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The Minimalist approach to acid/base disturbances

The Minimalist approach to acid/base disturbances. ABG’s so easy….a GEICO rep can do it. OBJECTIVES. Acid/Base terminology and definitions Normal physiologic acid/base regulation Acid/base disturbances ABG interpretation Case studies. Acid/Base terminology and definitions.

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The Minimalist approach to acid/base disturbances

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  1. The Minimalist approach to acid/base disturbances • ABG’s so easy….a GEICO rep can do it

  2. OBJECTIVES • Acid/Base terminology and definitions • Normal physiologic acid/base regulation • Acid/base disturbances • ABG interpretation • Case studies

  3. Acid/Base terminology and definitions • Acid/Base regulation / Control of( H+)concentration • Acid—proton (H+) donor increases (H+) • Base—proton (H+) acceptor decreases (H+) • Ph is the logarithmic/exponential representation of (H+) concentration in eq/liter

  4. What is log???? • 10 =10(1) log 10= 1 • 100=10(2) log 100=2 • 1000=10 (3) log1000=3 • 0.1= 10 (-1) log 0.1 = -1 • 0.01 = 10 (-2) log 0.01= -2

  5. ph = - Log (H+) • ph = 6 (H+) = 10 (-6) eq/liter 0.000001 eq/liter • ph = 7 (H+) = 10 (-7) eq/liter 0.0000001eq/liter • ph = (H+) = 10(-8) eq/liter 0.00000001 eq/liter

  6. H20 <<<<< (H+) + ( OH-) • (H+) = 10(-7) eq/liter • Pure Water has a ph 7(chemical neutral)

  7. Physiologic regulation of extracellular ph • Normal physiologic ph = 7.4 • Essential for normal enzymatic reactions

  8. Control Mechanisms/ Normal physiology • Buffer systems (immediate) • Respiratory (seconds) • Renal (hours-days)

  9. Buffer solutions • Solution of two or more compounds • Prevent marked changes in ph when acid or base is added

  10. Bicarbonate Buffer • H20 + CO2 <<< H2CO3 <<( H+)+ HCO3-

  11. Henderson/Haselbalch Equation • ph = 6.1 + log HCO3-/ CO2 • HCO3-/CO2= 20 • Ph= 7.4

  12. Ph is dependent on HCO3- (direct) and CO2(inverse) • HCO3 increases--- ph increases • HCO3 decreases ---- ph decreases • CO2 increases---- ph decreases • CO2 decreases--- ph increase

  13. Physiologic Response to chronic acid load • Bicarbonate Buffer system (immediate) • HCL + NaHCO3 >>>H2CO3 (CO2) +NaCL • Pulmonary Control (seconds to minutes) • Ventilation of newly created CO2 in buffer system • Renal control (hours to days) • Secretes H+ to reabsorb and regenerate HCO3 consumed by buffer

  14. Pathophysiology/ Acid/Base disturbances • Acidosis---any process that lowers ph • Lowers HCO3- or raises PCO2 • Alkalosis--- any process that raises ph • Raises HCO3- or lowers PCO2

  15. Definitions • Neutral • Ph 7.35-7.45 • Acidemia • Ph < 7.35 • Alkalemia • Ph > 7.45

  16. More Definitons • Isoelectric principle • + ions (cations) = -ions (anions) • Anion Gap—(Na + K) – (CL + HCO3) nl =15 • Measures minor/unmeasured anions • Endogenous/exogenous anions

  17. Acid/Base disturbances • Primary • Secondary- a response to a primary disturbance • Opposite direction from primary • Compensation is partial and incomplete

  18. Acid/Base disturbaces • Respiratory--- alteration in pCO2 • Metabolic --- alteration in HCO3

  19. Acute Respiratory Acidosis-Elevation in PCO2 • CNS vs. pulmonary • CNS – coma, stroke • Pulmonary-asthma, COPD, pneumonia (very late) • .08 ph drop for every 10mm increase in PCO2 • Typical abg’s 7.32/50/62 7.24/60/47 • No metabolic compensation/ HCO3 unchanged • Treatment- Fix the problem • Bronchodilators/cpap/bipap/intubation

  20. Chronic Respiratory Acidosis • Chronic/slow PCO2 elevation • COPD/sleep apnea/obesity • Compensatory metabolic alkalosis a major component • Kidneys increase H+ secretion –increase HCO3 • .02 ph drop / 10 mm increase in PCO2 • Typical abg’s 7.38/50 /50 7.36/60/55 • Treat underlying condition • Beware excess O2

  21. Acute Respiratory Alkalosis/ decreased PCO2 • CNS vs pulmonary • CNS stress, drugs, anxiety, sepsis, toxins • Pulmonary pneumonia/asthma/pulmonary embolism • .08 ph increase 10 mm decrease in PCO2 • Typical abg’s 7.48/30/62 • No metabolic compensation • Treat underlying condition • Supplement O2 • Beware of paper bag

  22. Chronic Respiratory Alkalosis • Extremely rare…some other time….

  23. Acute Metabolic Acidosis • Decreased HCO3 • Direct HCO3 loss via GI/kidneys • Decreased HCO3 from H+ buffering

  24. Metabolic Acidosis - defined by the associated anion • Anion gap acidosis- increased minor anions • Endogenous/Exogenous • Non anion gap acidosis-hyperchloremic • HCO3 loss from GI/kidneys • Increased reabsorption of CL-

  25. Acute metabolic acidosis • Compensatory respiratory alkalosis • Compensation is rapid but incomplete • PCO2 drop= HCO3 drop • Typical abg 7.34/35/98 serum HCO3 =20

  26. Treatment of Metabolic Acidosis • Define and treat the underlying condition • Watch for elevations in K+ • Beware HCO3-

  27. Pitfalls of HCO3- treatment • Paradoxical CNS and intracellular acidosis • Over correction alkalosis • Aggressive Na load

  28. Indications for HCO3- therapy • HCO3 < 5 • Ph <7.10 • Cardiovascular instablity or irritability • More likely to use on ventilated patients as CO2 is “blown off”

  29. Anion Gap Acidosis MUDPILES • Methanol • Uremia • Diabetic (ketones) • Paraldehyde • Iron • Lactate • Ethylene glycol • Salicylate

  30. Alternative Classification of anion gap acidosis • Ketoacidosis • Lactic acidosis • Exogenous poisonings • Uremia

  31. Ketoacidosis • Diabetic • alcoholic

  32. Diabetic ketoacidosis • Insulin lack • Hyperglycemia • Fatty acid breakdown Ketone accumulation • Treatment • Correct underlying disorder • fluids • insulin • maintenance of electrolytes especially K+

  33. Alcoholic Ketoacidosis • EtOH use followed by vomiting/starvation • Excessive ketone accumulation • Dehydration • Hypo or normoglycemia • Treatment • Fluids • Maintenance of electrolytres • Glucose

  34. Lactic Acidosis • Type A tissue hypoxia/underperfusion • Type B abnormal lactate utilization

  35. Treatment • Correct underlying cause • Antibiotics • Blood transfusion • Goal directed sepsis therapy • Fluids/pressors

  36. Exogenous toxins • Alcohols • Methanol/ethylene glycol • Salicylate

  37. Alcohol poisoning • Ethanol not usually a major cause of acidosis • MethanolEthylene Glycol • Formate/Oxylate accumulation • Increased osmolar gap • Renal failure/oxylate crystals • Treatment for methanol/ethylene glycol • Ethanol drip • Dialysis

  38. Clues to diagnosis for ethylene glycol/methanol intox • History • “sterno” or anti freeze ingestion • Altered mental status • Unexplained visual disturbances/coma • Unexplained anion gap acidosis • Renal failure

  39. Diagnostic Aides • ABG/ HCO3 • Serum osm • ETOH level • Osmolar gap • Oxylate crystals

  40. Salicylate Toxicity • Altered mental status • Anion gap acidosis • Primary respiratory alkalosis as well • Typical abg 7.35/20/110 serum HCO3 15 • Treatment • Alkalinazation of the urine • K+ replacement • Dialysis

  41. Lab Evaluation of anion gap Metabolic Acidosis • Ketones • Salicylate • Lactate • Etoh/serum osm • Osmolar gap = measured-calc serum osm

  42. Non Anion Gap-Hyperchloremic acidosis • GI or renal HCO3- loss • Compensatory CL- resorption • Usually associated with hypo/hyperkalemia

  43. Metabolic Alkalosis • Primary elevation in extracellular HCO3- • H + losses from GI (vomiting) • Excessive renal H+ excretion/ elevated HCO3 resorpbtion • Exogenous ingestion • Respiratory compensation • If HCO3 goes up by 10 pco2 goes up by 7 • Typical abg 7.47/47/100 HCO3 =34

  44. Metabolic Alkalosis • Physical effects • Tetany • Neuromuscular hyperactivity • Seizures • Decreased K+/ionized Ca 2+ • Treatment • Correct underlying cause • Acetazolamide---causes renal HCO3 loss • Correct electrolytes

  45. ABG interpretation • Step 1 obtain ABG/ HCO3- (electrolytes SMA7/Istat) • Step 2 ph determination to determine • ph 7.35-7.45 = nuetral • ph < 7.35 = academia • ph > 7.45 + alkalemia

  46. Acidemia Flow chart to determine underlying process • HCO3 low- primary process is metabolic acidosis • pCO2 elevated- primary process is respiratory acidosis

  47. Metabolic Acidosis • What is the anion gap???? • Is the respiratory compensation appropriate • Appropriate PCO2/HCO3 =1 HCO3 =15/ PCO2 =30 • PCO2/HCO3 > 1 resp alkalosis HCO3=15 PCO2= 20 • PCO2/HCO3 < 1 resp acidosis HCO3= 15 PCO2 =43

  48. Respiratory Acidosis • Is the acidosis acute/chronic? • Acute –ph down .08/10 mmPCO2 elevation • Chronic ph down .03/10mm PCO2 elevation • Acute on chronic 7.35/50/50 • Outside limits—second primary process

  49. Alkalemia flow chart • HCO3 elevated- primary metabolic alkalosis • pCO2 decreased- primary respiratory alkalosis

  50. Metabolic Alkalosis • Is the respiratory response appropriate? • PCO2/HCO3 = .7 7.48/47/75 HCO3 =35

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