Oxygen-Haemoglobin Dissociation Curve

OVERVIEW

• sigmoid shape of the oxy-Hb dissociation curve results from the allosteric interactions of the globin monomers that make up the haemoglobin tetramer as each one binds O2
• multiple factors can affect the affinity of Hb for oxygen, thus causing the curve to shift to the left (increased oxygen affinity) or to the right (decreased O2 affinity)

OXY-HB DISSOCIATION CURVE

• pulmonary veins
  PO2 95 -> SO2 97%
• pulmonary arteries
  PO2 40 -> SO2 75%

Table comparing SpO2 to PaO2

SpO2 (%)	PaO2 (mmHg)
97	95
92	60
89	50
75	40
50	27

• the amount of O2 bound to Hb is determined by the PO2 in a relationship termed the oxy-Hb dissociation curve.
• though atmospheric O2 concentration changes markedly, the buffering of Hb maintains constant tissue PO2.

Flat, upper portion of curve

-> if PO2 in alveolar falls, loading of O2 will be unaffected
-> a large partial pressure difference between blood & alveolar gas once most of gas has been transferred (diffusion hastened).

Middle, steep lower portion of curve

-> peripheral tissues can withdraw large amounts of O2 for a small drop in PO2 (assists O2 diffusion into tissue).

Utilization co-efficient = % of blood that gives up its O2 as it passes through tissue

• normal is 25%
• during exercise 75 to 85% (even higher)

FACTORS THAT SHIFT THE DISSOCIATION CURVE

Carbon monoxide

• interferes with O2 transport
• 240 x the affinity for Hb than O2 -> small amounts of CO can tie up large portions of Hb -> decreases O2 concentration
• oxy-Hb dissociation curve shifts to left -> favours uploading of O2
• can test for on co-oximetry

Temperature

• increased temperature shifts curve to right.

Carbon dioxide

• the Bohr effect
• high CO2 & H+ ion concentration
  -> as O2 is given up in tissues
  -> CO2 begins to bind & form carbonic acid
  -> shifts curve to right
  -> enhancing O2 off loading.
• blood passing through lungs gives up CO2 & H+ ions in the form of carbonic acid
  -> shifts O2 dissociation curve to left
  -> quantity of O2 binding increases at any given PO2
  -> increased O2 transport to tissues.

Hydrogen ion concentration

• see above

2,3 Diphosphoglycerate

• produced in response to hypoxia (after a few hours) or anaemia.
  -> right shift in curve
• formed in RBCs from 3-phosphoglyceraldehyde (a product of glycolysis)
• binds to the beta chains of deoxy Hb
• more O2 released into tissues, but also more difficult for O2 to bind with Hb in lungs.
• O2 released to tissues at as much as 10mmHg higher tissue O2 pressure than would be without increase in 2,3 DPG.
• in stored blood: 99% @ 7 days, 50% @ 14 days, 5% @ 28 days.
• can test for in blood

CLINICAL APPLICATION

• at present there is not enough data to support manipulation of O2-Hb curve to improve O2 delivery
• if arterial PO2 is critically low then O2 binding in the lungs may be impaired by a shift to the right -> this may seriously impair tissue oxygenation

References and Links

Journal articles

• Morgan TJ. The oxyhaemoglobin dissociation curve in critical illness. Crit Care Resusc. 1999 Mar;1(1):93-100. PubMed PMID: 16599868. [Free Full Text]