1. Unit I – Problem 3 – Clinical Acid-Base Disturbances
Kingdom of Bahrain Arabian Gulf University College of Medicine and Medical Sciences
Unit I – Problem 3 – Clinical
Acid-Base Disturbances
Prepared by: Ali Jassim Alhashli
Based on: Kaplan Step 2 CK Internal Medicine

2. Acid/Base Disturbances (ABG Interpretation)
First of all, you must know the normal values of ABG:
pH = 7.35 – 7.45… (7.4).
PCO2 = 35 – 45 mmHg… (40 mmHg).
HCO3 = 22 – 26 mEq/L… (24 mEq/L).
What are the steps which you must follow when interpreting an ABG result?
First, look to the pH and comment if the patient has acidosis or alkalosis?
Acidosis: pH > 7.35
Alkalosis: pH < 7.45
Then, comment whether this acid-base disturbance is respiratory or metabolic?
Respiratory acidosis (↑PCO2): look for any causes of hypoventilation which leads to accumulation of CO2 that is considered to be an acid:
COPD.
Obesity.
Sleep apnea.
Depression of respiratory drive by opiates.
Effusions.
Suffocation.
Respiratory alkalosis (↓PCO2): look for any causes of hyperventilation which leads to washout of CO2 from the lungs:
Pain.
Anxiety.
Severe anemia.
Pulmonary embolism.
Acid/Base Disturbances (ABG Interpretation)

3. Acid/Base Disturbances (ABG Interpretation)
What are the steps which you must follow when interpreting an ABG result?
First, look to the pH and comment if the patient has acidosis or alkalosis?
Acidosis: pH > 7.35
Alkalosis: pH < 7.45
Then, comment whether this acid-base disturbance is respiratory or metabolic? (continued)
Metabolic acidosis (↓HCO3): causes are classified according to the anion gap (anion gap = (Na) – (HCO3 + Cl)… normal value = 8-12):
Low anion gap:
Multiple myeloma.
Low albumin level.
Lithium.
Normal anion gap:
Diarrhea.
Renal tubular Acidosis (RTA).
Ureterosigmoidostomy (it is a surgical procedure in which ureter are connected to sigmoid colon and considered as a part of treatment of bladder cancer in which urinary bladder has to be removed).
High anion gap (mnemonic: LA MUD PIE):
L: Lactate.
A: Aspirin.
M: Methanol.
U: Uremia.
D: DKA.
P: Propylene glycol.
I: Isoniazid and Isopropyl alcohol.
E: Ethyelen glycol.
Metabolic alkalosis (↑HCO3): can be caused by the following:
Loss of H: vomiting, Conn syndrome or diuretics.
HCO3 retention: bicarboate administration.
Movement of H into cells: hypokalemia.
Acid/Base Disturbances (ABG Interpretation)

4. Acid/Base Disturbances (ABG Interpretation)
What are the steps which you must follow when interpreting an ABG result?
First, look to the pH and comment if the patient has acidosis or alkalosis?
Acidosis: pH > 7.35
Alkalosis: pH < 7.45
Then, comment whether this acid-base disturbance is respiratory or metabolic?
After that, look if there is compensation by body systems trying to bring the pH back to normal.
There are 2classifications:
Full compensation: in which pH completely returns back to normal range.
Partial compensation: in which pH doesn’t return back to normal range.
Notice that with presence of compensation, both CO2 and HCO3 should move in the same direction (for example, if there is respiratory acidosis (↑CO2) → there will be retention of HCO3). If they do not move in the same direction → this indicated the presence of a mixed disorder.
Compensation of respiratory acidosis:
Acute respiratory acidosis: HCO3 increases 1 mEq/L for each 10 mmHg increase in PCO2.
Chronic respiratory acidosis: HCO3 increases 4 mEq/L for each 10 mmHg increase in PCO2.
Compensation of respiratory alkalosis:
Acute respiratory alkalosis: HCO3 decreases 2 mEq/L for each 10 mmHg decrease in PCO2.
Chronic respiratory alkalosis: HCO3 decreases 5 mEq/L for each 10 mmHg decrease in PCO2.
Compensation of metabolic acidosis:
Expected PCO2 = 1.5 x (HCO3) + 8 (± 2)
Compensation for metabolic alkalosis:
Expected PCO2 = 0.7 x (HCO3) + 20 (± 5)
Acid/Base Disturbances (ABG Interpretation)

5. Acid/Base Disturbances (ABG Interpretation)

6. Acid/Base Disturbances (ABG Interpretation)
Case (1):
An elderly female from a nursing home was transferred to hospital because of profound weakness and areflexia. Her oral intake had been poor for a few days. Current medication was a diuretic tablet.
Arterial Blood Gases:
pH = 7.58
pCO2 = 50 mmHg
pO2 = not provided.
HCO3 = 44.4 mmol/l
Patient has metabolic alkalosis because HCO3 is high and explains pH value.
Compensation for her metabolic alkalosis:
Expected pCO2 = 0.7 (HCO3) + 20 mmHg (range: ±5)
0.7 (44) + 20 = 55 (50-55)
Therefore, pCO2 is within the expected range.
Final comment: pure partially compensated metabolic alkalosis.
Case (2):
A known case of COPD with the following ABG results:
pH = 7.34
pCO2 = 44 mmHg
HCO3 = 28 mmol/l
Patient has chronic respiratory acidosis because pCO2 is high and explains pH value.
Compensation for chronic respiratory acidosis:
HCO3 increases 4 mEq/l for each 10 mmHg rise in pCO2
In this patient, pCO2 is 5 mmHg above the baseline. Therefore, HCO3 should increase by 2 mEq/l ( = 26 mmol/l). You can notice that patients HCO3 is more than 26 mmol/l (indicating the presence of metabolic alkalosis)
Final comment: chronic respiratory acidosis with metabolic alkalosis.
Acid/Base Disturbances (ABG Interpretation)