- 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%
|SpO2 (%)||PO2 (mmHg)|
- 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
- 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
- increased temperature shifts curve to right.
- 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
- 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
- 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
- Morgan TJ. The oxyhaemoglobin dissociation curve in critical illness. Crit Care Resusc. 1999 Mar;1(1):93-100. PubMed PMID: 16599868. [Free Full Text]
Chris is an Intensivist and ECMO specialist at the Alfred ICU in Melbourne. He is also a Clinical Adjunct Associate Professor at Monash University. He is a co-founder of the Australia and New Zealand Clinician Educator Network (ANZCEN) and is the Lead for the ANZCEN Clinician Educator Incubator programme. He is on the Board of Directors for the Intensive Care Foundation and is a First Part Examiner for the College of Intensive Care Medicine. He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives.
After finishing his medical degree at the University of Auckland, he continued post-graduate training in New Zealand as well as Australia’s Northern Territory, Perth and Melbourne. He has completed fellowship training in both intensive care medicine and emergency medicine, as well as post-graduate training in biochemistry, clinical toxicology, clinical epidemiology, and health professional education.
He is actively involved in in using translational simulation to improve patient care and the design of processes and systems at Alfred Health. He coordinates the Alfred ICU’s education and simulation programmes and runs the unit’s education website, INTENSIVE. He created the ‘Critically Ill Airway’ course and teaches on numerous courses around the world. He is one of the founders of the FOAM movement (Free Open-Access Medical education) and is co-creator of litfl.com, the RAGE podcast, the Resuscitology course, and the SMACC conference.
His one great achievement is being the father of three amazing children.
On Twitter, he is @precordialthump.