Practice Problems Acid-Base Imbalances

  1. You need to know the normal values of pH, partial pressure of carbon dioxide (PCO2), and bicarbonate (HCO3-).
  2. You need to look at the patient's ABG's - arterial blood gases - to determine what's abnormal, and whether the abnormal values are too high or too low.
  3. You need to correlate the abnormal values of PCO2 and HCO3- to the abnormality of pH.
  4. You need to name the disorder, the cause, and the source of any compensation.
  5. You need to suggest possible causes of the disorder.

 Steps:

  1. Normal values for ABG's:
  • pH range 7.35 - 7.45
  • carbon dioxide partial pressure, 35 - 45 mm Hg
  • bicarbonate, 24 - 28 mEq/L
  1. Evalutate the patient's ABG's:
  • is the pH normal? Is it too high or too low? Is it acidosis or alkalosis?
  • Is the CO2 normal? Is it too high or too low? Will it cause acidosis or alkalosis? Will it correct acidosis or alkalosis?
  • Is the HCO3- normal? Is it too high or too low? Will it cause acidosis or alkalosis? Will it correct acidosis or alkalosis?
  1. Correlate the abnormal values:
  • If only one of the two parameters (CO2 or HCO3-) is abnormal, then its value should be consistent with the pH (for example, if the CO2 is high, since that causes a drop in pH, the pH should be low).
  • If both of the parameter are abnormal, then usually one is CAUSING the problem, and the other is trying to CORRECT (COMPENSATE FOR) the problem. (For example, if the CO2 is high, and is causing the pH imbalance, then the pH must be low, since CO2 behaves as an acid. If HCO3- level is also abnormal, then usually it will be high, to compensate for the low pH, since it is a base.)
  1. Name the disorder:
  • Respiratory acidosis (with or without renal compensation)
  • Respiratory alkalosis (with or without renal compensation)
  • Metabolic acidosis (with or without respiratory compensation)
  • Metabolic alkalosis (with or without respiratory compensation)
  1. Suggest a possible cause. For example, a cause of chronic respiratory acidosis is emphysema.

Practice Problem 1

ABG's:  pH 7.31   PCO2 55 mm Hg   HCO3- 28 mEq/L

ANSWER:

  1. Normal values for ABG's:
  • pH range 7.35 - 7.45
  • carbon dioxide partial pressure, 35 - 45 mm Hg
  • bicarbonate, 24 - 28 mEq/L
  1. pH is too low - acidosis;  PCO2 is too high, would cause acidosis or correct alkalosis; HCO3- is normal, neither causing nor correcting imbalance
  2. high PCO2 is correllated with low pH, which is consistent with patient's report
  3. because PCO2 is causing the problem, this is respiratory acidosis; because bicarbonate is normal, there is no compensation
  4. possible cause: cardiac arrest

Practice Problem 2   (Answers below)

ABG's:  pH 7.31   PCO2 55 mm Hg   HCO3- 35 mEq/L

Practice Problem 3

ABG's:  pH 7.31   PCO2 35 mm Hg   HCO3- 20 mEq/L

Practice Problem 4

ABG's:  pH 7.31   PCO2 25 mm Hg   HCO3- 20 mEq/L

Practice Problem 5

ABG's:  pH 7.48   PCO2 25 mm Hg   HCO3- 28 mEq/L

Practice Problem 6

ABG's:  pH 7.48   PCO2 25 mm Hg   HCO3- 20 mEq/L

Practice Problem 7

ABG's:  pH 7.48   PCO2 45 mm Hg   HCO3- 33 mEq/L

Practice Problem 8

ABG's:  pH 7.48   PCO2 55 mm Hg   HCO3- 33 mEq/L

 

ANSWERS

Practice Problem 2

ABG's:  pH 7.31   PCO2 55 mm Hg   HCO3- 35 mEq/L

pH low = acidosis

PCO2 high = respiratory acidosis

HCO3 - high = renal compensation

 

Practice Problem 3

ABG's:  pH 7.31   PCO2 35 mm Hg   HCO3- 20 mEq/L

pH low = acidosis

PCO2 normal; no compensation

HCO3 - low = metabolic acidosis

 

Practice Problem 4

ABG's:  pH 7.31   PCO2 25 mm Hg   HCO3- 20 mEq/L

pH low = acidosis

PCO2 low = respiratory compensation

HCO3 - low = metabolic acidosis

 

Practice Problem 5

ABG's:  pH 7.48   PCO2 25 mm Hg   HCO3- 28 mEq/L

pH high = alkalosis

PCO2 low = respiratory alkalosis

HCO3 - normal; no compensation

 

Practice Problem 6

ABG's:  pH 7.48   PCO2 25 mm Hg   HCO3- 20 mEq/L

pH high = alkalosis

PCO2 low = respiratory alkalosis

HCO3 - low = renal compensation

 

Practice Problem 7

ABG's:  pH 7.48   PCO2 45 mm Hg   HCO3- 33 mEq/L

pH high = alkalosis

PCO2 normal; no compensation

HCO3 high = metabolic alkalosis

 

Practice Problem 8

ABG's:  pH 7.48   PCO2 55 mm Hg   HCO3- 33 mEq/L

pH high = alkalosis

PCO2 high = respiratory compensation

HCO3 high = metabolic alkalosis