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.

 

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. Arterial PaCO2 = 40mmHg

NB. Arterial 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,

=> CaO2 - CvO2 = VO2 / Q

=> CvO2 = CaO2 - VO2/Q

O2 Concentration = 1.39 x [Hb] (g/100mL) x Sat% + 0.003 x PO2 (mmHg)

Thus, 1.39 x Hbg x SvO2 = 1.39 x Hb x SaO2 - VO2/Q [ignoring effectos of dissolved O2]

=> SvO2 = SaO2 - VO2/(Qx1.39x[Hb])

NB: Different from examiner's comment.

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/(Qx1.39x[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 moves, 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

Similar to 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

Oxygen transport

O2 is carried in blood in two forms:

1. Dissolved
2. Bound with haemoglobin

1. O2 dissolved

Dissolved O2 follows Henry's Law

=> amount dissolved is proportional to partial pressure

For each mmHg of PAO2, 0.003 mL of O2 is dissolved in 100 mL of blood

2. O2 bound to Hb

2.1. O2 capacity

O2 capacity = maximum amount of O2 that can be bound to Hb

=> 1g of Hb can combine with 1.39mL of O2

(per West. Alternative values are 1.34 or 1.36mL)

(per Nunn 1.31mL of O2 per gram of Hb)

=> =1.39 x [Hb] (g/100mL)

Normal blood has 15grams of Hb per 100mL

=> O2 capacity = 20.85mL O2 per 100mL blood

2.2. O2 saturation (S)

O2 saturation (%) = O2 combined with Hb / O2 capacity

=> = (O2 content - dissolved O2)/O2 capacity

 

NB. West defined O2 capacity as maximum O2 bound to Hb, thus doesn't include dissolved O2.

NB. Nunn includes dissolved O2 in its definition of O2 capacity.

Functional vs Fractional

Functional saturation = [HbO2]/([HbO2]+[DeoxyHb])

Fractional saturation = [HbO2]/[Total Hb]

Total Hb include HbO2, DeoxyHb, MetHb, COHb

Pulse oximeter measures HbO2 and deoxyHb, but may be calibrated to allow for MetHb and COHb

2.3. O2 concentration  (C)

O2 concentration (mL/100mL) 
= 1.39 x [Hb](g/100mL)x Sat(%) + 0.003x PO2(mmHg)

Remember this is NOT content.

Unit for O2 concentration is mL of O2 per 100mL of blood.

Once the volume of blood is known, then the content of O2 can be calculated.

O2 dissociation curve

Sigmoid shaped.

Important points (PO2 to SO2)

[Assuming temp = 37C, pH = 7.40, Base excess = 0]

• 0mmHg = 0%
• 10mmHg = 10%
• 60mmHg = 91%
• 150mmHg = 98.8%

[See diagram 20050305(1) - Oxygen dissociation curve]

P50

• The PO2 at which oxygen-carrying proteins is 50% saturated.
• It is a value on X-axis, not a point on the curve.
• It is used as an index of oxygen affinity
• Higher P50 => right-shifted curve => LOWER O2 affinity

In adult, P50 = 27mmHg

With foetal haemoglobin, P50 is lower at about 18mmHg

=> due to reduced binding of 2,3 DPG

Significance of the shape of ODC

Two portions:

1. Flat upper portion
2. Steep lower portion

Flat upper portion

Fall in alveolar PO2

=> little change in O2 sats.

Steep lower portion

As peripheral tissues extract O2

=> drop in PO2 is not too great

=> maintains O2 diffusion gradient

 

NB. positive cooperativity - when one of the 4 globin chains in Hb binds with O2, it causes structural changes which increase O2 affinity of the haem of the remaining chains => cause the sigmoid shape

Effects of carbon monoxide

• 240 times the affinity to O2
• Ties up Hb and make them unavailable for O2 uptake
• Presence of COHb also interfere with unloading of O2
• COHb = carboxyhaemoglobin

Two effects

1. ODC is shifted to left
   (binding of CO causes conformational change in Hb, and increase O2 affinity by other subunits)
2. O2 content is reduced

Carboxyhaemoglobin dissociation curve is extremely left-shifted and is a rectangular hyperbola.

Myoglobin

• Contains single chain.
• Very low P50 of 2.75mmHg.
• ODC for Myoglobin is a rectangular hyperbola

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