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Arterial and Venous Blood Gases: Indications, Interpretations, and Clinical Applications

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October 2009 (Vol 31, No 10)

NOTE: You have already passed this course.

NOTE: This course is no longer valid for accreditation purposes.

Arterial and Venous Blood Gases: Indications, Interpretations, and Clinical Applications

1. A pH of 7.40 means the

0
HCO₃^- concentration is within normal limits.
PCO₂ is within normal limits.
patient does not have an acid-base disorder.
HCO₃^-:PCO₂ ratio is normal.

2. A simple metabolic acidosis is characterized by a low pH and a

0
decreased BEecf, HCO₃^- concentration, and PCO₂.
normal BEecf and HCO₃^- concentration and increased PCO₂.
decreased TCO₂.
normal BEecf and HCO₃^- concentration and decreased PCO₂.

3. Hypoventilation (↑Pco₂) is potentially dangerous because it

0
correlates with a faster respiratory rate and an increased respiratory effort.
causes an increase in the A-a gradient.
by itself causes the pH and the PaO₂ to decrease.
is unresponsive to oxygen supplementation.

4. The BEecf is the most effective parameter to measure the metabolic component of acid-base disorders because

0
it is the only useful parameter when calculating the amount of sodium bicarbonate to administer to severely acidemic patients.
it is a direct measurement, not a calculation.
it standardizes for the respiratory contribution (PCO₂ of 40 mm Hg) to the acid-base balance.
a negative value always indicates a metabolic acidosis, not metabolic compensation for a respiratory alkalosis.

5. Simple acid-base disturbances are typically characterized by which of the following?

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an abnormal pH with PCO₂ and HCO₃^- concentration values changing in opposite directions (e.g., ↓HCO₃^- and ↑PCO₂)
an acid-base disorder (metabolic or respiratory in origin) and no apparent change in the opposing system
a neutral pH (7.4) with abnormal PCO₂ or HCO₃^- concentration values
an acid-base disorder (metabolic or respiratory in origin) and a quantifiable parallel compensation in the opposing system (e.g., ↓HCO₃^- and ↑PCO₂)

6. Consider the following:
Jack, an adult Labrador retriever, presented with a 2-day history of weakness, excessive thirst, and urination. On examination, he was found to be 8% to 10% dehydrated. Please explain Jack's acid-base status from his initial venous blood gas results:

The expected respiratory compensation for this metabolic acidosis (within a margin of variance of ±3 mm Hg) can be calculated as follows:

Expected Pco₂ decrease from midpoint reference value = ∆ HCO₃^- (decrease in HCO₃^- concentration from a midpoint reference value associated with the metabolic acidosis) × 0.7 (expected mm Hg decrease in Pco₂ for each 1.0 mEq/L decrement in HCO₃^-)

In this case: (22 - 5.4) × 0.7
16.6 × 0.7 = 11.6 mm Hg

Expected Pco₂ = Pco₂ midpoint reference range - Expected change in Pco₂

In this case:
38 mm Hg - 11.6 = 26.4 mm Hg

Therefore, Jack's expected Pco₂ is 26.4 mm Hg, and his measured Pco₂ is 24.2 mm Hg.

After reviewing Jack's measured and expected Pco₂ values, we can conclude that Jack's respiratory compensation for his metabolic acidosis is

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adequate (within the ±3 margin of variance).
inadequate (outside the ±3 margin of variance).
acute.
chronic.

7. Consider the following: Bonbon, a shih tzu puppy, presented with a 2-day history of vomiting and anorexia. On physical examination, a foreign body was palpated in the cranial abdomen. Please explain Bonbon's acid-base status from her initial venous blood gas results: Bonbon's metabolic alkalosis could be explained by

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an upper GI obstruction with loss of gastric juices in the vomitus.
pain-associated hypoventilation.
compensation for the primary respiratory acidosis.
lactic acidosis from shock.

8. Consider the following:
Please explain the acid-base status of Lucy, a geriatric dog with a 3-day history of progressive increased respiratory effort. On presentation, Lucy's temperature was 103.6°F (39.8°C), her respiratory rate was 42 breaths/min, and her heart rate was 140 bpm. Her physical examination revealed inspiratory stridor on auscultation.

The expected metabolic compensation for this chronic respiratory acidosis (within a margin of variance of ± 2 mEq/L) can be calculated as follows:

Expected HCO₃^- increase from midpoint reference value = ∆ Pco₂ (increase in Pco₂ from midpoint reference range associated with the respiratory acidosis) × 0.35 (expected mEq/L increase in HCO₃^- for each 1.0 mm Hg increment in Pco₂):

In this case: (67.6 - 38) × 0.35
29.6 × 0.35 = 10.4 mEq/L

Expected HCO₃^- = HCO₃^- midpoint reference range + Expected increase in HCO₃^-

In this case:
22 + 10.4 = 32.4 mEq/L

Therefore, Lucy's expected HCO₃^- is 32.4 mEq/L, and her measured HCO₃^- is 30.4 mEq/L.

Lucy's acid-base status can be described as

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a mixed acid-base disorder.
respiratory acidosis with adequate metabolic compensation (within the ±2 margin of variance).
metabolic alkalosis with respiratory compensation.
respiratory acidosis with no compensation (outside the ±2 margin of variance).

9. Consider the following: Please explain Lucy's oxygenation status from her arterial blood gas results. The sample was collected before oxygen supplementation was instituted. The a-a gradient calculation is: Pao₂ = 150 - 1.2 (Paco₂) Pao₂ = 150 - 1.2 (67.6) Pao₂ = 68.9 a-a = Pao₂ - Pao₂ a-a = 68.9 - 62.1 = 6.8 Lucy's respiratory evaluation reveals

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hypoxemia secondary to ventilation-perfusion abnormalities and hypoventilation.
hypoxemia secondary to hypoventilation only.
hypoxemia secondary to ventilation-perfusion abnormalities only.
hypoxemia secondary to pulmonary disease.

10. Consider the following: Bob is a young adult mixed-breed dog that presented after being hit by a car. Physical examination revealed extensive abrasions on his thorax. His temperature was 102.8°F (39.4°C), his respiratory rate was 62 breaths/min, and his heart rate was 140 bpm. Please explain Bob's oxygenation status from the arterial gas data collected on presentation: The a-a gradient is: Pao₂ = 150 - 1.2 (Paco₂) Pao₂ = 150 - 1.2 (25.9) Pao₂ = 119 a-a = Pao₂ - Pao₂ a-a = 119 - 63 = 56 Bob's respiratory evaluation reveals

0
hypoxemia secondary to ventilation-perfusion abnormalities and hypoventilation.
hypoxemia secondary to hypoventilation only.
hypoxemia secondary to ventilation-perfusion abnormalities only.
hypoxemia secondary to decreased inspired oxygen.

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