OXYHEMOGLOBIN DISSOCIATION CURVE Chemeketa Community College

Oxygen-hemoglobin dissociation curve

Oxy/hemo Curve u The ability of oxygen to bind with and dissociate from hemoglobin u How shifts change affinity

Oxy/hemo Curve u 97% O2 on Heme of Hgb u 3% in plasma –3% is available in anemia –Harmful in toxicity

Oxy/hemo Curve u O2 is “loosely” attached to heme u Easily formed and dissolved

Oxy/hemo Curve u Heme can carry 4 O2 molecules u Each site is affected by the other 3 u As they bind, space is decreased

Oxy/hemo Curve u The more they bind, the harder it becomes to bind

Oxy/hemo Curve u Two transfer sites exist u Alveolar-capillary site u Capillary-tissue site

O2 Saturation Monitoring u ABGs, pulse oximetry u Venous sats

O2 Sat. Monitoring u Does not tell tissue oxygenation u Patient may have tissue hypoxia in spite of monitors

Oxy/hemo Curve u Normal curve uses O2 Sats and PaO2 to reflect amount of oxygen available to the tissues

Oxy/hemo Curve-Normals u 37 degrees, pH 7.40, PaCO2 40 mm/hg u Deviation causes a shift

Oxygen-hemoglobin dissociation curve

Oxy/hemo Curve u Upper-flat portion is lungs u Steep portion is tissues u Body can hold 96-97% down to 80 mm/hg

Oxy/hemo Curve u Results of tissue transfer- –Venous blood at 63% –At 27 mm/hg the Sat is 50%

Changes in Affinity u pH, PaCO2, carbon monoxide, abnormal Hgb., temp, intracellular compounds, 2,3-DPG

The Bohr effect u Oxygenated Hgb = stronger acid than deoxygenated Hgb u Change in pH facilitates release of oxygen

The Bohr effect u Acid becomes weaker u Blood picks up CO2 u Transports to lungs and process reverses

Relationship of hemoglobin sat. and pH

Temperature u Decrease causes increased affinity –Shift to left u Increase causes decreased affinity –Shift to right

Relationship of hemoglobin sat. and Temperature

2,3 DPG (diphosphoglycerate) u An enzyme that affects binding directly u Competes with oxygen

2,3 DPG (diphosphoglycerate) u More 2,3 DPG =decreased affinity u Less = increased affinity

Carbon Monoxide (CO) u CO has > 200 times greater affinity than oxygen u Always causes lower oxygen sats

Abnormal Hemoglobin u May have greater or lesser affinity

Left Shift u Increased affinity for O2 u At any PaO2, % is higher

Left Shift u Easier to “hook-on” u Harder to “un-hook”

Left Shift-clinical situations u Alkalosis, hypocapnia, hypothermia u Decreased DPG, CO poisoning u Blood transfusion, fetal Hgb

Clinical example u 56 yo woman with ICP elevated u Craniotomy for CVA bleed/ aneurysm u Hyperventilated to vasoconstrict

Her ABGs u pH = 7.53, Pa CO2 = 21 mm/hg u PO2 = 118 mm/hg, HCO3 = 17.8 mEq/L u O2 Sat = 99.1%, Temp =37.6

What does it mean? u Left shift makes it hard to “un-hook” u Tissue hypoxia must be watched for-even if readings indicate high sats

Right shift u Decreased affinity for O2 u At any PO2, sat % is decreased u Harder to “hook-on” u Easy to “un-hook”

Clinical situations u Acidosis, hypercapnia, hyperthermia u Elevated DPG u Hyperthyroidism, anemia, chronic hypoxia

Clinical example u 25 yo with ARDS u Secondary to staph pneumonia u 100% O2, PPV

ABGs u pH = 7.27, PaCO2 = 51.2 mm/hg u PO2 = 40 mm/hg, HCO3 = 23.6 mEq/L u O2 Sat = 76.2%, Temp =39.7

Clinical example u Right shift is protective if- additional O2 is given

Summary u The curve helps us appreciate factors that affect the oxygenation status of critical patients. u ydisso/oxydisso.html ydisso/oxydisso.html

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