The therapist is using a Servo ventilator to ventilate a patient with heliox. How should the therapist anticipate the flow sensors on this ventilator to be affected by heliox?

A. Because the flow sensors operate on the principle of thermal conductivity, they will be affected by heliox.
B. Because the flow sensors are influenced by viscosity, they will be less affected by heliox.
C. They are affected by gas density and will read erroneously.
D. They will sense the flow of heliox accurately.

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.