OXYHEMOGLOBIN DISSOCIATION CURVE

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OXYHEMOGLOBIN DISSOCIATION CURVE
Chemeketa Community College
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Oxygen-hemoglobin dissociation curve
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Oxy/hemo Curve

• The ability of oxygen to bind with and dissociate
  from hemoglobin
• How shifts change affinity

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Oxy/hemo Curve

• 97 O2 on Heme of Hgb
• 3 in plasma
• 3 is available in anemia
• Harmful in toxicity

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Oxy/hemo Curve

• O2 is loosely attached to heme
• Easily formed and dissolved

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Oxy/hemo Curve

• Heme can carry 4 O2 molecules
• Each site is affected by the other 3
• As they bind, space is decreased

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Oxy/hemo Curve

• The more they bind, the harder it becomes to bind

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Oxy/hemo Curve

• Two transfer sites exist
• Alveolar-capillary site
• Capillary-tissue site

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O2 Saturation Monitoring

• ABGs, pulse oximetry
• Venous sats

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O2 Sat. Monitoring

• Does not tell tissue oxygenation
• Patient may have tissue hypoxia in spite of
  monitors

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Oxy/hemo Curve

• Normal curve uses O2 Sats and PaO2 to reflect
  amount of oxygen available to the tissues

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Oxy/hemo Curve-Normals

• 37 degrees, pH 7.40, PaCO2 40 mm/hg
• Deviation causes a shift

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Oxygen-hemoglobin dissociation curve
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Oxy/hemo Curve

• Upper-flat portion is lungs
• Steep portion is tissues
• Body can hold 96-97 down to 80 mm/hg

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Oxy/hemo Curve

• Results of tissue transfer-
• Venous blood at 63
• At 27 mm/hg the Sat is 50

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Changes in Affinity

• pH, PaCO2, carbon monoxide, abnormal Hgb., temp,
  intracellular compounds, 2,3-DPG

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The Bohr effect

• Oxygenated Hgb stronger acid than deoxygenated
  Hgb
• Change in pH facilitates release of oxygen

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The Bohr effect

• Acid becomes weaker
• Blood picks up CO2
• Transports to lungs and process reverses

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Relationship of hemoglobin sat. and pH
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Temperature

• Decrease causes increased affinity
• Shift to left
• Increase causes decreased affinity
• Shift to right

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Relationship of hemoglobin sat. and Temperature
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2,3 DPG (diphosphoglycerate)

• An enzyme that affects binding directly
• Competes with oxygen

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2,3 DPG (diphosphoglycerate)

• More 2,3 DPG decreased affinity
• Less increased affinity

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Carbon Monoxide (CO)

• CO has gt 200 times greater affinity than oxygen
• Always causes lower oxygen sats

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Abnormal Hemoglobin

• May have greater or lesser affinity

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Left Shift

• Increased affinity for O2
• At any PaO2, is higher

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Left Shift

• Easier to hook-on
• Harder to un-hook

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Left Shift-clinical situations

• Alkalosis, hypocapnia, hypothermia
• Decreased DPG, CO poisoning
• Blood transfusion, fetal Hgb

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Clinical example

• 56 yo woman with ICP elevated
• Craniotomy for CVA bleed/ aneurysm
• Hyperventilated to vasoconstrict

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Her ABGs

• pH 7.53, Pa CO2 21 mm/hg
• PO2 118 mm/hg, HCO3 17.8 mEq/L
• O2 Sat 99.1, Temp 37.6

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What does it mean?

• Left shift makes it hard to un-hook
• Tissue hypoxia must be watched for-even if
  readings indicate high sats

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Right shift

• Decreased affinity for O2
• At any PO2, sat is decreased
• Harder to hook-on
• Easy to un-hook

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Clinical situations

• Acidosis, hypercapnia, hyperthermia
• Elevated DPG
• Hyperthyroidism, anemia, chronic hypoxia

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Clinical example

• 25 yo with ARDS
• Secondary to staph pneumonia
• 100 O2, PPV

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ABGs

• pH 7.27, PaCO2 51.2 mm/hg
• PO2 40 mm/hg, HCO3 23.6 mEq/L
• O2 Sat 76.2, Temp 39.7

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Clinical example

• Right shift is protective if-additional O2 is
  given

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Summary

• The curve helps us appreciate factors that affect
  the oxygenation status of critical patients.
• http//www.ventworld.com/resources/oxydisso/oxydis
  so.html

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Summary

• Diseases or treatments shift the curve
• Understanding allows for more appropriate
  interventions

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