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Transport of O2 & CO2 ABG Interpretation, acidosis & alkalosis

Dr. Dalal AL- Matrouk. Transport of O2 & CO2 ABG Interpretation, acidosis & alkalosis. PART I Oxygen & CO2 Transport. Oxygen Transport. O 2 is transported by the blood either: Combined with haemoglobin ( Hb ) in the red blood cells (>98%) or, Dissolved in the blood plasma (<2%).

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Transport of O2 & CO2 ABG Interpretation, acidosis & alkalosis

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  1. Dr. Dalal AL-Matrouk Transport of O2 & CO2ABG Interpretation, acidosis & alkalosis

  2. PART IOxygen & CO2 Transport

  3. Oxygen Transport • O2 is transported by the blood either: • Combined with haemoglobin (Hb) in the red blood cells (>98%) or, • Dissolved in the blood plasma (<2%).

  4. 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 • In terms of total oxygen transport, a relatively small percentage of O2 is transported in the form of dissolved O2.

  5. Dissolved Oxygen • Dissolved Oxygen=.003 mls x PAo2 = .003 x 100=.3mls

  6. 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.

  7. Hemoglobin Haemoglobin molecules can transport up to four O2’s

  8. Haemoglobin Saturation • Haemoglobin saturation is the amount of oxygen bound by each molecule of hemoglobin • When oxygen binds to hemoglobin, it forms OXYHAEMOGLOBIN • Haemoglobin that is not bound to oxygen is referred to as DEOXYHAEMOGLOBIN.

  9. Quantity of O2 Bound to Hb • Each g% of Hb is capable of carrying approximately 1.34 ml of O2

  10. The Oxygen Dissociation Curve • Reveals the amount of haemoglobin saturation at different PO2 values.

  11. Oxygen Dissociation Curve • 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.

  12. Significance of the Flat Portion • The 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 • At 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.

  13. 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.

  14. 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.

  15. The Oxygen Transport Variables • Oxygen Content [CaO2] • Oxygen Delivery [DO2] • Oxygen Uptake [VO2] • Extraction Ratio [ER]

  16. Oxygen Content (1) The oxygen in the blood is either bound to hemoglobin or dissolved in plasma • the Sum of these two fractions is called the Oxygen Content CaO2:the Content of Oxygen in Arterial Blood Hb = Hemoglobin (14 g/dl) SaO2 = Arterial Saturation (98 %) PaO2 = Arterial PO2 (100 mmHg)

  17. Oxygen Content (2) CaO2 = (1.34 x Hb x SaO2) + (0.003 xPaO2) amount carried by Hbamount dissolved in plasma CaO2 = (1.34 x 14 x 0.98) + (0.003 x 100) CaO2 = 18.6 ml/dl

  18. Oxygen Content (3) • Note that the PaO2 contributes little to the Oxygen Content ! • Despite it’s popularity, the PaO2 is NOT an important measure of arterial oxygenation ! • The SaO2 is the more important blood gas variable for assessing the oxygenation of arterial blood ! the PaO2 should be reserved for evaluating the efficiency of pulmonary gas exchange

  19. Oxygen Delivery (1) DO2: the Rate of Oxygen Transport in the Arterial Blood * it is the product of Cardiac Output & Arterial Oxygen Content DO2 = Q x CaO2 By using a factor of 10, we can convert vol % to ml/min

  20. Oxygen Delivery (2)example of calculation DO2 = Q x CaO2 DO2 = 3 x (1.34 x Hb x SaO2) x 10 DO2 = 3 x (1.34 x 14 x .98) x 10 DO2 = 551 ml/min/m2

  21. Oxygen Uptake (1) oxygen uptake is the final step in the oxygen transport pathway and it represents the oxygen supply for tissue metabolism The Fick Equation: Oxygen Uptake is the Product of Cardiac Output and the Arteriovenous Difference in Oxygen Content VO2 = Q x [(CaO2 - CvO2)]

  22. Oxygen Uptake (2)

  23. Oxygen Uptake (3)example of calculation The Fick Equation: VO2 = Q x (CaO2 - CvO2) VO2 = Q x [(1.34 x Hb) x (SaO2 - SvO2) x 10] VO2 = 3 x [ (1.34 x 14) x (.98 - .73) x 10 ] VO2 = 3 x [ 46 ] VO2 = 140 ml/min/m2

  24. Extraction Ratio the fractional uptake of oxygen from the capillary bed O2ER: derived as the Ratio of Oxygen Uptake to Oxygen Delivery O2ER = VO2 / DO2 x 100 Normal Extraction 22 -32%

  25. Control of Oxygen Uptake • the uptake of oxygen from the microcirculation is maintained by adjusting the Extraction Ratioto match changes in oxygen delivery • the ability to adjust O2Extraction can be impaired in serious illness

  26. The Normal Response: O2ER The Normal Response to a Decrease in Blood Flow is an Increase in O2 Extraction sufficient enough to keep VO2 in the normal range VO2 = Q x Hb x 13.4 x (SaO2 - SvO2) • Q = 3; VO2 = 3 x 14 x 13.4 x (.97 - .73) = 110 ml/min • Q = 1; VO2 = 1 x 14 x 13.4 x(.97 -.37) = 109 ml/min

  27. The Normal Response: O2ER • Note the drop in SvO2 from 97 % to 37 % !! • This association between SvO2 & O2ER is the Basis for SvO2 Monitoring The Ability to Adjust Extraction is a feature of all vascular beds except the Coronary Circulation & the Diaphragm !

  28. Factors that affect O2ER • Factors that increase O2ER: • decreased cardiac output • increased oxygen consumption • anemia • decreased arterial oxygenation • Factors that decrease O2ER: • increased cardiac output • skeletal relaxation • peripheral shunting • hypothermia • increased Hb • increased arterial oxygenation

  29. The DO2 - VO2 Curve (1)

  30. Oxygen Transport Variables ParameterNormal Range Delivery (DO2)500 - 800 ml/min/m2 (900-1100 ml/min) Uptake (VO2) 110 - 160 ml/min/m2 (200-270 ml/min) Extraction Ratio (ER) 20 - 30 % Mixed Venous SO268 - 77 %

  31. Tissue Hypoxia • Tissue hypoxia means that the amount of oxygen available for cellular metabolism is inadequate. • Hypoxia leads to anaerobic mechanisms that eventually produces lactic acid and cause the blood pH to decrease.

  32. Tissue Hypoxia • There are four main types of hypoxia: • hypoxic hypoxia 2. anemic hypoxia 3. circulatory hypoxia 4. histotoxic hypoxia

  33. 1. Hypoxic Hypoxia • = hypoxemic hypoxia refers to the condition in which the PaO2 and CaO2 are abnormally low. • better known as hypoxemia (low oxygen concentration in the blood). • Causes

  34. How Does Hypoventilation Cause Hypoxemia? • The alveolar gas equation PaO2= FIO2 (Patm-PH2O) – PCO2/R If PaCO2=40 PaO2= 0.21(760-47) – 40/0.8 = 100 If PaCO2=80 PaO2= 0.21(760-47) – 80/0.8= 50

  35. Causes of hypoxemia………..important………

  36. Example:Causes of post-op Arterial Hypoxemia • Hypoventilation • Residual narcotics • Residual benzos • Residual inhaled anesthetics • Residual muscle relaxants • Airway obstruction • Pain, splinting • Restrictive Conditions, abdominal wall binding, abdominal distension • Bronchospasm • Diseases affecting brainstem,spinal cord, NMJ & resp muscles; chest wall abnormalities • V/Q mismatch and Shunt • Atelectasis • Pulmonary edema • Aspiration Pneumonitis/ pneumonia • PE • ARDS • COPD/lung fibrosis/ asthma • pneumothorax

  37. 2. Anemic Hypoxia • 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.

  38. 3. Circulatory Hypoxia • normal O2 tension and content, but the amount 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

  39. 4. HistotoxicHypoxia • 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. • Example: cyanide poisoning.

  40. CO2 Transport

  41. Carbon Dioxide Transport Carbon dioxide is carried in the blood primarily in three ways… 1. Dissolved in plasma (7 – 10%) 2. As bicarbonate ions resulting from the dissociation of carbonic acid 3. Bound to haemoglobin.

  42. Carbon Dioxide

  43. PART IIABG interpretation, acidosis & alkalosis

  44. Normal values • pH: 7.35-7.45 • PaCO2 35-45 mm Hg (4.7-6.0) • PaO2  80-100 mm Hg (9.3–13.3 kPa) • HCO3  22-26 mEq/L • BE  +/- 3

  45. Four-Step Guide to ABG Analysis: • Is the pH normal, acidotic or alkalotic? • Are the pCO2 or HCO3 abnormal?  Which one appears to influence the pH? • If both the pCO2 and HCO3 are abnormal, the one which deviates most from the norm is most likely causing an abnormal pH. • Check the pO2.  Is the patient hypoxic?

  46. Additional steps in case of metabolic acidosis • Anion gap calculation • If high AG, calculate the osmolalgap, gap-gap

  47. H+ conc. is regulated to maintain the arterial pH in a range of 7.35 – 7.45 Acid base balance

  48. Alteration in H conc. will affect: • Cellular metabolism • Cycoskeletal structure • Muscle contractility • Cell-cell coupling • Membrane conductance • Intercellular messengers • Cell activation, growth, and proliferation • Cell volume regulation • intracellular membrane flow

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