#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 

PAINE #PANCE Pearl – Renal



Question

 

In an oliguric patient, what is the first step in differentiating between prerenal and intrinsic renal causes?

 



Answer

 

The fractional excretion of sodium (FENa) can help differentiate prerenal from intrinsic renal causes in patient with oliguria.  If the FENa is < 1%, it suggests prerenal and if it is > 2%, it suggests intrinsic renal causes.  There are many limitations to using this calculation, but it is a good first step in determining the cause of a patient’s oliguria.  Other laboratory studies, such as BUN/Cr, can be useful as well.

 

Image result for fractional excretion of sodium