Practice Problems Acid-Base Imbalances

interpretation of Results of Arterial Blood Gases (ABG)

Getting an arterial blood gas sample

Radial Artery

Ulnar Artery

Blood Gas ReportAcid-Base Information pH (H+ concentration) 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

Respiratory acidosis PCO2 up HCO3 up

Respiratory alkalosis PCO2 down HCO3 down

Metabolic acidosis HCO3 down PCO2 down

Metabolic 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

Know normal values of pH, H+, (PCO2) & (HCO3-)

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

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

Name the A/B disorder & source of any compensation

Name the cause of the disorder

1- Normal Values for ABG's: pH range: 7.35 - 7.45 H+ : 36 – 44 nmol/L PCO2 30 -40 mmHg

HCO3- 20 – 24 mmol/L

2- Evaluate the Patient's ABG's: Is the pH (or H+) 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 PCO2 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. It causes a drop in pH. So, pH should be low).

If both of the parameter are abnormal, then usually

One is CAUSING the problem

Other is trying to COMPENSATE the problem (correct)

For example,

If the PCO2 is high, and is causing the pH imbalance,

then the pH must be low (as CO2 behaves as an acid).

If HCO3- level is also abnormal, then usually it will be high, to

compensate for the low pH (as HCO3- is a base).

4- Name the Acid-Base Disorder: (and a source of any compensation)

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 Cause of the disorder:For example, a possible cause of chronic respiratory acidosis is emphysema.

Practice Problem 1 ABG:  H+ 78 nmol/L   PCO2

55 mmHg   HCO3

- 24 mmol/L

Normal values for ABG's: H+ range 36 – 44 nmol/LPCO2 35 - 45 mmHg

HCO3- 20 - 24 mmol/L

H+ is too high So, it is a case of acidosis; 

high PCO2 is correlated with low pH as PCO2

is the cause

So, this is respiratory acidosis Because bicarbonate is normal, there is no compensation

Practice Problem 2 ABG:H+ 65 nmol/L PCO2 55 mmHg HCO3- 35 mmol/L

H+ high = acidosis PCO2 high = respiratory

acidosis HCO3 high = renal compensation

Practice Problem 3 ABG:H+ 50 nmol/LPCO2 35 mmHg HCO3- 11 mmol/L

H+ high = acidosis PCO2 normal = no

compensation HCO3 low = metabolic acidosis

Practice Problem 4 ABG:H+ 55 nmol/LPCO2 25 mmHg HCO3- 12 mmol/L

H+ high = acidosis PCO2 low = respiratory

compensation HCO3 low = metabolic acidosis

Practice Problem 5 ABG: H+ 32 nmol/L PCO2 25 mmHg HCO3- 24 mmol/L

H+ low= alkalosis PCO2 low = respiratory

alkalosis HCO3 normal = no

compensation

Practice Problem 6 ABG: H+ 52 nmol/L PCO2 25 mm Hg HCO3- 18 mmol/L

H+ high = alkalosis PCO2 low = respiratory alkalosis HCO3 low = renal compensation

Practice Problem 7 ABG: H+ 32 nmol/L PCO2 40 mmHg HCO3- 33 mmol/L

H+ low= alkalosis PCO2 normal = no

compensation HCO3 high = metabolic

alkalosis

Practice Problem 8 ABG's: H+ 33 nmol/L PCO2 55 mm Hg HCO3 33 mmol/L

H+ low = alkalosis PCO2 high = respiratory

compensation HCO3 high = metabolic

alkalosis