Mixed venous blood and O2 tension

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

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:

1. superior vena cava
2. inferior vena cava
3. coronary sinus

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 of coronary sinus blood

• High O2 extraction ratio (55-65%)
• Typically low: 20mmHg
• Increase in myocardial oxygen consumption can only be met by increasing coronary blood flow

PO2 of SVC and IVC

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.

PvO2 and SvO2 of mixed venous blood

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

 

PvCO2 and SvCO2 of mixed venous blood

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

• PaCO2 = 40mmHg
• CaCO2 = 48mLs/100mL

 

Fick's principle

"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"

Fick equation

=> VO2 = Q (CaO2 - CvO2)

• VO2 = O2 consumption per minute (mL O2/time)
• Q = pulmonary blood flow (mL/time)
• CaO2 = O2 concentration in blood leaving lung (mL/100mL)
• CvO2 = O2 concentration in mixed venous blood (mL/100mL)

 

Factors affecting mixed venous O2 tension

From Fick equation

VO2 = Q x (CaO2 - CvO2)

--> CvO2 = CaO2 - VO2/Q

--> SvO2 = SaO2 - VO2/(Q x 1.34 x [Hb])

NB:

• SvO2 is derived so O2 dissociation curve (which is SpO2 vs PO2) can be used

 

When O2 dissociation curve is fixed

SvO2 = SaO2 - VO2/(Qx1.34x[Hb])

SvO2 is increased when:

• SaO2 is increased
• O2 consumption (VO2) is decreased
• cardiac output (Q) is increased
• Hb concentration is increased

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 O2 dissociation curve shifts to the right

When ODC shifts to right, given the same SvO2
--> PvO2 will increase

PvO2 increase when ODC moves to the right due to:

• increased PvCO2
• increased [H+] (i.e. drop in pH)
• increased temperature
• increased red cell 2,3 DPG

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)

 

Factors affecting mixed CO2 tension

Fick's principle

Production of CO2 = Elimination of CO2

VCO2 = Q (CaCO2-CvCO2)

=> CvCO2 = CaCO2 - VCO2/Q

When CO2 dissociation curve is fixed

CvCO2 is increased when

• CaCO2 is increased
  --> Heavily influenced by alveolar ventilation
• CO2 output is decreased
• cardiac output is increased

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 shifts to the right

When CO2 dissociation curve moves, given the same SvCO2, PvCO2 will increase

--> PvCO2 increase when dissociation curve moves to right due to:

• ? increased PvO2
• ? decreased [Hb]

In summary

Mixed venous O2 tension is increased by:

• increased SaO2
• decreased O2 consumption
• increased cardiac output
• increased Hb concentration
• right shift in ODC, due to:
  * increased PvCO2
  * increased [H+]
  * increased temperature
  * increased red cell 2,3DPG

 

Additional notes

Examiner's comment

• The site from which mixed venous blood gas is obtained
• Fick equation
  => ????? SvO2 = SaO2 - (O2 consumption/cardiac output x Hb x 1.3)
• relationship between PO2 and O2 content in venous blood
• relationship between mixed venous O2 content, arterial O2 content, oxygen consumption, and cardiac output
• A right shift of Hb O2 curve increases mixed venous PO2.
• (common errors) confusing saturation with mixed venous oxygen tension
• ???? "a common error was to say that mixed venous oxygen tension is determined by fraction of inspired oxygen, or that alveolar PO2 is an important determinant of PvO2."
• HbO2 dissociation curve is a very small part of the answer.
• (extra) reciprocal relationship between O2 extraction and cardiac output. i.e. if cardiac output falls, oxygen extraction increases and PvO2 falls.
• (common error) confusing content and concentration, tension and saturation, mixed venous O2 or saturation and partial pressure

To be added later

Question about the correctness of the formula in examiner's comment.

? effects of carbon monoxide