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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

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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

 

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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

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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

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

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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

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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