NBME 21 Mixed venous oxygen tension (NBME Answers)

Retired NBME 21 Answers

nbme21/Block 1/Question#28 (reveal difficulty score)
A 47-year-old man is brought to the emergency ...
Mixed venous oxygen tension 🔍 / 📺 / 🌳 / 📖
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submitted by ∗nicnac20(21)

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Mixed venous oxygen tension is the amount of oxygen bound to hemoglobin as it returns to the right side of the heart after traveling to the tissues.

Decreased oxygen delivery to the tissues due to decreased cardiac output leads to the tissues extracting more oxygen from the passing RBCs than they normally would, which decreases the overall mixed venous oxygen tension.

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pparalpha A helpful equation is CO=rate of O2 consumption/(arterial O2 content-venous O2 content): Fick principle If CO is decreased, then the difference between arterial O2 content and venous O2 content is increased +19

misterdoctor69 To clarify a little bit, the tissues wouldn't necessarily extract MORE oxygen from the passing RBCs than they normally would. They would extract the same amount as they normally would to carry out their own functions. However, because the starting amount of O2 delivered to tissues is lower, you'd end up with less O2 bound to hemoglobin returning to the right heart. +5

doso2222 Heart failure --> decreased cardiac output --> slower blood circulation time (blood is more stagnant) --> translate to increased oxygen extraction by tissues and therefore lowers oxygen tension by the time it returns to the right heart. +

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submitted by ∗kentuckyfan(47)

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I get why the mixed venous oxygen tension decreased. However,, isn't the systemic vascular resistance also decreased?

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yb_26 no, decreased CO => peripheral vasoconstriction => SVR will be increased +8

yssya1992 No SVR will increase due to RAAS and SAN thats why we decrease afterload in HF treatment ( ACEI, ARBs ) +6

snafull Wouldn't pulmonary vascular resistance also be decreased here due to pulmonary vasodilation in the setting of an MI? +

cienfuegos @snafull: my initial thought is that we would see pulmonary vasoconstriction because of the relatively low oxygen tension (that results from the low cardiac output). +3

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submitted by paloma(10)

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What about pulmonary vascular resistance? It follows the systemic vascular resistance?

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cooldudeboy1 crackles are heard bilaterally so there is pulm patho which leads to increased pulm vascular resistance, since systemic blood flows into the lungs. any block in the flow ahead (lungs) will increase resistance in flow behind ( systemic ) +4

mrglass This patient is hypoxic increased diffusion distance. This causes pulmonary vasoconstriction. Ordinarily this response is designed to shunt blood to parts of the lungs that are well ventilated, but the response is maladaptive in global hypoxia +4

happyhib_ I dont believe decreased venous oxygen tension would lead to pulmonary vasoconstriction (this is typically in the setting of low PAO2 you see this; shunting blood away from poorly oxygenated alveoli). You can get to increased pulmonary vascular resistance due to pulm edema from Left heart failing leading to fluid overload in pulm vasculature). +

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submitted by ∗hayayah(1212)

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Mixed venous oxygen saturation (SvO2) is measured in the pulmonary artery. SvO2 samples the true mixed venous blood leaving the right heart. Measurement of mixed venous oxygen saturation (SvO2) from the pulmonary artery has been advocated as an indirect index of tissue oxygenation.

In cardiogenic shock you have decreased CO --> decreased O2 delivery --> decreased SvO2.

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