Physics and the Anesthesia Machine

Question 1

What is the pressure and volume in a full oxygen cylinder?

Answer 1

2200 psi

700 L

Question 2

What is the pressure and volume in a full nitrous oxide cylinder?

Answer 2

750 psi

1600 L

Question 3

What is the pressure from the wall source of oxygen or nitrous oxide?

Answer 3

50 psi

Question 4

What does the first stage pressure regulator bring the pressure from the O2 and nitrous oxide tanks down to?

Answer 4

45 psi

Question 5

When does the pressure in a nitrous oxide tank start to fall?

Answer 5

When it is 75% depleted (i.e., 400 L)

Question 6

What does the O2 fail-safe valve do?

Answer 6

Prevents delivering a hypoxic mixture by decreasing the flow of all other gases

Question 7

What are the risks of using the O2 flush during positive-pressure ventilation?

Answer 7

barotrauma - unless an inspiratory pressure limit is set

volutrauma - if used during inspiration when pressure relief valve is CLOSED

Question 8

What indicates a possible hole in the bellows?

Answer 8

high tidal volumes
high flows
high FiO2 (in machines using 100% O2 to drive the bellows)

Question 9

Is laminar gas flow more dependent on viscosity or density of the gas? Turbulent flow?

Answer 9

laminar: viscosity-dependent
turbulent: density-dependent

Question 10

What will you notice on the ventilator with a malfunctioning inspiratory valve? Expiratory valve?

Answer 10

increased inspired CO2 for both

Question 11

What is the active base in CO2 adsorbents?

Answer 11

calcium hydroxide

Question 12

What is the flow rate when you use the O2 flush?

Answer 12

35-75 L/min at 50 psi

Question 13

What is the primary determinant of vapor pressure?

Answer 13

temperature

Question 14

Why do vaporizers need a temperature compensation mechanism?

Answer 14

• As more molecules of volatile anesthetic are vaporized, temperature in the vaporizer will drop.
• As temperature drops, so does vapor pressure.

Question 15

How do very low fresh gas flow rates affect vaporizer output? Why?

Answer 15

Decreases output because flow is too slow to pull up the vapor.

Question 16

How do very high fresh gas flow rates affect vaporizer output? Why?

Answer 16

Decreases output because turbulent flow doesn’t pick up the vapor effectively.

Question 17

Does using nitrous oxide as a carrier gas increase or decrease vaporizer output?

Answer 17

decrease

Question 18

List common anesthetic gases in order of increasing vapor pressure.

Answer 18

sevoflurane (160 torr) < enflurane (172) < isoflurane (240) < halothane (243) < desflurane (680)

Question 19

How should you adjust a variable bypass vaporizer at high altitude? Below sea level?

Answer 19

Leave the setting the same because the partial pressure of gas delivered won’t change (e.g., for sevoflurane and isoflurane).

Question 20

How should you adjust a desflurane vaporizer at high altitude? Below sea level?

Answer 20

Turn up the dial at high altitude and turn down the dial below sea level to achieve the same partial pressure that you would at sea level.

Question 21

How does mesh size affect airflow resistance through the CO2 absorbent canister?

Answer 21

Increasing mesh size -> smaller particles -> increased resistance to airflow