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ABG INTERPRETATION

ABG INTERPRETATION. Debbie Sander PAS-II. Objectives. What’s an ABG? Understanding Acid/Base Relationship General approach to ABG Interpretation Clinical causes Abnormal ABG’s Case studies Take home. What is an ABG. Arterial Blood Gas Drawn from artery- radial, brachial, femoral

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ABG INTERPRETATION

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  1. ABG INTERPRETATION Debbie Sander PAS-II

  2. Objectives • What’s an ABG? • Understanding Acid/Base Relationship • General approach to ABG Interpretation • Clinical causes Abnormal ABG’s • Case studies • Take home

  3. What is an ABG Arterial Blood Gas Drawn from artery- radial, brachial, femoral It is an invasive procedure. Caution must be taken with patient on anticoagulants. Helps differentiate oxygen deficiencies from primary ventilatory deficiencies from primary metabolic acid-base abnormalities

  4. What Is An ABG? pH [H+] PCO2 Partial pressure CO2 PO2 Partial pressure O2 HCO3 Bicarbonate BE Base excess SaO2 Oxygen Saturation

  5. Acid/Base Relationship • This relationship is critical for homeostasis • Significant deviations from normal pH ranges are poorly tolerated and may be life threatening • Achieved by Respiratory and Renal systems

  6. Case Study No. 1 60 y/o male comes ER c/o SOB. Tachypneic, tachycardic, diaphoretic and Cyanotic. Dx acute resp. failure and ABG’s Show PaCO2 well below nl, pH above nl, PaO2 is very low. The blood gas document Resp. failure due to primary O2 problem.

  7. Case Study No. 2 60 y/o male comes ER c/o SOB. Tachypneic, tachycardic, diaphoretic and Cyanotic. Dx acute resp. failure and ABG’s Show PaCO2 very high, low pH and PaO2 is moderately low. The blood gas document Resp. failure due to primarily ventilatory insufficiency.

  8. Buffers • There are two buffers that work in pairs • H2CO3 NaHCO3Carbonic acid base bicarbonate • These buffers are linked to the respiratory and renal compensatory system

  9. Respiratory Component • function of the lungs • Carbonic acid H2CO3 • Approximately 98% normal metabolites are in the form of CO2 CO2 + H2O  H2CO3 • excess CO2 exhaled by the lungs

  10. Metabolic Component • Function of the kidneys • base bicarbonate Na HCO3 • Process of kidneys excreting H+ into the urine and reabsorbing HCO3- into the blood from the renal tubules 1) active exchange Na+ for H+ between the tubular cells and glomerular filtrate 2) carbonic anhydrase is an enzyme that accelerates hydration/dehydration CO2in renal epithelial cells

  11. Acid/Base Relationship H2O + CO2  H2CO3  HCO3 + H+

  12. Normal ABG values pH 7.35 – 7.45 PCO2 35 – 45 mmHg PO2 80 – 100 mmHg HCO3 22 – 26 mmol/L BE -2 - +2 SaO2 >95%

  13. Acidosis Alkalosis pH > 7.45 PCO2 < 35 HCO3 > 26 pH < 7.35 PCO2 > 45 HCO3 < 22

  14. Respiratory Acidosis • Think of CO2 as an acid • failure of the lungs to exhale adequate CO2 • pH < 7.35 • PCO2 > 45 • CO2 + H2CO3  pH

  15. Causes of Respiratory Acidosis • emphysema • drug overdose • narcosis • respiratory arrest • airway obstruction

  16. Metabolic Acidosis • failure of kidney function •  blood HCO3 which results in  availability of renal tubular HCO3 for H+ excretion • pH < 7.35 • HCO3 < 22

  17. Causes of Metabolic Acidosis • renal failure • diabetic ketoacidosis • lactic acidosis • excessive diarrhea • cardiac arrest

  18. Respiratory Alkalosis • too much CO2 exhaled (hyperventilation) •  PCO2, H2CO3 insufficiency =  pH • pH > 7.45 • PCO2 < 35

  19. Causes of Respiratory Alkalosis • hyperventilation • panic d/o • pain • pregnancy • acute anemia • salicylate overdose

  20. Metabolic Alkalosis •  plasma bicarbonate • pH > 7.45 • HCO3 > 26

  21. Causes of Metabolic Alkalosis •  loss acid from stomach or kidney • hypokalemia • excessive alkali intake

  22. How to Analyze an ABG • PO2 NL = 80 – 100 mmHg • 2. pH NL = 7.35 – 7.45 • Acidotic <7.35 • Alkalotic >7.45 • PCO2 NL = 35 – 45 mmHg • Acidotic >45 • Alkalotic <35 • HCO3 NL = 22 – 26 mmol/L • Acidotic < 22 • Alkalotic > 26

  23. Four-step ABG Interpretation • Step 1: • Examine PaO2 & SaO2 • Determine oxygen status • Low PaO2 (<80 mmHg) & SaO2 means hypoxia • NL/elevated oxygen means adequate oxygenation

  24. Four-step ABG Interpretation • Step 2: • pH acidosis <7.35 alkalosis >7.45

  25. Four-step ABG Interpretation • Step 3: • study PaCO2 & HCO 3 • respiratory irregularity if PaCO2 abnl & HCO3 NL • metabolic irregularity if HCO3 abnl & PaCO2 NL

  26. Four-step ABG Interpretation Step 4: Determine if there is a compensatory mechanism working to try to correct the pH. ie: if have primary respiratory acidosis will have increased PaCO2 and decreased pH. Compensation occurs when the kidneys retain HCO3.

  27. ~ PaCO2– pH Relationship 80 7.2060 7.30407.4030 7.5020 7.60

  28. ABG Interpretation Acidosis CO2 Change CO2 CO2 Change c/w Normal opposes Abnormality Abnormality CO2 CO2 CO2 Metabolic Compensated Metabolic Acidosis More Abnormal Expected Less Abnormal Metabolic Acidosis Compensated Respiratory Mixed Respiratory Acidosis Respiratory Acidosis Metabolic Acidosis

  29. ABG Interpretation Alkalosis CO2 Change CO2 CO2 Change c/w Normal opposes Abnormality Abnormality CO2 CO2 CO2 Metabolic Compensated More Abnormal Expected Less Abnormal Alkalosis Metabolic Alkalosis Compensated Respiratory Mixed Respiratory Alkalosis Respiratory Alkalosis Metabolic Alkalosis

  30. Respiratory Acidosis pH 7.30 PaCO2 60 HCO3 26

  31. Respiratory Alkalosis pH 7.50 PaCO2 30 HCO3 22

  32. Metabolic Acidosis pH 7.30 PaCO2 40 HCO3 15

  33. Metabolic Alkalosis pH 7.50 PCO2 40 HCO3 30

  34. What are the compensations? Respiratory acidosis  metabolic alkalosis Respiratory alkalosis  metabolic acidosis In respiratory conditions, therefore, the kidneys will attempt to compensate and visa versa. In chronic respiratory acidosis (COPD) the kidneys increase the elimination of H+ and absorb more HCO3. The ABG will Show NL pH, CO2 and HCO3. Buffers kick in within minutes. Respiratory compensation is rapid and starts within minutes and complete within 24 hours. Kidney compensation takes hours and up to 5 days.

  35. Mixed Acid-Base Abnormalities Case Study No. 3: 56 yo  neurologic dz required ventilator support for several weeks. She seemed most comfortable when hyperventilated to PaCO2 28-30 mmHg. She required daily doses of lasix to assure adequate urine output and received 40 mmol/L IV K+ each day. On 10th day of ICU her ABG on 24% oxygen & VS:

  36. ABG Results pH 7.62 BP 115/80 mmHg PCO2 30 mmHg Pulse 88/min PO2 85 mmHg RR 10/min HCO3 30 mmol/L VT 1000ml BE 10 mmol/L MV 10L K+ 2.5 mmol/L Interpretation: Acute alveolar hyperventilation (resp. alkalosis) and metabolic alkalosis with corrected hypoxemia.

  37. Case study No. 4 27 yo retarded  with insulin-dependent DM arrived at ER from the institution where he lived. On room air ABG & VS: pH 7.15 BP 180/110 mmHg PCO2 22 mmHg Pulse 130/min PO2 92 mmHg RR 40/min HCO3 9 mmol/L VT 800ml BE -30 mmol/L MV 32L Interpretation: Partly compensated metabolic acidosis.

  38. Case study No. 5 74 yo  with hx chronic renal failure and chronic diuretic therapy was admitted to ICU comatose and severely dehydrated. On 40% oxygen her ABG & VS: pH 7.52 BP 130/90 mmHg PCO2 55 mmHg Pulse 120/min PO2 92 mmHg RR 25/min HCO3 42 mmol/L VT 150ml BE 17 mmol/L MV 3.75L Interpretation: Partly compensated metabolic alkalosis with corrected hypoxemia.

  39. Case study No. 6 43 yo  arrives in ER 20 minutes after a MVA in which he injured his face on the dashboard. He is agitated, has mottled, cold and clammy skin and has obvious partial airway obstruction. An oxygen mask at 10 L is placed on his face. ABG & VS: pH 7.10 BP 150/110 mmHg PCO2 60 mmHg Pulse 150/min PO2 125 mmHg RR 45/min HCO3 18 mmol/L VT ? ml BE -15 mmol/L MV ? L . Interpretation: Acute ventilatory failure (resp. acidosis) and acute metabolic acidosis with corrected hypoxemia

  40. Case study No. 7 17 yo, 48 kg  with known insulin-dependent DM came to ER with Kussmaul breathing and irregular pulse. Room air ABG & VS: pH 7.05 BP 140/90 mmHg PCO2 12 mmHg Pulse 118/min PO2 108 mmHg RR 40/min HCO3 5 mmol/L VT 1200ml BE -30 mmol/L MV 48L Interpretation: Severe partly compensated metabolic acidosis without hypoxemia.

  41. Case No. 7 cont’d This patient is in diabetic ketoacidosis. IV glucose and insulin were immediately administered. A judgement was made that severe acidemia was adversely affecting CV function and bicarb was elected to restore pH to  7.20. Bicarb administration calculation: Base deficit X weight (kg) 4 30 X 48 = 360 mmol/L Admin 1/2 over 15 min & 4 repeat ABG

  42. Case No. 7 cont’d ABG result after bicarb: pH 7.27 BP 130/80 mmHg PCO2 25 mmHg Pulse 100/min PO2 92 mmHg RR 22/min HCO3 11 mmol/L VT 600ml BE -14 mmol/L MV 13.2L

  43. Case study No. 8 47 yo  was in PACU for 3 hours s/p cholecystectomy. She had been on 40% oxygen and ABG & VS: pH 7.44 BP 130/90 mmHg PCO2 32 mmHg Pulse 95/min, regular PO2 121 mmHg RR 20/min HCO3 22 mmol/L VT 350ml BE -2 mmol/L MV 7L SaO2 98% Hb 13 g/dL

  44. Case No. 8 cont’d Oxygen was changed to 2L N/C. 1/2 hour pt. ready to be D/C to floor and ABG & VS: pH 7.41 BP 130/90 mmHg PCO2 10 mmHg Pulse 95/min, regular PO2 148 mmHg RR 20/min HCO3 6 mmol/L VT 350ml BE -17 mmol/L MV 7L SaO2 99% Hb 7 g/dL

  45. Case No. 8 cont’d What is going on?

  46. Case No. 8 cont’d If the picture doesn’t fit, repeat ABG!! pH 7. 45 BP 130/90 mmHg PCO2 31 mmHg Pulse 95/min PO2 87 mmHg RR 20/min HCO3 22 mmol/L VT 350ml BE -2 mmol/L MV 7L SaO2 96% Hb 13 g/dL Technical error was presumed.

  47. Case study No. 9 67 yo  who had closed reduction of leg fx without incident. Four days later she experienced a sudden onset of severe chest pain and SOB. Room air ABG & VS: pH 7.36 BP 130/90 mmHg PCO2 33 mmHg Pulse 100/min PO2 55 mmHg RR 25/min HCO3 18 mmol/L BE -5 mmol/L MV 18L SaO2 88% Interpretation: Compensated metabolic acidosis with moderate hypoxemia. Dx: PE

  48. Case study No. 10 76 yo  with documented chronic hypercapnia secondary to severe COPD has been in ICU for 3 days while being tx for pneumonia. She had been stable for past 24 hours and was transferred to general floor. Pt was on 2L oxygen & ABG &VS: pH 7.44 BP 135/95 mmHg PCO2 63 mmHg Pulse 110/min PO2 52 mmHg RR 22/min HCO3 42 mmol/L BE +16 mmol/L MV 10L SaO2 86% . Interpretation: Chronic ventilatory failure (resp. acidosis) with uncorrected hypoxemia

  49. Case No. 10 cont’d She was placed on 3L and monitored for next hour. She remained alert, oriented and comfortable. ABG was repeated: pH 7.36 BP 140/100 mmHg PCO2 75 mmHg Pulse 105/min PO2 65 mmHg RR 24/min HCO3 42 mmol/L BE +16 mmol/L MV 4.8L SaO2 92% . Pt’s ventilatory pattern has changed to more rapid and shallow breathing. Although still acceptable the pH and CO2 are trending in the wrong direction. High-flow oxygen may be better for this pt to prevent intubation

  50. Take Home Message: • Valuable information can be gained from an • ABG as to the patients physiologic condition • Remember that ABG analysis if only part of the patient • assessment. • Be systematic with your analysis, start with ABC’s as always and look for hypoxia (which you can usually treat quickly), then follow the four steps. • A quick assessment of patient oxygenation can be achieved with a pulse oximeter which measures SaO2.

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