A. List the normal values for mixed venous blood gases and briefly explain the factors determining mixed venous oxygen tension.(1996)
B. Briefly describe the factors that influence the partial pressure of oxygen in mixed venous blood (00A2)
C. What factors influence the mixed venous CO2 tension. Briefly explain how these factors exert their influence. (1993)
Mixed venous blood - mixture of all the systemic venous blood draining from all the tissue capillary beds of the body, excluding shunted blood (i.e. central or peripheral shunt). Pulmonary venous blood is not included.
Has 3 major component:
Mixed venous blood is best taken from pulmonary artery as adequate mixing has occurred. Sometimes right ventricle can be used. In right atrium, the bloods are not adequately mixed.
PO2 from IVC is normally higher (SO2 77%) than from SVC (SO2 71%)
=> because kidney takes 25% of cardiac output but use only 7-8% of body's O2 consumption
=> IVC receives blood more oxygen rich.
With severe haemorrhage,
=> PO2 from SVC may be higher because of renal vasoconstriction.
Typical value: 40mmHg or 75% oxygen saturation
NB: mixed venous blood point does not lie on the standard oxygen dissociation curve because at mixed venous blood level, the curve is right-shifted because of increased PvCO2 and decreased pH
CvO2 = 15mL/100mL
Typical value: 46mmHg or 52mLs of CO2 per 100mL blood
=> (assuming SvO2 of 75%)
If SvO2 is 97%,
=> at PvCO2 of 46mmHg, CvCO2 = 50mLs/100mL
=> due to Haldane effect
NB
"Amount of O2 extracted from respired gases equals the amount added to the blood that flows through the lung"
i.e. "O2 consumption per unit time = O2 taken up by pulmonary blood flow per unit time"
=> VO2 = Q (CaO2 - CvO2)
From Fick equation
VO2 = Q x (CaO2 - CvO2)
--> CvO2 = CaO2 - VO2/Q
--> SvO2 = SaO2 - VO2/(Q x 1.34 x [Hb])
NB:
SvO2 = SaO2 - VO2/(Qx1.34x[Hb])
SvO2 is increased when:
As SvO2 increase, PO2 is increased.
However, at mixed venous blood level of PO2 (40mmHg), changes in SvO2 doesn't have as great an effect on PvO2 as it would at higher level of PO2.
When ODC shifts to right, given the same SvO2
--> PvO2 will increase
PvO2 increase when ODC moves to the right due to:
Bohr effect - CO2 loading assisting in O2 unloading from Hb
(for comparison, Haldane effect is when O2 unloading from Hb helps with CO2 loading)
Majority of Bohr effect is due to pH change caused by changes in PO2)
Fick's principle
Production of CO2 = Elimination of CO2
VCO2 = Q (CaCO2-CvCO2)
=> CvCO2 = CaCO2 - VCO2/Q
CvCO2 is increased when
NB. CO2 output and production are the same in steady state, but different in dynamic state because much of CO2 produced is diverted into body stores.
When CO2 dissociation curve moves, given the same SvCO2, PvCO2 will increase
--> PvCO2 increase when dissociation curve moves to right due to:
Mixed venous O2 tension is increased by:
Question about the correctness of the formula in examiner's comment.
? effects of carbon monoxide
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