#43 – ABG Interpretation



Ground Rules

  1. Accurate assessment of a patient’s acid/base status requires a measurement of arterial pH and PCO2
    1. bedside analyzers directly measure these and then use the Henderson-Hasselbach equation to calculate the HCO3-
    2. You can use venous blood, but must make adjustments:
      1. pH is 0.03-0.04 lower than arterial
      2. PCO2 is 7-8 mmHg higher than arterial
      3. HCO3- is 2 mEq/L higher than arterial
    3. A primary disturbance is usually accompanied by a compensatory response, but does not fully correct the pH


There are four main ways pH can change:

  • Metabolic Alkalosis
    • increase in HCO3- and increase in pH
  • Metabolic Acidosis
    • Decrease in HCO3- and decrease in pH
  • Respiratory Alkalosis
    • Decrease in PCO2 and increase in pH
  • Respiratory Acidosis
    • Increase in PCO2 and decrease in pH

 

 

Remember, that the lungs can compensate considerable FASTER than the kidneys.

  • Lungs can excrete 12 moles of acid per day as CO2
  • Kidneys can excrete 0.1 moles of acid per day as ammonia, but can secrete HCO3- to buffer

 


Respiratory Acidosis

  • Decrease in minute ventilation (TV x RR)
  • Causes
    • Normal Lungs
      • CNS depression (drugs, head trauma)
      • Neuromuscular impairment (GBS, MG)
      • Thoracic restriction
    • Abnormal Lungs
      • Obstruction
      • Alveoli dysfunction (ARDS)
      • Perfusion defect (cardiac arrest, PTE)
  • Compensation
    • Acute
      • Increase of 10 mmHg of PaCO2 will increase serum HCO3- by 1 mEq/L
    • Chronic
      • Increase of 10 mmHg of PaCO2 will increase serum HCO3- by 3 mE1/L

Respiratory Alkalosis

  • Increase in minute ventilation (TV x RR)
  • Causes
    • Cardiac
    • Hypoxemia
    • Anemia
    • Medications
    • Pregnancy
    • Iatrogenic
    • Obstruction
    • Neurologic
    • Stress
  • Compensation
    • Acute
      • Decrease of 10 mmHg of PaCO2 will decrease serum HCO3- by 2 mEq/L
    • Chronic
      • Decrease of 10 mmHg of PaCO2 will decrease serum HCO3- by 4 mEq/L

If the condition is 100% acute respiratory, then the pH will change 0.08 for every 10 mmHg change in PaCO2



Metabolic Acidosis

  • Decreased pH due to decreased HCO3-
  • First step after determining that a patient has a metabolic acidosis is to calculate the anion gap
  • High Anion Gap Metabolic Acidosis (HAGMA)
    •  Causes
      • Carbon monoxide, cyanide
      • Aminoglycosides
      • Theophyline, toluene
      • Methanol
      • Uremia
      • Diabetic ketoacidosis
      • Propylene glycol
      • Inborn errors of metabolism
      • Lactic acidosis
      • Ethylene glycol, ethanol
      • Salicylates

  • Calculating a Delta Gap

  • Normal Anion Gap Metabolic Acidosis (NAGMA)
    • Causes
      • Ureteric diversion
      • Small bowel fistulae
      • Excessive saline
      • Diarrhea
      • Carbonic anhydrase inhibitors
      • Renal tubular acidosis
      • Adrenal insufficiency
      • Pancreatic fistulae

  • Respiratory Compensation
    • Expected PaCO2 = 8 + (1.5 x HCO3-) ± 2

Metabolic Alkalosis

  • Increased pH due to increased HCO3-
  • Causes
    • Contraction
    • Licorice
    • Endocrine
    • Vomiting
    • Excessive NG suction
    • Ringer’s solution
    • Post-hypercapnia
    • Diuretics
  • Respiratory Compensation
    • Expected PaCO2 = 20 + (0.7 x HCO3-) ± 5 

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