OXYHEMOGLOBIN DISSOCIATION

CURVE

Chemeketa Community College

Oxygen-hemoglobin dissociation curve

Oxy/hemo Curve

The ability of oxygen to bind with and dissociate from hemoglobin

How shifts change affinity

Oxy/hemo Curve

97% O2 on Heme of Hgb 3% in plasma

–3% is available in anemia

–Harmful in toxicity

Oxy/hemo Curve

O2 is “loosely” attached to heme

Easily formed and dissolved

Oxy/hemo Curve

Heme can carry 4 O2 molecules

Each site is affected by the other 3

As they bind, space is decreased

Oxy/hemo Curve

The more they bind, the harder it becomes to bind

Oxy/hemo Curve

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

O2 Saturation Monitoring

ABGs, pulse oximetry

Venous sats

O2 Sat. Monitoring

Does not tell tissue oxygenation

Patient may have tissue hypoxia in spite of monitors

Oxy/hemo Curve

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

Oxy/hemo Curve-Normals

37 degrees, pH 7.40, PaCO2 40 mm/hg

Deviation causes a shift

Oxygen-hemoglobin dissociation curve

Oxy/hemo Curve

Upper-flat portion is lungs Steep portion is tissues Body can hold 96-97% down

to 80 mm/hg

Oxy/hemo Curve

Results of tissue transfer-

–Venous blood at 63%

–At 27 mm/hg the Sat is 50%

Changes in Affinity

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

The Bohr effect

Oxygenated Hgb = stronger acid than deoxygenated Hgb

Change in pH facilitates release of oxygen

The Bohr effect

Acid becomes weaker

Blood picks up CO2

Transports to lungs and

process reverses

Relationship of hemoglobin sat. and pH

Temperature

Decrease causes increased affinity–Shift to left

Increase causes decreased affinity–Shift to right

Relationship of hemoglobin sat. and Temperature

2,3 DPG (diphosphoglycerate)

An enzyme that affects binding directly

Competes with oxygen

2,3 DPG (diphosphoglycerate)

More 2,3 DPG =decreased affinity

Less = increased affinity

Carbon Monoxide (CO)

CO has > 200 times greater affinity than oxygen

Always causes lower oxygen sats

Abnormal Hemoglobin

May have greater or lesser affinity

Left Shift

Increased affinity for O2 At any PaO2, % is higher

Left Shift

Easier to “hook-on” Harder to “un-hook”

Left Shift-clinical situations Alkalosis, hypocapnia,

hypothermia Decreased DPG, CO

poisoning Blood transfusion,

fetal Hgb

Clinical example

56 yo woman with ICP elevated

Craniotomy for CVA bleed/ aneurysm

Hyperventilated to vasoconstrict

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

What does it mean? Left shift makes it

hard to “un-hook” Tissue hypoxia must

be watched for-even if readings indicate high sats

Right shift

Decreased affinity for O2 At any PO2, sat % is

decreased Harder to “hook-on” Easy to “un-hook”

Clinical situations

Acidosis, hypercapnia, hyperthermia

Elevated DPG Hyperthyroidism, anemia,

chronic hypoxia

Clinical example

25 yo with ARDS Secondary to staph

pneumonia 100% O2, PPV

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

Clinical example

Right shift is protective if-additional O2 is given

Summary

The curve helps us appreciate factors that affect the oxygenation status of critical patients.

http://www.ventworld.com/resources/oxydisso/oxydisso.html

Summary

Diseases or treatments shift the curve

Understanding allows for more appropriate interventions