BLOOD GAS QUIZ

Lawrence Martin, M.D.
 Chairman, Dept. of Medicine, UH-Richmond Medical Center
Richmond Hts., Ohio 44143
Clinical Professor of Medicine
Case Western Reserve University School of Medicine, Cleveland
Questions (below) posted February 1, 2009
Answers posted May 1, 2009
Winners posted May 10, 2009

1) Given the following arterial blood gas values:

pH: 7.56

PaCO2: 31 mm Hg

HCO3: 27 mEq/l

PaO2: 56 mm Hg

What is(are) the most likely acid-base state(s) in the patient?

a) acute respiratory alkalosis

b) chronic respiratory alkalosis

c) respiratory alkalosis and metabolic alkalosis

d) respiratory acidosis and metabolic acidosis

e) respiratory alkalosis and metabolic acidosis

2) A healthy young woman, toward the end of a mile run in the gym, has increased her heart rate by 50% over baseline. At that point, which one of the following statements is least likely to be true.

a) She is tachypneic and/or hyperpneic.

b) She is hyperventilating.

c) Her alveolar ventilation is increased above its resting baseline value.

d) Her CO2 production is increased above its resting baseline value.

e) Her PaO2 is normal.

3) Which of the following changes will most increase arterial oxygen delivery?

a) PaO2 from 60 to 95 mm Hg

b) cardiac output from 4 to 5 L/min

c) hemoglobin from 9 to 10 grams%

d) atmospheric pressure from 1 to 2 atmospheres

e) arterial pH from 7.30 to 7.50

4) Given the following arterial blood gas values:

pH: 7.40

PaCO2: 20 mm Hg

HCO3: 12 mEq/l

What is(are) the most likely acid-base state(s) in the patient?

a) metabolic acidosis with full compensation

b) respiratory alkalosis with full compensation

c) metabolic acidosis and respiratory alkalosis

d) metabolic acidosis and metabolic alkalosis

e) normal acid-base state

5) All of the following are true about SpO2 as measured by pulse oximeters that utilize two wavelengths of light (i.e., most pulse oximeters in use today) except:

a) Can can be normal even when PaCO2 is >200 mm Hg.

b) Is not affected by anemia.

c) Does not differentiate oxyhemoglobin from carboxyhemoglobin.

d) Is not affected by excess methemoglobin.

e) Requires a detectable pulse.

6) All of the following are true about non-arterial assessment of oxygenation and acid-base balance except:

a) if venous CO2 (measured as part of standard electrolyte panel) is truly abnormal, the patient has some type of acid-base disorder.

b) in a hemodynamically stable patient, venous blood gases measured from central venous blood can be used to assess acid-base status.

c) venous blood can be used in lieu of arterial blood to measure blood carboxyhemoglobin.

d) the bicarbonate gap, which is (Na+ - Cl- - 39), can diagnose a metabolic alkalosis even when there is increased anion gap.

e) in a stable patient with normal lungs, the difference between PaCO2 and end-tidal PCO2 is 10 to 15 mm Hg.

7) One or more of the following statements (A, B, C) about PaO2 may be correct. Choose the single letter answer (a - e) that reflects which statement(s) is(are) correct.

A) If the lungs and heart are normal, then PaO2 is affected only by factors that affect alveolar PO2.
B) In a person with normal heart and lungs, anemia should not affect PaO2.
C) The reason PaO2 falls with increasing altitude is because barometric pressure falls.

a) only A) is correct.

b) A) and B) are correct.

c) A) and C) are correct.

d) B) and C) are correct

e) A), B) and C) are correct.

8) A patient presents with the following arterial blood gases, drawn on room air (FIO2 = .21).

pH: 7.40

PCO2: 40 mm Hg

PO2: 82 mm Hg

HCO3: 24 mEq/L

Which of the following statements is most accurate?

a) The patient does not have an acid-base disorder.

b) To determine if there is an acid base disorder you need to know the measured serum bicarbonate, since the HCO3 from blood gases is only a calculation.

c) Need to know patient's respiratory rate, as PaCO2 may be inappropriately "normal" and signify a respiratory acidosis.

d) A patient can have normal blood gases if two metabolic acid-base disorders oppose to give normal bicarbonate.

e) The PaO2 is lower than predicted for a patient breathing room air.

9) A patient with respiratory failure has the following room air (FIO2 = .21) arterial blood gases:

pH: 7.20

PCO2: 70 mm Hg

PO2: 60 mm Hg

SaO2: 86%

Which of the following is the least likely cause of these abnormal blood gases?

a) congestive heart failure

b) narcotic overdose

c) myasthenia gravis

d) flail chest

e) Guillain Barre' Syndrome

10) A 42-year-old man is admitted to the hospital with dehydration and hypotension.

Electrolytes show:
Na+165 mEq/L
K+4.0 mEq/L
CO232 mEq/L
Cl-112 mEq/L

No arterial blood gas is obtained. Which statement best applies about this patient's acid-base status?

a) Electrolytes indicate the presence of metabolic acidosis.

b) Electrolytes indicate the presence of metabolic alkalosis.

c) Electrolytes indicate the presence of both metabolic acidosis and metabolic alkalosis.

d) Need serum albumin to make any clinically useful assessment of his metabolic acid-base disorders.

e) Need serum lactate to make any clinically useful assessment of his metabolic acid-base disorders.

11) A 30-year-old man, previously healthy, is brought to the ED after suffering smoke inhalation. Measured carboxyhemoglobin is 20% and hemoglobin content = 15 gm%. He has following blood gas values:

PaO2: 80 mm Hg (on room air at sea level)
PaCO2: 32 mm Hg
pH: 7.34
SaO2: 96% (calculated)

Exam shows clear lungs to auscultation and his chest x-ray is normal. From this information alone, you can determine that:

a) His actual SaO2 is much lower than the calculated value.

b) There is no lung abnormality present, though pulmonary disease could develop in the ensuing 24 hours.

c) He has a mild metabolic acidosis associated with an increased anion gap.

d) His arterial oxygen content is in the normal range.

e) None of the above.

12) Below are two sets of blood gases:

Patient A: pH 7.48, PaCO2 34 mm Hg, PaO2 85 mm Hg, SaO2 95%, Hemoglobin 7 gm%

Patient B: pH 7.32, PaCO2 74 mm Hg, PaO2 55 mm Hg, SaO2 85%, Hemoglobin 15 gm%

Which is the most correct statement?

a) B is more hypoxemic because PaO2 is lower than A.

b) B is more hypoxemic because SaO2 is lower than A.

c) A is more hypoxemic because A-a gradient is higher than B.

d) A is more hypoxemic because O2 content is lower than B.

e) The differences balance out and neither A nor B is more hypoxemic than the other.

13) State which one of the following situations would be expected to lower a patient's arterial PO2.

a) anemia
b) carbon monoxide poisoning
c) an abnormal hemoglobin that holds oxygen with half the affinity of normal hemoglobin
d) an abnormal hemoglobin that holds oxygen with twice the affinity of normal hemoglobin
e) lung disease with more than a normal amount of ventilation-perfusion imbalance

14) A 40 year-old patient is admitted to the ICU with the following lab values:

BLOOD GASES
pH: 7.40
PCO2: 38 mm Hg
HCO3: 24 mEq/L
PO2: 88 mm Hg (on room air)

ELECTROLYTES, BUN & CREATININE
Na: 149 mEq/L
K: 3.8 mEq/L
Cl: 100 mEq/L
CO2: 24 mEq/L
BUN: 110 mg%
Creatinine: 8.7 mg%

Which statement best describes the disorder(s)?

a) Normal electrolytes, normal blood gases

b) Abnormal electrolytes and abnormal blood gases

c) Metabolic acidosis

d) Metabolic alkalosis

e) Metabolic acidosis and metabolic alkalosis

15) All of the following are true about cyanosis except:

a) For cyanosis to manifest there needs to be 5 gm% of deoxygenated hemoglobin in the capillaries.
b) Patients with normal hemoglobin manifest cyanosis at higher SaO2 values than patients with anemia.
c) Cyanosis can be caused by excess methemoglobin, which is HbFe+3.
d) For methemoglobin to cause cyanosis, the PaO2 generally has to be <80 mm Hg.
e) Some drugs may cause cyanosis without causing vasoconstriction, or any impairment in PaO2, SaO2, or oxygen content.

16) Since the early 1980s, climbers have summited Mt. Everest without supplemental oxygen. Since the barometric pressure on the summit is only 253 mm Hg, summiting (without extra O2) has only been possible due to prolonged acclimitization at altitude and profound hyperventilation. Indeed, if a a climber maintained PaCO2 of 40 mm Hg and an alveolar-arterial PO2 difference of 5 mm Hg, what would be his/her theoretical PaO2?

a) 25 mm Hg
b) 15 mm Hg
c) 5 mm Hg
d) -5 mm Hg
e) -10 mm Hg

17) All are true about excess carbon monoxide except:

a) shifts the oxygen dissociation curve to the left
b) lowers the PaO2
c) lowers the oxygen saturation
d) lowers the arterial oxygen content
e) is not accounted for when SaO2 is calculated from arterial blood gas measurements

18) A mountain climber ascends from sea level to 18,000 feet over a two day period, without supplemental oxygen. With ascent all of the following factors will decrease except:

a) Fraction of inspired oxygen (FIO2)
b) Barometric pressure
c) PaO2
d) PaCO2
e) Arterial hydrogen ion concentration

19) You are scuba diving to a depth of 99 feet in the ocean, breathing compressed air from a tank. At this depth, compared to the surface, your arterial PO2 will be approximately:

a) the same
b) 2x the surface value
c) 3x the surface value
d) 4x the surface value
e) dependent on amount of air pressure in the tank

20) While all of the following conditions could possibly be managed without measuring arterial blood gases, in which one would blood gases be most helpful?

a) A 40-year-old woman suffering an asthma attack. Her peak expiratory flow rate is 65% of predicted and SpO2 is 95% on room air.

b) A 17-year-old-high school student who presents to the ED with hyperpnea and tachypnea; history reveals he became "excited" during a church service. He has some tetanic contractions of his hands, his lungs are clear and pulse oxygen saturation is 98% on room air.

c) A 68-year-old hypertensive patient has been feeling "weak" for a few days. She has been taking her anti-hypertensive medications. Electrolyte measurements show:
Na+148 mEq/L
K+4.0 mEq/L
CO224 mEq/L
Cl-102 mEq/L

d) A 24-year-old insulin-dependent diabetic comes to the ED, complaining of lethargy; she has not used insulin in several days. Her pulse oximeter oxygen saturation on room air is 98%. Lab values show:
Glucose750 mg%
Na+135 mEq/L
K+4.5 mEq/L
CO210 mEq/L
Cl-100 mEq/L
Urine4+ ketones

e) A 25 year old woman comes to the ED with chest pain. She does not smoke. SaO2 is 96% on room air and V/Q scan is read as low probablity for pulmonary embolism. EKG is normal.

21) The factor 0.863 in the PCO2 equation:

a) equates dissimilar units for CO2 production and alveolar ventilation into mm Hg used for PCO2.
b) accounts for the difference between alveolar ventilation and total or minute ventilation.
c) accounts for the dissolved fraction of carbon dioxide.
d) factors in base excess.
e) is not explained by any of the above.

22) The limit of human hyperventilation is a PaCO2 of about 8 mm Hg. What is the highest PaO2 (mm Hg) a patient with normal lungs could achieve breathing room air (FIO2=.21) at sea level?

a) 100
b) 122
c) 135
d) 142
e) 150

23) A 45 year-old-man is treated in a hyperbaric chamber for severe carbon monoxide toxicity. Assume he is breathing 100% oxygen at 3 atmospheres of pressure, that he has normal lungs, and that hemoglobin=15 gm%, carboxyhemoglobin=40%. What is his approximate arterial oxygen content in ml/dl?

a) 10
b) 13
c) 15
d) 20
e) 21

24) Which one of the following statements is not true?

a) If nothing else changes, as PaCO2 goes up alveolar PO2 and arterial PO2 go down.
b) PaO2 is inversely related to blood pH: as pH goes up PaO2 also increases.
c) If PaCO2 increases while HCO3- remains unchanged, pH always goes down.
d) A high bicarbonate could reflect metabolic alkalosis or respiratory acidosis or metabolic acidosis+metabolic alkalosis.
e) The SaO2 is related to hemoglobin-bound arterial oxygen content on a linear scale (i.e., a straight-line relationship).

25) Which one of the following sets of blood gas values most likely represents a lab or transcription error? (PaCO2 and PaO2 in mm Hg, HCO3 in mEq/L, SaO2 in %. Assume all blood gases drawn at sea level.)

pH PaCO2 HCO3 PaO2 SaO2 FIO2
a) 7.40 75 24 70 75 0.21
b) 7.22 20 8 160 98 0.50
c) 7.59 25 23 60 90 0.28
d) 6.65 265 28 200 96 1.00
e) 7.48 33 24 90 60 0.21

Tie Breaker
(Tie-breaker was used for scoring quiz when first posted for prizes, February 1 - April 30, 2009)

26) A 54-year-old man with chronic obstructive pulmonary disease is seen in the Emergency Department for respiratory distress. Arterial blood gases show: pH 7.38, PaCO2 70 mm Hg, PaO2 38 mm Hg, SaO2 52% on room air (FIO2=.21, at sea level). At the same time, his oxygen saturation measured by pulse oximetry (SpO2) is 65%. Explain, in physiologic terms, reason(s) for both the low PaO2 and low SaO2.

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email: Lawrence Martin, M.D.
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