Free Anesthesiology Assistant Sample Questions
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First Free Anesthesiology Assistant Question
The alveolar ventilation (VA) and partial pressure of carbon dioxide (PACO2) of a mechanically ventilated patient is represented by the black circle in the image below. If alveolar ventilation increases by changing the respiratory rate, which of the following BEST describes the curve that would result?
- A. A
- B. B
- C. C
- D. D
- E. E
The Answer and Explanation
Did you get it right? The correct answer is: C
As VA increases, PACO2 decreases non-linearly, asymptotically approaching zero.
Alveolar ventilation is the volume per unit time of gas that participates in gas exchange. It can be related to alveolar carbon dioxide in the following manner. Since carbon dioxide (CO2) is so readily diffusible across the alveolar membrane, it is assumed that the alveolar and arterial partial pressures of CO2 are equal (PACO2 = PaCO2). The elimination of CO2 (VCO2) is therefore directly proportional to the product of the alveolar ventilation and alveolar carbon dioxide partial pressure: VCO2 α VA * PACO2.
Solving for PACO2, it is rewritten as:
PACO2 = 0.863 * VCO2 / VA
where PACO2 is expressed in mm Hg, VCO2 in mL/min, and VA in L/min. The constant 0.863 adjusts for this units conversion.
At steady state, the elimination of CO2 equals its production. For a 70-kg resting adult, VCO2 is 200 mL/min, assuming an oxygen consumption of ~3.5 mL/min/kg and a respiratory quotient (R) of 0.8. Therefore, the equation above would appear as:
PACO2 ≈ 173 / VA
If this is plotted on the axes in the Figure below, the curve (red) that results is of the form y = k / x, which is a rectangular hyperbola; i.e. a curve that decreases rapidly at first and then gradually, and is asymptotic to both vertical and horizontal axes.
Physiologically, this relationship makes sense. At low ventilation rates, PACO2 is high. Increasing ventilation and therefore the amount of CO2 that is removed per unit time decreases alveolar CO2 content; however, since CO2 is constantly produced, even at high ventilation rates the alveolar content can never be nil.
If CO2 production is held constant, then as ventilation changes the PACO2 changes along the curve. If CO2 production increases, then the curve is shifted to the right (Figure, green). Exertion, sepsis, fever, and an increased carbohydrate fraction in diet or total parenteral nutrition can all increase CO2 production.
Even though the stem only depicted one section of the graph, it is important to be familiar with the entire shape of the curve and how it may vary.
Figure. Alveolar partial pressure of carbon dioxide (PACO2, mm Hg) as a function of alveolar ventilation (VA, L/min) for a sample patient. As VA increases, PACO2 decreases along a hyperbolic curve. The red curve represents a basal CO2 production of 200 mL/min. If CO2 production increases, the curve is shifted to the right (green).
Incorrect Answer Explanations
Answer A: PACO2 decreases, not increases as alveolar ventilation increases.
Answers B: A right shift of the curve would occur if CO2 production increased, not with a change in ventilation.
Answer D: Although sometimes the relationship between VA and PACO2 is approximated to be linear over small intervals, it is in fact not so, and the curve can never cross zero.
Answer E: A right shift of the curve would occur if CO2 production increased, not with a change in ventilation.
Bottom Line
PACO2 and VA have an inverse relationship: as VA increases, PACO2 decreases rapidly at first and gradually later. Increased CO2 production causes a right-shift of the curve.
For more information:
ABA Gaps In Knowledge Report, ITE 2016. “As minute ventilation increases linearly in a mechanically ventilated patient, PACO2 decreases asymptotically, with a more rapid decline initially.”
American Board of Anesthesiology Keyword. “Relationship Of Alveolar Vent To PaC (A)”
Miller Anesthesia. 8th edition. Pages 447, 2930.
Second Free Anesthesiology Assistant Question
The medial calf has sensory innervation from which of the following nerves?
- A. Femoral
- B. Sciatic
- C. Sural
- D. Tibial
The Answer and Explanation
Did you get it right? The correct answer is: A
The femoral nerve supplies cutaneous innervation via its branches to the anteromedial thigh (anterior cutaneous branches of the femoral nerve) and the medial side of the lower leg and foot (saphenous nerve).
Incorrect Answer Explanations
Answer B: The sciatic nerve innervates the skin of the foot, as well as most of the lower leg (except for its medial side), through its two branches: the common peroneal and the tibial nerves.
Answers C: The sural nerve provides sensory innervation to the lateral posterior portion of the leg.
Answer D: The tibial nerve is a branch of the sciatic nerve. Its sensory innervation includes the heel and plantar regions of the foot.
Bottom Line
The medial lower leg is innervated by the saphenous nerve, which is a branch of the femoral nerve.
For more information:
American Board of Anesthesiology Keyword. “Lower Extremity Sensory Anatomy (B)”
Miller’s Anesthesia. Chapter 57: Peripheral Nerve Blocks. 8th Edition. Pages 1738-1748.
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