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Old 09-23-2012
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Arrow Gyrus Daily Questions; Internal Medicine #23

A 24-year-old woman is brought to the emergency room after attempting suicide with an overdose of heroin. On arrival at the emergency department in Jacksonville, FL, she is obtunded and has a respiratory rate of 6 breaths/min. She is hypotensive with a blood pressure of 84/60 mmHg and a heart rate of 80 beats/min. The oxygen saturation is 70% on room air. An arterial blood gas is performed showing the following: pH 7.09, PaCO2 80 mmHg, PaO2 42 mmHg. Which of the following statements is true regarding the patient’s arterial blood gas?


A. The patient is hypoxic due to hypoventilation with an increased A – a (alveolar-arterial) gradient.

B. The patient is hypoxic due to hypoventilation with a normal A – a gradient.

C. The patient is hypoxic due to shunt with an increased A – a gradient.

D. The patient is hypoxic due to ventilation-perfusion(V• /Q• ) mismatch with an increased A – a gradient.
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Answer B

Heroin overdose causing respiratory depression .A-a gradient is normal but the lack of ventilation is causing C02 build up and poor Oxygenation .
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Quote:
Originally Posted by step_enhancer View Post
Answer B

Heroin overdose causing respiratory depression .A-a gradient is normal but the lack of ventilation is causing C02 build up and poor Oxygenation .
Answer is correct..


The patient in this presentation is presenting after a narcotic overdose, which leads to hypoxia because of hypoventilation. The major causes of hypoxemia are hypoventilation, shunt, and V • /Q• mismatch. Diffusing defects can also
cause hypoxemia but are a much less frequent cause. A final cause of hypoxemia to consider is decreased concentration of oxygen in inspired air, which is only present at altitude or in the setting of medical equipment malfunction. When evaluating a patient with hypoxia, it is important to consider whether the alveolar-arterial oxygen gradient is normal or elevated. Of the causes of hypoxia, only hypoventilation and decreased fraction of inspired oxygen will cause hypoxia with a normal A – a gradient. The formula for calculating

the alveolar oxygen concentration is:

PAO2 = ((Patm – PH2O)*(FIO2)) – (PaCO2/R),

where Patm = atmospheric pressure,
PH2O = water vapor pressure,
FIO2 = fraction of inspired oxygen, and
R = respiratory quotient.

When values are substituted assuming usual conditions at sea level and with the patient
breathing room air, the equation is simplified to:

PAO2 = (760 – 47)(0.21) – (PaCO2/0.8) = 150 – PaCO2/0.8

In this patient, the calculated PAO2 is 50. Thus, the A – a gradient is 8 mmHg (normal
value <15 mmHg) and is normal. Thus, the only correct answer is B.
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Quick answers .Nice
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A–a gradient is approximated as: (150 − 5/4(PCO2)) – PaO2 at sea level
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Internal-Medicine-, Pulmonology-, Step-2-Questions

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