Practice Problems Acid-Base Imbalances

interpretation of Arterial Blood Gases (ABG)

Getting an arterial blood gas sample

Radial Artery

Ulnar Artery

Blood Gas ReportAcid-Base Information•pH•PCO2

•HCO3 [calculated vs measured]

Oxygenation Information•PO2 [oxygen tension]•SO2 [oxygen saturation]

PRIMARY AND SECONDARY ACID-BASE DERANGEMENTS

End-Point: A Constant PCO2/[HCO3- ] Ratio Acid-Base Disorder Primary Change Compensatory

Change

Respiratory acidosis PCO2 up HCO3 upRespiratory alkalosis PCO2 down HCO3 downMetabolic acidosis HCO3 down PCO2 downMetabolic alkalosis HCO3 up PCO2 up

STATES OF COMPENSATION * PaCO2 30-40mmHg HCO3- 20-24 mmol/L pH 7.36-7.44 Uncompensated 80 mmHg N 22 mmol/L 7.06 Partial Comp. 80 mmHg 36 mmol/L 7.30 Respiratory Fully Comp. 80 mmHg 48 mmol/L N 7.40 Acidemia Uncompensated 20 mmHg N 22 mmol/L 7.66 Partial Comp. 20 mmHg 16 mmol/L 7.53 Respiratory Fully Comp. 20 mmHg 12 mmol/L N 7.40 Alkalemia Uncompensated N 35 mmHg 12 mmol/L 7.16 Partial Comp. 23 mmHg 12 mmol/L 7.34 Metabolic Fully Comp. 20 mmHg 11 mmol/L N 7.40 Acidemia Uncompensated N 35 mmHg 48 mmol/L 7.70 Partial Comp. 60 mmHg 48 mmol/L 7.53 Metabolic Fully Comp. 80 mmHg 48 mmol/L N 7.40 Alkalemia

Steps for interpretation of ABG

1. Know normal values of pH, (PCO2), and (HCO3-).

2. Look at the patient's ABG's to determine what's abnormal high or low.

3. Correlate the abnormal values of PCO2 and HCO3- to the abnormality of pH.

4. Name the disorder, the cause, and the source of any compensation.

 Steps1- Normal values for ABG's: pH range 7.35 - 7.45 PCO2 40 mm Hg HCO3

- 24 mEq/L

2- Evalutate the patient's ABG's: is the pH normal? Is it too high or too low? Is it 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?

Is the CO2 normal? Is it too high or too low? Will it cause acidosis or alkalosis? Will it correct acidosis or alkalosis?

3-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.)

4- 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)

5- Suggest a possible cause. For example, a cause of chronic respiratory acidosis is emphysema.

Practice Problem 1ABG's:  pH 7.31   PCO2

55 mm Hg   HCO3- 28

mEq/L

Normal values for ABG's: pH range 7.35 - 7.45 PCO2 35 - 45 mm Hg HCO3

-, 24 - 28 mEq/L pH is too low - acidosis;  PCO2

is too high, would cause acidosis or correct alkalosis; HCO3

- is normal, neither causing nor correcting imbalance

high PCO2 is correlated with low pH, which is

consistent with patient's report because PCO2

is causing the problem, this is respiratory acidosis; because bicarbonate is normal, there is no compensation

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- 24 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 40 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