[CK] [ ] Q oxyhemoglobin curve. MVO2 > 70% as our goal !(65% - 70%)

OxyHemoglobin Dissociation Curve 

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This curve describes the relationship between available oxygen and amount of oxygen carried by hemoglobin.

• The horizontal axis is Pa02, or the amount of oxygen available.
• The vertical axis is SaO2, or the amount of hemoglobin saturated with oxygen.

• Once the PaO2 reaches 60 mm Hg the curve is almost flat, indicating there is little change in saturation above this point.
  • So, PaO2 of 60 or more is usually considered adequate.
  • But, at less than 60 mm Hg the curve is very steep, and small changes in the PaO2 greatly reduce the SaO2.
• The term "affinity" is used to describe oxygen's attraction to hemoglobin binding sites.
  

  • Affinity changes with:
    • variation in pH,
    • temperature,
    • CO2 and,
    • 2,3,-DPG
      • a metabolic by-product which competes with O2 for binding sites.
  • Traditionally the curve starts with:
    • pH at 7.4,
    • temperature at 37 Centigrade, and
    • PaCO2 at 40.
  • Changes from these values are called "shifts".
    
    Click a button to shift the curve.
• A left shift will increase oxygen's affinity for hemoglobin.
  • In a left shift condition (alkalosis, hypothermia, etc.) oxygen will have a higher affinity for hemoglobin.
  • SaO2 will increase at a given PaO2, but more of it will stay on the hemoglobin and ride back through the lungs without being used. This can result in tissue hypoxia even when there is sufficient oxygen in the blood.
• A right shift decreases oxygen's affinity for hemoglobin.
  • In a right shift (acidosis, fever, etc.) oxygen has a lower affinity for hemoglobin. Blood will release oxygen more readily.
• This means more O2 will be released to the cells, but it also means less oxygen will be carried from the lungs in the first place.

ARTERIAL: 90%

MIXED VENOUS: SvO2(pulmonary artery <- including coronary sinus)

                             > 60%(SCVo2 > 70%) 

OXYGEN: ScvO₂ and SvO₂ 

Hemoglobin in arterial blood is highly saturated with oxygen (SaO2).  Normal levels of SaO2 delivered to tissues are 95 - 98%.

Central venous (ScvO2) and mixed venous oxygen saturation (SvO2) are measurements of the relationship between oxygen consumption and oxygen delivery in the body.   Normal values of mixed venous oxygen saturation (SvO2) are 60 - 80%.  Central venous oxygen saturation (ScvO2) values represent regional venous saturations with a normal value of ~70%.  ScvO2 usually measures slightly higher than SvO2 as it has not mixed with the venous blood from the coronary sinus.  Although the values may differ, they trend together.

OXYGEN: BALANCE Oxygen balance is necessary to sustain life. SvO2 is the "watchdog" of this balance and is often called the "fifth vital sign." When monitored, it serves as an early trouble indicator and can help clinicians adjust therapies. It is also a means of interpreting other clinical variables. In a healthy individual, normal SvO2 is between 60 percent and 80 percent. If SvO2 is low (below 60 percent) then the oxygen supply is insufficient or the oxygen demand has increased. If SvO2 is high (above 80 percent) then the oxygen demand has declined or the oxygen supply has increased.

OXYGEN: BALANCING SUPPLY AND DEMAND <=> CARDIOGENIC SHOCK + SEPSIS !!! BOTH CAN DECREASE IT .  When the oxygen supply and demand balance is threatened, the body will compensate in the following three ways: Increased cardiac output and/or heart rate: Initial response to decreasing supply or increasing demand. Increased oxygen extraction: Tissues take more oxygen from arterial blood. This results in lower return of oxygen to the venous system and therefore a lower SvO2 reading. Diverted blood flow: Blood flow is redirected to areas where the oxygen is needed the most.  Redirected blood flow cannot be measured clinically and therefore does not provide early warning signals.

GLOBAL TISSUE HYPOXIA An indicator of serious illness, global tissue hypoxia is a key development preceding multiorgan failure and death.  In cases of circulatory abnormalities, an imbalance between systemic oxygen delivery and oxygen demand, results in global tissue hypoxia.