Anesthesiology Written Board Exam Sample Questions
Book 1 - 2000
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QUESTION

47. How long after the termination of an anesthetic that included nitrous oxide, should you be concerned about diffusion hypoxia?

    A)1 minute
    B) 2-3 minutes
    C) 5-10 minutes
    D) 15-20 minutes
    E) 30 minutes

ANSWER

47. C 5-10 minutes

Diffusion hypoxia can occur whenever nitrous oxide (N2O) is used to supplement an anesthetic.  N2O has a low blood:gas coefficient (i.e., low blood solubility) which means that, during induction, the blood and alveoli rapidly equilibrate. After N2O is discontinued, the diffusion gradient reverses, filling the alveoli with large amounts of N2O, thereby, displacing oxygen. Breathing room air during this time can cause alveolar hypoxia.

The peak effect of this phenomenon occurs about 2-5 minutes after the cessation of N2O, and lasts approximately 5-10 minutes.  The large influx of N2O also displaces alveolar carbon dioxide, thereby decreasing ventilatory drive, and potentially worsening the hypoxia.

1. Stoelting R.  Pharmacology and Physiology in Anesthetic Practice.  2nd Edition.  J.B. Lippincott Co. Philadelphia PA.  1991:30.

2. Miller R: Anesthesia.  5th Edition.  Churchill-Livingstone.  New York. 2000:583-4.

 

QUESTION

63. Which of the following is/are hypobaric spinal anesthetic(s)?

    A) 0.5% bupivacaine in saline
    B) 2% lidocaine in saline
    C) 2.5% procaine in distilled water
    D) 0.5% tetracaine in 5% glucose
    E) All of the above

ANSWER

63. C 2.5% procaine in distilled water

Distilled water is used to prepare hypobaric solutions. Hypobaric spinal anesthetics have a density of less than 0.9998 and a baricity (density of the local anesthetic / density of CSF) of less than 1.0000. Hypobaric solutions gravitate to the least dependent areas of the subarachnoid space.

CSF, with a density of 0.9998-1.0008 has a baricity of 1.0000 and is, therefore, isobaric.  Isobaric solutions tend to remain in the area of injection and are unaffected by the shape of the spinal canal. Most less-concentrated solutions mixed with saline are isobaric (e.g., 2% lidocaine in saline).  Diluting local anesthetic solutions with cerebrospinal fluid will also make them isobaric.

Glucose is added to make a hyperbaric solution (density of greater then 1.0008). Hyperbaric solutions gravitate to the most dependent areas of the subarachnoid space.

1. Miller R. Anesthesia.  5th Edition.  Churchill-Livingstone.  New York. 2000:1505.

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