged equation can be rewritten as: 

SVO₂ = Sao₂ - VO₂/(Q_T x Hgb x 1.36)

where SVO₂ is the fractional saturation of hemoglobin in mixed venous blood, Sao₂ is the fractional saturation of hemoglobin in arterial blood, and Hgb is the concentration of hemoglobin in blood. Thus it can be seen that SVO₂ is a function of V02(ie, metabolic rate), Q_T,Sao₂, and Hgb. Accordingly, subnormal values of SVO₂ can be caused by a decrease in Q_T (eg, due to heart failure or hypovolemia), a decrease in Sao₂ (eg, due to intrinsic pulmonary disease), a decrease in Hgb (ie, anemia), or an increase in metabolic rate (eg, due to seizures or fever).