Arterial Blood Gases

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Arterial Blood Gas interpretation made easy.

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  • June 2002 Remind class of the basics of hemoglobin: Carries Oxygen Necessary component in determining oxygen delivery
  • June 2002 Normal pH is maintained by balancing the H2CO3 (carbonic acid) and HCO3- (bicarb) Carbonic acid (H 2 CO 3 ) is central to our understanding and evaluation of acid-base disturbances. The dissociation products and the ionization products are normally in equilibrium:
  • June 2002 Normal pH is maintained by balancing the H2CO3 (carbonic acid) and HCO3- (bicarb)
  • June 2002 Oxygen is not very soluble in water. Hemoglobin is thus necessary to carry oxygen.
  • June 2002 Don Samples Questions
  • June 2002 The horizontal axis is Pa02 , or the amount of oxygen available. The vertical axis is SaO2 , or the amount of hemoglobin saturated with oxygen. “ Affinity” is used to describe oxygen's attraction to hemoglobin. Affinity can change with changes: 2,3,-DPG = a metabolic by-product which competes with O2 for binding sites.
  • June 2002 1. Respiratory Acidosis: Respiratory acidosis results from hypoventilation which is manifested by the accumulation of CO 2 in the blood and a drop in blood pH. Examples of specific causes can be categorized as follows:           Central Nervous System Depression (Sedatives, CNS disease, Obesity Hypoventilation syndrome)           Pleural Disease (Pneumothorax)           Lung Disease (COPD, pneumonia)           Musculoskelatal disorders (Kyphoscoliosis, Guillain-Barre, Myasthenia Gravis, Polio)
  • June 2002 2. Respiratory Alkalosis: Respiratory alkalosis results from hyperventilation which is manifested by excess elimination of CO 2 from the blood and a rise in the blood pH. Examples of specific causes are listed below:           Catastrophic CNS event (CNS hemorrhage)           Drugs (salicylates, progesterone)           Pregnancy (especially the 3 rd trimester)           Decreased lung compliance (interstitial lung disease)           Liver cirrhosis           Anxiety
  • June 2002 3. Metabolic Alkalosis Metabolic alkalosis results from elevation of serum bicarbonate. Examples of specific causes:           Volume contraction (vomiting, overdiuresis, ascites)           Hypokalemia           Alkali ingestion (bicarbonate)           Excess gluco- or mineralocorticoids           Bartter’s syndrome
  • June 2002 4. Metabolic Acidosis Anion gap acidosis results from accumulation of acidic metabolites and is manifested by a low HCO 3 - and an anion gap > 12 (anion gap calculation discussed in step 3). Examples of specific causes:           Uremia           Ketoacidosis (diabetic hyperglycemia, EtOH withdrawal)           Alcohol poisons or drug intoxication (methanol, ethylene glycol, paraldehyde, salicylates)           Lactic acidosis (sepsis, left ventricular failure) One may use a mnemonic device to remember these items. MULEPAK is a mnemonic commonly used ( M ethanol, U remia, L actic acidosis, E thylene glycol intoxication, P araldehyde intoxication, A spirin, K etoacidosis). 5. Non-Anion Gap Acidosis Non-anion gap acidosis results from loss of bicarbonate or external acid infusion and is manifested by a low HCO 3 - , but the anion gap is <12 (anion gap calculation is discussed in step 3). Examples of specific causes:           GI loss of HCO 3 - (diarrhea)           Renal loss of HCO 3 -           Compensation for respiratory alkalosis           Carbonic anhydrase inhibitor (Diamox)           Renal tubular acidosis           Ureteral diversion           Other causes: HCl or NH 4 Cl infusion, Cl gas inhalation, Hyperalimentation A mnemonic device may be used to remember this list of causes. The commonly used mnemonic is ACCRUED ( A cid infusion, C ompensation for respiratory alkalosis, C arbonic anhydrase inhibitor, R enal tubular acidosis, U reteral diversion, E xtra alimentation or hyperalimentation, D iarrhea).
  • June 2002 The horizontal axis is Pa02 , or the amount of oxygen available. The vertical axis is SaO2 , or the amount of hemoglobin saturated with oxygen. “ Affinity” is used to describe oxygen's attraction to hemoglobin. Affinity can change with changes: 2,3,-DPG = a metabolic by-product which competes with O2 for binding sites.
  • Arterial Blood Gases

    1. 1. Arterial Blood Gas Interpretation Sherry L. Knowles, RN, CCRN, CRNI Orlando Regional Medical Center 2009
    2. 2. Objectives <ul><li>Recognize Signs & Symptoms of Respiratory Failure </li></ul><ul><li>Understand Ventilation and Perfusion Mechanics </li></ul><ul><li>Analyze and Interpret Acid Base Disturbances </li></ul><ul><li>Identify Appropriate Treatments for Abnormal ABG’s </li></ul>
    3. 3. Objectives <ul><li>Recognize normal and abnormal values for pH, PaO 2 , PaCO 2 , SaO 2 and HCO 3 . </li></ul><ul><li>Relate the pH scale to acidosis and alkalosis. </li></ul><ul><li>Discuss the respiratory and metabolic mechanisms involved in controlling the body's acid-base balance. </li></ul><ul><li>Interpret basic arterial blood gas values and relate these values to patient conditions. </li></ul><ul><li>Anticipate appropriate therapies for acid-base correction. </li></ul>
    4. 4. Why Do ABG’s? <ul><li>1) Check oxygenation </li></ul><ul><li>2) Check the pH ( acid base balance ) </li></ul><ul><li>3) Define the problem </li></ul><ul><li>4) Determine the treatment </li></ul>
    5. 5. Fundamentals <ul><li>All human cells require oxygen. </li></ul><ul><li>Breathing (ventilation) brings oxygen in and CO 2 out of the lungs. </li></ul><ul><li>Oxygen is absorbed into the </li></ul><ul><li>bloodstream through the alveoli. </li></ul><ul><li>Hemoglobin molecules carry oxygen to the tissues. </li></ul>
    6. 6. Hemoglobin <ul><li>Carries Oxygen </li></ul><ul><li>Has 4 Binding Sites </li></ul>Hemoglobin + 4 Oxygen = Oxyhemoglobin
    7. 7. Hemoglobin Binding Sites <ul><li>When all of 4 sites are occupied, the hemoglobin molecule cannot hold any more. </li></ul><ul><li>Molecules, other than oxygen, can attach to the oxygen binding sites. </li></ul><ul><li>If enough hemoglobin binding sites are occupied with molecules other than oxygen, severe tissue hypoxia can result. </li></ul><ul><li>Hypoxia can occur even in the presence of 100% oxygen. This can be a life-threatening condition. </li></ul>
    8. 8. Carboxyhemoglobin (HbCO) <ul><li>Carboxyhemoglobin (HbCO) is a hemoglobin molecule that has carbon monoxide attached where the oxygen should be. </li></ul><ul><li>Exposure to car exhaust, or other chemicals can cause carbon monoxide (CO) to attach to hemoglobin binding sites, instead of oxygen and thus compete with the oxygen for the limited number of binding sites. </li></ul><ul><li>The blood will exhibit a cherry red color. </li></ul>
    9. 9. Methemoglobin (MetHb) <ul><li>Methemoglobin (MetHb) is produced when exposed to certain poisons or due to a genetic condition that affects the hemoglobin molecule. </li></ul><ul><li>The hemoglobin molecule is saturated with methemoglobin (MetHb) and competes with oxygen for the hemoglobin binding sites. </li></ul><ul><li>Methemoglobin (MetHb) changes blood to a brownish color. </li></ul>
    10. 10. Oxygen and Hemoglobin <ul><li>CO 2 's affinity for hemoglobin is much less than O 2 's affinity for hemoglobin. </li></ul><ul><li>When CO 2 and O 2 are both available, hemoglobin will accept oxygen rather than CO 2 . </li></ul><ul><li>In the oxygen rich environment of the alveoli, hemoglobin carries oxygen. </li></ul><ul><li>Oxygenated blood then travels through the body. </li></ul>
    11. 11. Acid Base Balance <ul><li>Understanding the cause of an acid-base imbalance is the key to treating it. </li></ul><ul><li>The Respiratory component of acid base balance affects the pH within minutes . </li></ul><ul><li>The Metabolic component of acid base balance can take days to affect pH. </li></ul>
    12. 12. Buffer System <ul><li>Carbonic Acid - Bicarbonate Buffer System </li></ul><ul><li>CO 2 + H 2 O <--> H 2 CO 3 <--> ( HCO 3 - ) + ( H+ ) </li></ul><ul><li>carbon dioxide + water <--> carbonic acid <--> bicarbonate + hydrogen ion </li></ul><ul><li>Note: The two headed arrows indicate that the process is reversible </li></ul>
    13. 13. CO 2 <ul><li>When combined with water, carbon dioxide becomes carbonic acid ( H2CO 3 ) </li></ul><ul><li>Carbon dioxide is an acid when dissolved in water. </li></ul><ul><li>Carbon dioxide is a product of metabolism. </li></ul><ul><li>As long as cells are functioning, CO 2 is produced. </li></ul>
    14. 14. HCO 3 <ul><li>Bicarbonate = HCO 3. </li></ul><ul><li>HCO 3 increases in response to high CO 2. </li></ul><ul><li>Metabolic changes take days to affect pH. </li></ul>
    15. 15. Basic ABG Components <ul><li>pH </li></ul><ul><li>PaCO 2 </li></ul><ul><li>HCO 3 </li></ul><ul><li>PaO 2 </li></ul>
    16. 16. pH <ul><li>The pH scale ranges from 1 to 14. </li></ul><ul><li>pH 7 is Neutral </li></ul><ul><li>Low pH is Acid . </li></ul><ul><li>High pH is Alkaline . </li></ul>
    17. 17. pH <ul><li>Normal pH is maintained by balancing the H2CO3 (carbonic acid) and HCO3- (bicarb) </li></ul><ul><li>Normal blood pH = 7.35-7.45 </li></ul><ul><li>pH < 7.35 = acidosis </li></ul><ul><li>Ph > 7.45 = alkalosis </li></ul>
    18. 18. PaCO 2 <ul><li>CO2 has several forms in the blood. </li></ul><ul><li>Like oxygen, some is dissolved directly into the plasma. The PaCO2 is the measurement of the partial pressure of carbon dioxide dissolved in the plasma. It is measured in mm Hg (millimeters of mercury). </li></ul><ul><li>The rest is found in the red blood cells on a hemoglobin molecule. </li></ul>
    19. 19. PaO 2 <ul><li>About 3% of the body's oxygen is dissolved in the plasma. </li></ul><ul><li>PaO 2 is a measurement of the partial pressure of oxygen dissolved in the plasma only. It is measured in mm Hg. </li></ul><ul><li>The PaO 2 does not tell us about the body's total oxygen content, but it does indicate how much oxygen was available in the alveoli to dissolve in the blood. </li></ul>
    20. 20. SaO 2 <ul><li>The remainder of the body's oxygen is carried attached to hemoglobin molecules. </li></ul><ul><li>SaO 2 , or oxygen saturation , measures the degree to which oxygen is bound to hemoglobin. </li></ul><ul><li>Sa0 2 is expressed as a percentage. </li></ul>
    21. 21. Ventilation <ul><li>Controls CO 2 levels </li></ul><ul><li>CO 2 = Ventilation </li></ul><ul><li>High CO 2 = Hypoventilation </li></ul><ul><li>Low CO 2 = Hyperventilation </li></ul>
    22. 22. Respiratory Mechanism <ul><li>Respiratory Mechanism (depth and rate of breathing) controls CO2. </li></ul><ul><li>CO 2 in solution is an acid. </li></ul><ul><li>Higher PaCO2 causes acidosis (lower pH), or neutralizes alkalosis. </li></ul><ul><li>Lower PaCO2 causes alkalosis (raises pH.), or neutralizes acidosis. </li></ul>
    23. 23. Metabolic Mechanism <ul><li>Bicarbonate = Alkaline = HCO 3. </li></ul><ul><li>HCO 3 increases in response to high CO 2. </li></ul><ul><li>Metabolic changes take days to affect pH. </li></ul>
    24. 24. Compensation <ul><li>“ Compensation&quot; is the body's normal response to normalize pH </li></ul><ul><ul><li>By neutralizing the opposite acid base mechanism. </li></ul></ul><ul><li>Example: If the pH is high because of respiratory alkalosis (low CO 2 ): </li></ul><ul><ul><li>Alkaline HCO 3 - will decrease to neutralize the pH . </li></ul></ul><ul><ul><li>In this case, the abnormal bicarb is not a metabolic problem; it is a metabolic solution to a respiratory problem. </li></ul></ul><ul><li>It is important to determine which is the cause and which is the effect. </li></ul><ul><li>If you treat the compensatory abnormality, you make the pH even more abnormal. </li></ul>
    25. 25. Normal ABG Values   <ul><li>pH = 7.35-7.45 7.4 (+/- 0.5)  </li></ul><ul><li>PaCO 2  = 35-45 40 (+/- 5) </li></ul><ul><li>HCO 3  = 22-26 24 (+/- 2) </li></ul><ul><li>PaO 2  = 80-100 90 (+/- 10) </li></ul><ul><li>SaO 2  = 94-100 97 (+/- 3)   </li></ul> Norms Quick Reference
    26. 26. Steps to ABG Interpretation <ul><li>1) Determine Acidosis or Alkalosis . </li></ul><ul><li>2) Evaluate the Respiratory Mechanism </li></ul><ul><li>3) Evaluate the Metabolic Mechanism </li></ul>
    27. 27. STEP 1 <ul><li>Step 1. </li></ul><ul><li>Use pH to determine Acidosis or Alkalosis . </li></ul><ul><li>  Normal or Compensated = </li></ul><ul><li>    Acidosis = </li></ul><ul><li>Alkalosis = </li></ul><ul><li>  </li></ul>7.35-7.45 < 7.35 > 7.45  
    28. 28. STEP 1 <ul><li>Step 1. </li></ul><ul><li>Use pH to determine Acidosis or Alkalosis . </li></ul>    ph   < 7.35   7.35-7.45   > 7.45   Acidosis   Normal or Compensated   Alkalosis
    29. 29. STEP # 2 <ul><li>Step 2. </li></ul><ul><li>Use PaCO 2 to look at the </li></ul><ul><li>Respiratory Mechanism </li></ul>    PaCO2   < 35   35 -45 > 45 <ul><li>Tends toward </li></ul><ul><li>alkalosis </li></ul><ul><li>Causes high </li></ul><ul><li>pH </li></ul><ul><li>Neutralizes </li></ul><ul><li>low pH </li></ul>  Normal or Compensated <ul><li>  Tends toward </li></ul><ul><li>acidosis </li></ul><ul><li>Causes low </li></ul><ul><li>pH </li></ul><ul><li>Neutralizes </li></ul><ul><li>high pH </li></ul>
    30. 30. STEP 3 <ul><li>Step 3. </li></ul><ul><li>Use HCO 3 to look at the </li></ul><ul><li>Metabolic Mechanism </li></ul>    HCO 3   < 22   22-26 > 26 <ul><li>Tends toward </li></ul><ul><li>acidosis </li></ul><ul><li>Causes low </li></ul><ul><li>pH </li></ul><ul><li>Neutralizes </li></ul><ul><li>high pH </li></ul>  Normal or Compensated <ul><li>  Tends toward </li></ul><ul><li>alkalosis Causes high </li></ul><ul><li>pH </li></ul><ul><li>Neutralizes </li></ul><ul><li>low pH </li></ul>
    31. 31. Interpretation   High pH Low pH   Alkalosis Acidosis   High HCO 3 Low PaCO2 High PaCO2 Low HCO 3   Metabolic Respiratory Respiratory Metabolic
    32. 32. Compensation <ul><li>“ Compensation&quot; is the body's normal response to normalize pH </li></ul><ul><ul><li>By neutralizing the opposite acid base mechanism. </li></ul></ul><ul><li>Example: If the pH is high because of respiratory alkalosis (low CO 2 ): </li></ul><ul><ul><li>Alkaline HCO 3 - will decrease to neutralize the pH. </li></ul></ul><ul><ul><li>In this case, the abnormal bicarb is not a metabolic problem; it is a metabolic solution to a respiratory problem. </li></ul></ul><ul><li>It is important to determine which is the cause and which is the effect. </li></ul><ul><li>If you treat the compensatory abnormality, you make the pH even more abnormal. </li></ul>
    33. 33. Normal ABG Values   <ul><li> Norms Quick Reference </li></ul><ul><li>pH = 7.35-7.45 7.4 (+/- 0.5)  </li></ul><ul><li>PaCO 2  = 35-45 40 (+/- 5) </li></ul><ul><li>HCO 3  = 22-26 24 (+/- 2) </li></ul><ul><li>PaO 2  = 80-100 90 (+/- 10) </li></ul><ul><li>SaO 2  = 94-100 97 (+/- 3)   </li></ul>
    34. 34. Combined Disturbances <ul><li>A “ Combined Disturbance ” occurs when the PaCO 2 and HCO3- both alter the pH in the same direction. </li></ul><ul><li>A high PaCO2 and low HCO3- (acidosis). </li></ul><ul><li>Low PaCO2 and high HCO3- (alkalosis). </li></ul><ul><li>VERY RARE </li></ul>
    35. 35. OxyHemoglobin Dissociation Curve  <ul><li>This curve describes the relationship between available oxygen and amount of oxygen carried by hemoglobin. </li></ul><ul><li>Oxygen-Hemoglobin affinity changes with: </li></ul><ul><ul><li>variation in pH *CO2 *temperature *2,3,-DPG </li></ul></ul><ul><li>Once the PaO2 reaches 60 mm Hg the curve indicates that there is little change in saturation above this point. </li></ul><ul><ul><li>So, PaO2 of 60 or more is usually considered adequate. </li></ul></ul><ul><ul><li>At PaO2 of less than 60 even small changes will greatly reduce the SaO 2. </li></ul></ul>
    36. 36. Question 1 <ul><li>If the pH is 7.23, the PaCO 2 is 50, and the HCO 3 is 24 what is the likely diagnosis? </li></ul><ul><li>  </li></ul><ul><li> RESPIRATORY ACIDOSIS </li></ul>
    37. 37. Question #2 <ul><li>If the pH is 7.49, the PaCO 2 is 25, and the HCO 3 is 22 what is the likely diagnosis? </li></ul>RESPIRATORY ALKALOSIS
    38. 38. Question # 3 <ul><li>If the the pH is 7.56, the PaCO 2 is 39, and the HCO 3 is 38, what is the likely diagnosis? </li></ul>METABOLIC ALKALOSIS
    39. 39. Question # 4 <ul><li>If the pH is 7.35, the PaCO 2 is 25, and the HCO 3 is 9, what is the likely diagnosis? </li></ul><ul><li>   COMPENSTATED METABOLIC ACIDOSIS    </li></ul>
    40. 40. Question # 5 <ul><li>If the pH is 7.30, the PaCO 2 is 25, and the HCO 3 is 9, what is the likely diagnosis? </li></ul><ul><li>   PARTIALLY COMPENSTATED </li></ul><ul><li> METABOLIC ACIDOSIS    </li></ul>
    41. 42. OxyHemoglobin Dissociation Curve  Shift Left Interpretation Increased affinity for O2 Hemoglobin does not unload so easily SaO2 may be higher at given PaO2, but O2 delivery is less Causes pH PCO2 Temperature 2, 3-DPG (use of bank blood can cause this ) Shift Right Interpretation Decreased affinity for O2 Hemoglobin unloads O2 more easily than usual SaO2 may be less, but O2 delivery is higher Causes pH PCO2 Temperature 2, 3-DPG

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