Oxygen Transport
Oxygen Extraction Ratio
- (O2ER) is the amount of oxygen extracted by the peripheral tissues divided by the amount of O2 delivered to the peripheral cells. -O2ER = CaO2 - CvO2 / CaO2 -normal O2ER is .25 -Under normal circumstances an individuals Hb returns to the alveoli approximately 75% saturated with O2.
Oxygen Dissociation Curve
- O2 dissociation curve graphically illustrated the percentage of Hb that is chemically bound to O2 at each O2 pressure. -The curve is S-shaped with a steep slope between 10 and 60 mm Hg and a flat portion between 70 and 100 mm Hg. -The flat and steep portions of the curve each have a distinct clinical significance.
Oxygen Transport
- is essential to the study of pulmonary physiology and to the clinical interpretation of arterial and venous blood gases.
Anemic hypoxia
- is when the oxygen tension in the arterial blood is normal, but the oxygen-carrying capacity of the blood is inadequate. -This form of hypoxia can develop from: -a low amount of Hb in the blood -a deficiency in the ability of Hb to carry O2 -Increased cardiac output is the main compensatory mechanism for anemic hypoxia.
Tissue hypoxia
- means that the amount of oxygen available for cellular metabolism is inadequate. -There are four main types of hypoxia: -hypoxic hypoxia - circulatory hypoxia -anemic hypoxia - histotoxic hypoxia -Hypoxia leads to anaerobic mechanisms that eventually produces lactic acid and cause the blood pH to decrease.
circulatory hypoxia
- the arterial blood that reaches the tissue cells may have a normal O2 tension and content, but the amount of of blood--and therefore the amount of O2--is not adequate to meet tissue needs. -The two main causes of circulating hypoxia are: -stagnant hypoxia -arterial-venous shunting
The P50
-A common point of reference on the oxygen dissociation curve is the P50. -The P50 represents the partial pressure at which the hemoglobin is 50% saturated with oxygen, typically 26.6 mm Hg in adults. -The P50 is a conventional measure of hemoglobin affinity for oxygen.
O2 Dissolved in Plasma
-As O2 diffuses from the alveoli into the pulmonary capillary blood, it dissolves in the plasma of the blood. -At normal body and temperature about 0.003 ml of O2 will dissolve in 100 ml of blood for every 1 mm Hg of Po2. -Vol% represents the amount of O2 in milliliters that is in 100 ml of blood. -In terms of total oxygen transport, a relatively small percentage of O2 is transported in the form of dissolved O2.
True Shunt
-Clinical conditions that cause true shunt can be grouped under two major categories: 1. anatomic shunts: -congenital heart disease -intrapulmonary fistula - vascular lung tumors 2. capillary shunts -The sum of the anatomic and capillary shunts is referred to as true, or absolute shunt and refractory to O2.
Quantity of O2 Bound to Hb
-Each g% of Hb is capable of carrying approximately 1.34 ml of O2 thus: - O2 bound to Hb = 1.34 ml O2 x g% Hb -At a normal Pao2 of 100 MM Hg, hemoglobin saturation (Sao2) is about 97% because of normal physiologic shunts: -thebesian veins (Coronary circ.) - -bronchial venous drainage -V/Q mismatch
Factors that decrease O2ER:
-Factors that decrease O2ER: -increased cardiac output -skeletal relaxation -peripheral shunting -hypothermia - increased arterial oxygenation -increased Hb
Factors that decrease the C(a-v)O2:
-Factors that decrease the C(a-v)O2: -increased cardiac output -skeletal relaxation (drugs) -peripheral shunting -poisons -decreased temp
Mixed Venous O2 Saturation
-Factors that increase the SvO2: -increased CO - peripheral shunting -skeletal relaxation -hypothermia
Dissolved Oxygen
-Henry's Law states that the amount of gas that dissolves is proportional to its partial pressure. Dissolved Oxygen=.003 mls x Pao2 .003 x 100=.3mls of dissolved O2
Shifts in the P50
-In the presence of disease or other conditions that change the hemoglobin's oxygen affinity and, consequently, shift the curve to the right or left, the P50 changes accordingly. -An increased P50 indicates a rightward shift of the standard curve, which means that a larger partial pressure is necessary to maintain a 50% oxygen saturation, indicating a decreased affinity. -Conversely, a lower P50 indicates a leftward shift and a higher affinity.
Clinical Significance of Shifts
-Individuals with PaO2's within normal (80-100) limits are rarely affected by shift changes. -patients PaO2 falls below 80, a shift to the right or left can have remarkable effects on the hemoglobin's ability to pick up and release oxygen.
Left shift
-Left shift curves enhance the loading capability of oxygen enhance the loading capability of oxygen a the A-C membrane. -The total oxygen delivery may be higher than indicated by a particular Pao2 when the patient has some disease process that cause a left shift. -Left shift curves decreases the unloading of oxygen at the tissue level.
O2 Bound with Hemoglobin
-Most of the O2 that diffuses into the pulmonary capillary blood rapidly moves into the RBC's and chemically attaches to the hemoglobin. -Each RBC contains about 280 million Hb molecules, which are highly specialized to transport O2 and CO2. -The normal hemoglobin value for the adult male is 14 to 16 g% and female is 12 to 15 g%.
The Hemoglobin Molecule
-Normal adult hemoglobin consists of: -four hemo groups which are the pigmented, iron-containing non-protein portions. -four amino chains that collectively constitute globin (a protein). -At the center of each heme group, the iron molecule can combine with one O2 molecule to form oxyhemoglobin: - Hb + O2 <> HbO2 -The amount of O2 bound to Hb is directly related to the partial pressure of O2 in the plasma.
Mixed Venous O2 Saturation
-Normally, the SvO2 is about 75%, however, clinically an SvO2 of about 65% is acceptable. Factors that decrease the SvO2: -decreased CO -increased O2 consumption
Significance of Steep Portion
-PO2 reductions below 60 mm Hg produce a rapid decrease in the amount of O2 bound to hemoglobin. -Clinically, when the PO2 falls below 60 mm Hg, the quantity of O2 delivered to the tissue cells may be significantly reduced. -As oxygen partial pressures decrease in this steep area of the curve, the oxygen is unloaded to peripheral tissue readily as the hemoglobin's affinity diminishes.
Shunt Equation
-Pulmonary shunting and venous admixture are common complications in respiratory disorders, knowledge of the degree of shunting is often desirable when developing patient care plans. -The amount of intrapulmonary shunting can be calculated by using the classic shunt equation: -QS/QT = CcO2 - CaO2 / CcO2 - CvO2
Clinical Significance of Shunts
-Pulmonary shunting below 10% reflects normal lung status. -A shunt between 10 and 20% is indicative of an intrapulmonary abnormality, but is seldom of clinical significance. -Pulmonary shunting between 20 and 30% denotes significant intrapulmonary disease and may be life-threatening. -Pulmonary shunting greater than 30% is a potentially life-threatening situation and aggressive support is needed.
Mechanisms of Pulmonary Shunting
-Pulmonary shunting is defined as that portion of the cardiac output that enters the left side of the heart without exchanging gases with alveolar gases (true shunt) or as blood that does exchange gases with alveolar gases but does not obtain a PO2 that equals that of normal alveolus (shunt-like effect). -Regardless of the type of shunt the physiologic effect is the same, hypoxemia.
Right Shifts
-Right shift decrease the loading of oxygen onto Hb at the A-C membrane. -Decreased affinity -The total oxygen delivery may be much lower than indicated by a particular Pao2 when the patient has some disease process that causes a right shift. -Right shift curves enhance the unloading of oxygen at the tissue level.
Oxygenation Consumption
-The amount of oxygen extracted by the peripheral tissues during the period of one minute is called oxygen consumption or VO2. -Oxygen consumption is calculated by: -VO2 = QT [C(a-v)O2 x 10] -O2 consumption is commonly indexed by the patients body surface area (BSA) and calculated by: -VO2 / BSA -Normal VO2 index is between 125 - 165
Arterial-Venous Difference
-The arterial-venous oxygen content difference, C (a-v)O2, is the difference between the CaO2 and the CvO2. The normal C(a-v)O2 is about 5 vol%. -Clinically, the C(a-v)O2 can provide useful information regarding the patient's cardiopulmonary status.
Venous Admixture
-The end result of pulmonary shunting is venous admixture. -Venous admixture is the mixing of shunted, non-reoxygenated blood with reoxygenated blood distal to the alveoli. -When venous admixture occurs, shunted and oxygenated blood mixes until the PO2 throughout all plasma of the newly mixed blood is in equilibrium and all the Hb molecules carry the same number of oxygen molecules.
Fetal Vs. Adult Hemoglobin
-The globin, or protein portion of each adult Hb molecule consists of two identical alpha chains and two identical beta chains. -Normal fetal Hb (Hb F) has two alpha chains and two gamma chains. -These gamma chains increase hemoglobin's attraction to O2 and increases the transfer of O2 from the mom's Hgb across the placenta to the baby's blood. -Fetal Hb is gradually replaced with adult Hb (Hb A) over the first year of postnatal life.
Total Oxygen Delivery
-The total amount of oxygen delivered to the peripheral tissues is dependent on: -the body's ability to oxygenate blood -the hemoglobin concentration -the cardiac output -Total O2 delivery (DO2) is calculated: -DO2 = QT x (CaO2 x 10) -O2 delivery decreases when there is a decline in blood oxygenation, hemoglobin concentration, or cardiac output.
Oxygen Transport
-The transport of oxygen between the lungs and the cells of the body is a function of the blood and the heart. -Oxygen is carried in the blood in two forms: - as dissolved oxygen in the blood plasma - chemically bound to the hemoglobin -(Hb) that is encased in the erythrocytes, or RBC's
Total Oxygen Content
-To determine the total amount of O2 in 100 ml of blood, the dissolved O2 and the O2 bound to Hb must be added together. -The total oxygen content of specific blood is calculated as follows: - Cao2: (Hb x 1.34 x Sao2) + (Pao2 x 0.003) - Cvo2: (Hb x 1.34 x Svo2) + (Pvo2 x 0.003) -Cco2: (Hb x 1.34) + (PAo2 x .003)
Curve on the right
-decrease in pH -increase in DPG -increase in temperature
Histotoxic hypoxia
-develops in any condition that impairs the ability of tissue cells to utilize oxygen. -Clinically, the PaO2 and CaO2 in the blood are normal, but the tissue cells are extremely hypoxic. -The PvO2, CvO2 and SvO2 are elevated because oxygen is not utilized. -One cause of this type of hypoxia is cyanide poisoning.
Significance of the Flat Portion
-flat portion of the curve shows that the P02 can fall from 100 to 60 mmHg and the Hg will still be 90% saturated with 02. -pressures above 60mm Hg, the standard dissociation curve is relatively flat. -This means the oxygen content does not change significantly even with large changes in the partial pressure of oxygen.
Cyanosis
-hypoxemia is severe, signs of cyanosis may develop. - this term used to describe the blue-gray or purplish discoloration seen on the mucous membranes, fingertips, and toes whenever the blood in these areas is hypoxemic. -The recognition of cyanosis depends on the acuity of the observer, on the lighting conditions, and skin pigmentation.
Curve on the left
-increase in pH -decease in DPG -decrease in temperature
Factors that effect the 02 Dissociation
-pH- Change in the blood pH -Temperature-temp increases the curve moves to the right -2,3 Diphosphoglycerate-Increases 2,3 DPG results in decreased affinity. -Carbon monoxide
Shunt-Like Effect
-pulmonary capillary perfusion is in excess of alveolar ventilation, a shunt-like effect is said to exist. -Common causes of this form of shunting are: -hypoventilation -uneven distribution of ventilation -alveolar-capillary diffusion defects -Any of the previously mentioned phenomenon can be corrected by O2.
Hypoxic hypoxia or hypoxemic hypoxia
-refers to the condition in which the PaO2 and CaO2 are abnormally low. -This form of hypoxia is better known as hypoxemia (low oxygen concentration in the blood). -This form of hypoxia can develop from: -pulmonary shunting - low alveolar PO2 -diffusion impairment - V/Q mismatch
Polycythemia
-when pulmonary disorders produce chronic hypoxemia, the hormone erythropoietin responds by stimulating the bone marrow to increase RBC production. -An increased level of RBC's is called polycythemia. The polycythemia that results from hypoxemia is an adaptive mechanism designed to increase the oxygen-carrying capacity of the blood.
Normal Blood gas values
Arterial blood -pH=7.35-7.45 -Pco2=35-45mmHg -Po2=80-100mmHg -HCO3-=22-28mEq/L
Factors that decrease O2 consumption:
Factors that decrease O2 consumption: -skeletal relaxation -peripheral shunting -certain poisons -decreased temperature
Factors that increase O2 consumption:
Factors that increase O2 consumption: -exercise - seizures -shivering - increased temp
Factors that increase O2ER:
Factors that increase O2ER: -decreased cardiac output -increased oxygen consumption -anemia -decreased arterial oxygenation
Factors that increase the C(a-v)O2
Factors that increase the C(a-v)O2: -decreased cardiac output -increased O2 consumption -exercise -seizures -shivering -increased temp
Normal Blood gas values
Venous Blood - pH=7.30-7.40 - 42-48 mmHg -35-45 mmHg -24-30 mEq/L
