Answer to Question 1
ANS: B
A. Incorrect response: See explanation B.
B. Correct response: Heliox administration via mechanical ventilation can be problematic. Ventilators are designed to deliver a mixture of air and O2. The different density and viscosity of helium can affect the delivered Vt and the exhaled Vt. For some ventilators, no reliable Vt is delivered with heliox. For other ventilators, a Vt higher than desired may be delivered. This problem can be circumvented partially by selecting pressure ventilation rather than volume ventilation. Unlike flow sensors, pressure sensors are not affected by a different gas composition.
C. Incorrect response: See explanation B.
D. Incorrect response: See explanation B.
Answer to Question 2
ANS: B
A. Incorrect response: See explanation B.
B. Correct response: The effect of heliox on the ability of the ventilator to correctly monitor flow and Vt depends on the method that is used for this measurement. Monitoring devices that are density dependent are inaccurate in the presence of heliox. Devices that use the principle of thermal conductivity are also affected. However, devices that are influenced by gas viscosity rather than by gas density are affected to a lesser degree because the viscosity of helium is only slightly different from that of air or O2. Screen pneumotachometers such as those used in the Servo ventilators are affected by viscosity rather than density. Thus, the flow sensors in these ventilators are affected to a lesser degree than occurs in other ventilators.
C. Incorrect response: See explanation B.
D. Incorrect response: See explanation B.