Oxygen Dissociation Curve PDF

OXYGEN TRANSPORT Tamethia Perkins MS, RRT-NPS, RRT-ACCS RT 3005/6005 OXYGEN TRANSPORT ◼ How O2 is transported to the tissues ◼ O2 dissociation curve and its clinical significance ◼ O2 transport studies to identify patient’s cardiac and ventilatory status ◼ Major form...

OXYGEN TRANSPORT Tamethia Perkins MS, RRT-NPS, RRT-ACCS RT 3005/6005 OXYGEN TRANSPORT ◼ How O2 is transported to the tissues ◼ O2 dissociation curve and its clinical significance ◼ O2 transport studies to identify patient’s cardiac and ventilatory status ◼ Major forms of tissue hypoxia OXYGEN TRANSPORT ◼ Transported in two forms: ◼ Dissolved in the plasma ◼ Bound to Hgb DISSOLVED OXYGEN ◼ Once O2 enters the plasma, it moves freely throughout the plasma in its normal gaseous state. ◼ This is the portion of O2 measured clinically (PO2). ◼ At normal T, about 0.003mL of O2 will dissolve in 100 mL of blood for every 1 mmHg of PO2. ◼ 0.003 = solubility coefficient. ◼ Normal PO2 = 100 mmHg = 0.3 mL is dissolved in every 100 mL of blood. O2 BOUND TO HEMOGLOBIN HEMOGLOBIN ◼ Each RBC has ~280 million Hb molecules. ◼ Normal HbA consists of: ◼ 4 heme groups. ◼ Iron-containing non-protein portion ◼ 4 amino acid chains (polypeptide) that constitute a globin (protein). HEMOGLOBIN ◼ Iron can combine with one O2 molecule in a reversible reaction to form oxyHb. ◼ Hb (reduced Hb or deoxyHb) + O 2 = HbO2 ◼ When 4 molecules are bound to one Hb molecule: Hb is 100% saturated. ◼ Globin portion: 2 alpha and beta chains. ◼ Changes in the aa sequence alters normal structure of the Hb. ◼ Sickle cell Hb (HbS) = different B chain = changes morphology of the RBC.  HEMOGLOBIN ◼ Normal Hb: 14-16 g/dL to 12-15 g/dL. ◼ Normal infant Hb: 14-20 g/dL. OXYGEN BOUND TO Hb ◼ Each gram of Hb is capable of carrying 1.34 mL of O 2. ◼ If Hb is 15 g% and 100% saturated = 20 vol%. (HbO 2 = 1.34 mLO2 x 15g%) =20 vol %. ◼ Normal Hb sat is 97% ◼ Thebesian veins, bronchial veins, underventilated alveoli. TOTAL O2 CONTENT ◼ HbO2 + dissolved O2 ◼ CaO2 = (Hb x 1.34 x SaO2) + (PaO2 x 0.003) = normal 20 vol% O2 transport (DO2) ◼ Good indication of cardiopulmonary Function ◼ CO x (CaO2 x 10) ◼ Normal 900 – 1150 ml O2/min O2 DISSOCIATION CURVE Oxygen Dissociation Curve ◼ Illustrates percentage of Hb that is chemically bound to O2 at each PO2. ◼ S-shaped with a steep slope between 10 and 60 mmHg, and a flat portion between 70 and 100 mmHg. ◼ Steep: rapid combination of Hb and O2. ◼ Flat: further increase in PO2 induces a slight change in Hb saturation. Clinical Importance of the Flat Portion of the Curve ◼ PO2 can fall from 100 mmHg to 60 mmHg and Hb saturation will be ~90%. ◼ This plateau becomes a safety zone for uploading of O2 in the lungs. ◼ As Hb moves through the alveolo-capillary system to pick up O2, a significant PO2 continues to exist between alveolar gas and blood even after most of the O2 is transferred= enhances diffusion. Clinical Importance of the Flat Portion of the Curve ◼ Flat portion also means that increasing PO2 beyond 100 mmHg adds very little additional O2 to the blood. Clinical Importance of the Steep Portion of the Curve ◼ PO2 reductions below 60 mmHg cause a rapid decrease in the amount of O2 bound to Hb = decrease O2 delivery to the tissues. P50 ◼ Partial P of O2 at which the Hb is 50% saturated. ◼ NORMAL: 27 mmHg ◼ When P50 increases: ◼ Shift to the R: low affinity of Hb for O 2. ◼ When P50 decreases: ◼ Shift to the L: high affinity of Hb for O 2. FACTORS THAT SHIFT THE CURVE pH ◼ High [H+]= low pH = shift to R ◼ Improves unloading O 2 to the tissues. ◼ Low [H+] = high pH = shift to L ◼ Improves uploading of O2 in the lungs Temperature ◼ High T = shift to R ◼ Low T = shift to L ◼ Cyanosis while swimming in very cold water. ◼ PaO2 is normal but Hb is not releasing the O2. FACTORS THAT SHIFT THE CURVE CO2 ◼ High CO2 = low pH = shift to R ◼ Low CO2 = high pH = shift to L ◼ Improves uploading of O2 in the lungs. 2,3-Diphosphoglycerate (2,3-DPG) ◼ RBCs contain large amounts of 2,3-DPG. ◼ Formed during anerobic glycolisis. ◼ High 2,3-DPG = shift to R. ◼ Hypoxia, anemia, low pH = high 2,3-DPG. ◼ Stored blood = low 2,3-DPG = low PO2 tissues. FACTORS THAT SHIFT THE CURVE Fetal Hb (Hb F) ◼ HbF has greater affinity (L) for O2 than HbA. ◼ During fetal development, higher affinity enhances transfer of O2 from maternal blood to fetal blood. ◼ HbF progressively dissappears >1y. Carbon Monoxide Hb (COHb) ◼ CO has 210 times more affinity for Hb than O2. ◼ Small amounts of CO can tie Hb. ◼ Prevents O2 from binding to Hb. ◼ Shifts the curve to the L = limited unloading to the tissues. CLINICAL APPLICATION ◼ When PaO2 is normal (80-100 mmHg), a shift to the R or L does not significantly affect Hb’s ability to transport O2 to the tissues. ◼ Shifts in this pressure range happen on the flat portion of the curve. Clinical Application (cont.) ◼ The problem is a Change on the STEEP portion of the curve Right Shifts - Loading O2 in the Lungs Asthmatic with acute exacerbation ◼ PAO2 of 60 mmHg. ◼ PcO2 = 60 mmHg = HbO2 90% as it leaves the alveoli. ◼ If curve shifts to the R (pH:7.1)= HbO2 will be only 75% with the same 60 mmHg. Total O2 delivery may be lower than what is indicated by the PaO2. If R shift is accompanied by decreased cardiac output or reduced Hb, ability to transport O2 will be jeopardized even more. Right Shifts - Unloading O2 at the Tissues ◼ If tissue cells metabolize 5 vol% of O2 when O2 dissociation curve is in normal position, plasma PO2 must fall from 60 mmHg (HbO2 90%) to about 35 mmHg to free 5 vol% of O2 from the Hb. ◼ If curve shifts to the R (pH 7.1 – HbO2 75%) = PO2 at the tissues would only have to fall from 60 mmHg to about 40 mmHg to unload the same 5 vol%. Left Shifts - Loading O2 in the Lungs Asthmatic with acute exacerbation ◼ PAO2 of 60 mmHg. ◼ PcO2 = 60 mmHg = HbO2 90% as it leaves the alveoli. ◼ If curve shifts to the L (pH:7.6)= HbO2 will be 95% with the same 60 mmHg. Left Shifts - Unloading O2 at the Tissues ◼ If tissue cells metabolize 5 vol% of O2 when O2 dissociation curve is in normal position, plasma PO2 must fall from 60 mmHg (HbO2 95%) to about 35 mmHg to free 5 vol% of O2 from the Hb. ◼ If curve shifts to the L (pH 7.6 – HbO2 95%) = PO2 at the tissues would have to fall from 60 mmHg to about 30 mmHg to unload the same 5 vol%.