Arterial Blood Gas Practice Questions – Answers
1. 7.27/58/60 on 5L, HCO3- 26, anion gap is 10, albumin is 4.0
Step 2: pH = Acidemia (pH < 7.4)
CO2 = Acid (CO2>40) HCO3- = Base (HCO3- >24)
Step 3: Primary disturbance = Respiratory Acidosis
Step 4: If respiratory disturbance is it acute or chronic? ACUTE
i. CO2 has increased by 18
ii. If chronic the pH will decrease 0.05 (0.003 x 18 = 0.054)  pH would be 7.35
iii. If acute the pH will decrease 0.14 (0.008 x 18 = 0.144) pH would be 7.26.
Step 5: No anion gap metabolic acidosis is present; and no adjustment needs to be
made for albumin.
Step 6b (1) (a): There is an acute respiratory acidosis, is there a metabolic problem too?
i. ΔHCO 3 - = 1 mEq/L↑/10mmHg↑pCO 2
1. The HCO 3 - will go up 1mEq/L for every 10mmHg the pCO 2 goes up
above 40
ii. The pCO2 is up by 18  so it is expected that the HCO3- will go up by 1.8.
Expected HCO3- is 25.8, compared to the actual HCO3- of 26, so there is no
additional metabolic disturbance.
ACUTE RESPIRATORY ACIDOSIS
(secondary to pneumonia)
2. 7.54/24/99 on room air, HCO3- 20, anion gap is 10, albumin is 4.0.
Step 2: pH = Alkalemia (pH > 7.4)
CO2 = Base (CO2<40)
HCO3- = Acid (HCO3- <24)
Step 3: Primary disturbance = Respiratory Alkalosis
Step 4: If respiratory disturbance is it acute or chronic? ACUTE
i. CO2 has decreased by 16.
ii. If chronic the pH will increase 0.05 (0.003 x 16 = 0.048)  pH would be 7.45
iii. If acute the pH will increase 0.13 (0.008 x 16 = 0.128)  pH would be 7.53
Step 5: No anion gap metabolic acidosis is present; and no adjustment needs to be
made for albumin.
Step 6b (2)(a): There is an acute respiratory alkalosis, is there a metabolic problem too?
i. ΔHCO 3 - = 2 mEq/L↓/10mmHg↓pCO 2
1. HCO 3 - will go down 2 mEq/L for every 10mmHg the pCO 2 goes
down below 40
ii. The pCO2 is down by 16  so it is expected that the HCO3- will go down by 3.2.
Expected HCO3- is 20.8, compared to the actual HCO3- of 20, so there is no
additional metabolic disturbance.
ACUTE RESPIRATORY ALKALOSIS (secondary to hyperventilation from anxiety)
3. 7.58/55/80 on room air, HCO3- 46, anion gap is 10, albumin is 4.0.
Step 2: pH= Alkalemia (pH > 7.4)
CO2 = Acid (CO2>40)
HCO3-= Base (HCO3- >24)
Step 3: Primary disturbance = Metabolic Alkalosis
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Arterial Blood Gas Practice Questions – Answers
Step 4: Not applicable
Step 5: No anion gap is present; and no adjustment needs to be made for albumin.
Metabolic Alkalosis
Step 6a(2): There is a metabolic alkalosis, is there a respiratory problem also?
i. ∆ pCO2=0.7 x ∆ HCO31. The CO2 will increase for every 0.7 the HCO3- increases.
ii. The HCO3- is up by 22. CO2 will increase by 0.7x22 = 15.4. Expected CO2 is 55.4,
compared to the actual CO2 of 55, therefore there is no additional respiratory
disturbance.
METABOLIC ALKALOSIS
(secondary to vomiting)
4. 7.46/20/80 on room air, HCO3- 16, anion gap = 10, albumin = 4.0
Step 2: pH = Alkalemia (pH > 7.4)
CO2 = Base (CO2<40)
HCO3-= Acid (HCO3- <24)
Step 3: Primary disturbance = Respiratory Alkalosis
Step 4: If respiratory disturbance is it acute or chronic? Chronic
i. CO2 has decreased by 20.
ii. If chronic the pH will increase 0.06 (0.003 x 20 = 0.06)  pH would be 7.46.
iii. If acute the pH will increase 0.16 (0.008 x 20 = 0.16)  pH would be 7.56.
Step 5: No anion gap is present; and no adjustment needs to be made for albumin.
Step 6b (2)(b): There is a chronic respiratory alkalosis, is there a metabolic problem
also?
i. Chronic: ΔHCO 3 - = 4 mEq/L↓/10mmHg↓pCO 2
1. HCO 3 - will go down 4 mEq/L for every 10mmHg the pCO 2 goes
below 40.
ii. The pCO2 is down by 20  so it is expected that the HCO3- will go down by 8.
Expected HCO3- is 16, therefore there is no additional metabolic disorder.
CHRONIC RESPIRATORY ALKALOSIS
(secondary to chronic liver disease - cirrhosis)
5. 7.28/27/90 on room air, HCO3- 13, anion gap = 10, albumin = 4.0
Step 2: pH = Acidemia (pH < 7.4)
CO2 = Base (CO2<40)
HCO3-= Acid (HCO3- <24)
Step 3: Primary disturbance = Metabolic Acidosis
Step 4: Not Applicable
Step 5: No anion gap is present; and no adjustment needs to be made for albumin.
Non-Anion Gap Metabolic Acidosis
Step 6a1: There is a metabolic acidosis, is there a respiratory disturbance as well?
i. ∆ pCO2=1.2 x ∆ HCO31. the CO2will decrease for every 1.2 the HCO3- decreases
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Arterial Blood Gas Practice Questions – Answers
ii. The HCO3- decreased by 11  so the CO2 should go down by 13.2. The
expected CO2 is 26.8. The actual CO2 is 27 so there is no additional respiratory
disturbance.
Non-Anion Gap Metabolic Acidosis
(secondary to diarrhea)
6. 7.35/52/50 on room air, HCO3- 27, anion gap = 10, albumin = 4.0
Step 2: pH = Acidemia (pH < 7.4)
CO2= (CO2>40) HCO3-= Base (HCO3- >24)
Step 3: Primary disturbance = Respiratory Acidosis
Step 4: If respiratory disturbance is it acute or chronic? Chronic
i. CO2 has increased by 12
ii. If chronic the pH will decrease 0.04 (0.003 x 12 = 0.036)  pH would be 7.36
iii. If acute the pH will decrease 0.10 (0.008 x 12 = 0.096) pH would be 7.3
Step 5: No anion gap metabolic acidosis is present; and no adjustment needs to be
made for albumin.
Step 6b (1) (b): There is a chronic respiratory acidosis, is there a metabolic problem
too?
i. ΔHCO 3 - = 3 mEq/L↑/10mmHg↑pCO 2
1. The HCO 3 - will go up 3mEq/L for every 10mmHg the pCO 2 goes
up above 40
ii. The pCO2 is up by 12  3 x 1.2 = 3.6. So it is expected that the HCO3- will go up
by 3.6. Expected HCO3- is 27.6, compared to the actual HCO3- of 27, so there is
no additional metabolic disturbance.
Chronic Respiratory Acidosis
(secondary to pulmonary fibrosis)
7. 7.43/30/95 on room air, HCO3- 20, anion gap = 10, albumin = 4.0
Step 2: pH = Alkalemia (pH > 7.4)
CO2= Base (CO2<40)
HCO3-= Acid (HCO3- <24)
Step 3: Primary Disturbance = Respiratory Alkalosis
Step 4: If respiratory disturbance is it acute or chronic? Chronic
i. CO2 has decreased by 10.
ii. If chronic the pH will increase 0.03 (0.003 x 10 = 0.03)  pH would be 7.43.
iii. If acute the pH will increase 0.08 (0.008 x 10 = 0.08)  pH would be 7.48.
Step 5: No anion gap is present; and no adjustment needs to be made for albumin.
Step 6b (2)(b): There is a respiratory alkalosis, is there a metabolic problem also?
i. Chronic: ΔHCO 3 - = 4 mEq/L↓/10mmHg↓pCO 2
1. HCO 3 - will go down 4 mEq/L for every 10mmHg the pCO 2 goes
below 40.
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Arterial Blood Gas Practice Questions – Answers
ii. The pCO2 is down by 10  so it is expected that the HCO3- will go down by 4.
Expected HCO3- is 20 for a chronic process. Since the actual HCO 3 - is 20 there
is no additional metabolic disturbance.
Chronic Respiratory Alkalosis
(secondary to pregnancy)
8. 7.48/49/75 on room air, HCO3- 37, anion gap = 10, albumin = 4.0
Step 2: pH= Alkalemia (pH > 7.4)
CO2= Acid (CO2>40)
HCO3-= Base (HCO3- >24)
Step 3: Primary disturbance = Metabolic Alkalosis
Step 4: Not applicable
Step 5: No anion gap is present; and no adjustment needs to be made for albumin.
Metabolic Alkalosis
Step 6a(2): There is a metabolic alkalosis, is there a respiratory problem also?
i. ∆ pCO2=0.7 x ∆ HCO31. The CO2 will increase for every 0.7 the HCO3- increases.
ii. The HCO3- is up by 13. CO2 will increase by 0.7x13 = 9.1. Expected CO2 is 49,
compared to the actual CO2 of 49, therefore there is no additional respiratory
disturbance.
METABOLIC ALKALOSIS
(secondary to diuretic use - furosemide)
9. 7.21/64/70 on room air, HCO3- 26, anion gap = 10, albumin = 4.0
Step 2: pH = Acidemia (pH < 7.4)
CO2= Acid (CO2>40)
HCO3-= Base (HCO3- >24)
Step 3: Primary disturbance = Respiratory Acidosis
Step 4: If respiratory disturbance is it acute or chronic? Acute
i. CO2 has increased by 24
ii. If chronic the pH will decrease 0.072 (0.003 x 24 = 0.072)  pH would be 7.33
iii. If acute the pH will decrease 0.19 (0.008 x 24 = 0.19) pH would be 7.21
Step 5: No anion gap metabolic acidosis is present; and no adjustment needs to be
made for albumin.
Step 6b (1) (a): There is an acute respiratory acidosis, is there a metabolic problem too?
i. ΔHCO 3 - = 1 mEq/L↑/10mmHg↑pCO 2
1. The HCO 3 - will go up 1mEq/L for every 10mmHg the pCO 2 goes up
above 40
ii. The pCO2 is up by 24  so it is expected that the HCO3- will go up by 2.4.
Expected HCO3- is 26.4, compared to the actual HCO3- of 26, so there is no
additional metabolic disturbance.
ACUTE RESPIRATORY ACIDOSIS
(secondary to obstruction – stridor from anaphylaxis secondary to a latex allergy)
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Arterial Blood Gas Practice Questions – Answers
10. 7.19/35/60 on 7L, HCO3- 9, anion gap = 18, albumin = 4.0
Step 2: pH = Acidemia (pH < 7.4)
CO2= Base (CO2<40) HCO3- = Acid (HCO3- <24)
Step 3: Primary disturbance: Metabolic Acidosis
Step 4: Not applicable
Step 5: Anion Gap = 18 + Anion Gap (alb normal so no correction necessary)
i. Excess Gap = 18-10 = 8
ii. Potential HCO3- = 8 + 9 = 17 which is <22  Non-AG Met Acidosis
Step 6a(1): There is a metabolic acidosis, is there a respiratory problem too?
i. ∆ pCO2=1.2 x ∆ HCO31. The CO2will decrease for every 1.2 the HCO3- decreases.
ii. CO2 will decrease by 1.2 (∆HCO3-)  1.2 (24-9)  18. 40 – 18= 22 Actual CO2
is higher than expected  Respiratory Acidosis
ANION GAP METABOLIC ACIDOSIS
(secondary to diabetic ketoacidosis)
NON-ANION GAP METABOLIC ACIDOSIS
(secondary to chronic kidney disease or type IV Renal Tubular Acidosis (RTA 4)secondary to diabetic
nephropathy),\
This problem is very complicated. Since the diabetic ketoacidosis is the presenting problem, it is
therefore the primary disturbance. Presumably the CKD or RTA is a chronic issue that has been present
for some time and is therefore, secondary
RESPIRATORY ACIDOSIS
(secondary to a strep pneumoniae pneumonia – which probably triggered the DKA)
11. 7.54/80/65 on 2L, HCO3- 54, anion gap = 12, albumin = 4.0
Step 2: pH = Alkalemia (pH > 7.4)
CO2= Acid (CO2>40) HCO3- = Base (HCO3- >24)
Step 3: Primary disturbance: Metabolic Alkalosis
Step 4: Not applicable
Step 5: Anion Gap = 12 (alb normal so no correction necessary)
Step 6a(2): There is a metabolic alkalosis, is there a respiratory problem too?
i. ∆ pCO2=0.7 x ∆ HCO31. The CO2 will increase for every 0.7 the HCO3- increases.
ii. CO2 will increase by 0.7 (∆HCO3-)  0.7 (54-24)  21  40 + 21 = 61  Actual
CO2 is higher than expected  Respiratory Acidosis
METABOLIC ALKALOSIS
(secondary to contraction alkalosis from the furosemide)
RESPIRATORY ACIDOSIS
(secondary to COPD)
12. 7.6/30/83 on room air, HCO3- 28, anion gap = 12, albumin = 4.0
Step 2: pH = Alkalemia (pH > 7.4)
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Arterial Blood Gas Practice Questions – Answers
CO2= Base (CO2<40) HCO3- = Base (HCO3- >24)
Step 3: Primary Disturbance: Metabolic Alkalosis (this is the primary disturbance based
on the history of new onset vomiting).
Step 4: Not applicable
Step 5: Anion Gap = 12 (alb normal so no correction necessary)
Step 6: 6a(2): There is a metabolic alkalosis, is there a respiratory problem too?
i. ∆ pCO2=0.7 x ∆ HCO31. The CO2 will increase for every 0.7 the HCO3- increases.
ii. CO2 will increase by 0.7 (∆HCO3-)  0.7 (28-24)  2.8  40 + 2.8 = 42.8 
Actual CO2 is lower than expected  Respiratory Alkalosis
METABOLIC ALKALOSIS
(secondary to vomiting)
RESPIRATORY ALKALOSIS
(secondary to pregnancy)
13. 7.25/46/78 on 2L, HCO3- 20, anion gap = 10, albumin = 4.0
Step 2: pH = Acidemia (pH < 7.4)
CO2= Acid (CO2>40) HCO3- = Acid (HCO3- <24)
Step 3: Primary disturbance: Respiratory Acidosis (this is the primary disturbance
based on the history of new dyspnea).
Step 4: If respiratory disturbance is it acute or chronic? ACUTE (This makes sense given
the history of sudden onset of shortness of breath. Since the pH is lower than
expected and the HCO3- is low, there is clearly a secondary metabolic acidosis. See
below for clarification.)
i. CO2 has increased by 6
ii. If chronic the pH will decrease 0.02 (0.003 x 6 = 0.018)  pH would be 7.38
iii. If acute the pH will decrease 0.05 (0.008 x 6 = 0.048) pH would be 7.35.
Step 5: Anion Gap = 10 (alb normal so no correction necessary)
Step 6b (1) (a): There is an acute respiratory acidosis, is there a metabolic problem too?
i. ∆ HCO3- = 1 mEq/L↑/10mmHg↑pCO 2
1. The HCO 3 - will go up 1mEq/L for every 10mmHg the pCO 2 goes up
above 40
ii. The pCO2 is up by 6  so it is expected that the HCO3- will go up by 0.6.
Expected HCO3- is 24.6, compared to the actual HCO3- of 20. Since the HCO3- is
lower than expected  Non-Anion Gap Metabolic Acidosis (which we
suspected).
RESPIRATORY ACIDOSIS (secondary to pulmonary edema)
NON-ANION GAP METABOLIC ACIDOSIS (secondary to chronic kidney disease)
14. 7.15/22/75 on room air, HCO3- 9, anion gap = 10, albumin = 2.0
Step 2: pH = Acidemia (pH < 7.4)
CO2= Base (CO2<40) HCO3- = Acid (HCO3- <24)
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Arterial Blood Gas Practice Questions – Answers
Step
Step
Step
3: Primary disturbance: Metabolic Acidosis
4: If respiratory disturbance is it acute or chronic? N/A
5: Anion Gap = 15.5  Anion Gap Metabolic Acidosis
i. The anion gap decreases 2.5 for every drop in albumin by 1.0. 4.0 – 2.0 = 2. 2 x
2.5 = 5. Therefore add 5 to the measured anion gap. 10 + 5 = 15.
ii. Excess gap = 15 – 10 = 5
iii. Potential HCO3- = 5 + 9 = 14. 14 < 22  NAG Metabolic Acidosis
Step 6a(1): There is a metabolic acidosis, is there a respiratory problem too?
i. ∆ pCO2=1.2 x ∆ HCO31. The CO2will decrease for every 1.2 the HCO3- decreases.
ii. Expected pCO2 = 1.2 x ∆ HCO3-  1.2 (24 -9)  1.2 (15)  18. The expected
pCO2 is 22 mmHg. The actual pCO2 is 22, which is expected, so there is no
concomitant disorder.
ANION GAP METABOLIC ACIDOSIS
(secondary to lactic acidosis from ischemic bowel)
NON-ANION GAP METABOLIC ACIDOSIS
(secondary to a Type IV Renal Tubular Acidosis from her Diabetes Mellitus)
15. 7.38/23/84 on room air, HCO3- 16, anion gap = 18, albumin = 4.0
Step 2: pH = Acidemia (pH < 7.4)
CO2= Base (CO2<40) HCO3- = Acid (HCO3- <24)
Step 3: Primary disturbance: Metabolic Acidosis
Step 4: If respiratory disturbance is it acute or chronic? N/A
Step 5: Anion Gap = 18 + Anion Gap (alb normal so no correction necessary)
i. Excess Gap = 18-10 = 8
ii. Potential HCO3- = 8 (excess gap) + 16 (serum HCO3-) = 24  No concurrent
metabolic disturbance.
Step 6a(1): There is a metabolic acidosis, is there a respiratory problem too?
i. ∆ pCO2=1.2 x ∆ HCO31. The CO2will decrease for every 1.2 the HCO3- decreases.
ii. CO2 will decrease by 1.2 (∆HCO3-)  1.2 (24-16)  9.6. 40 – 9.6= 30.4  Actual
CO2 is 23, which is less than expected, so there is also a respiratory alkalosis.
ANION GAP METABOLIC ACIDOSIS
(secondary aspirin)
RESPIRATORY ALKALOSIS
(secondary to aspirin)
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