The Anesthesia Machine

Question 1

What is the single most common source of injury to the anesthesia machine?

Answer 1

The breathing circuit (40%).

Other sources include vaporizers, ventilators, oxygen supply.

Question 2

What are the basic functions of an anesthesia machine?

Answer 2

In its most basic form, the anesthesia machine receives medical gases from a gas supply, controls the flow and reduces the pressure of desired gases to a safe level, vaporizes volatile anesthetics into the final gas mixture, and delivers the gases to a breathing circuit connected to the patient’s airway.

Question 3

How many gas inlet pressure gauges does an anesthesia machine have?

Answer 3

2 for each gas, one for pipeline pressure and one for cylinder pressure.

Question 4

What are the power outlets that modern machines possess?

Answer 4

Electric and pneumatic (oxygen). They can be used to drive the ventilator.

Question 5

How do cylinders attach to the anesthesia machine?

Answer 5

Via pin index safety system. This is to prevent attachment of the wrong gas.

Question 6

How is cylinder gas pressure regulated?

Answer 6

The pressure in cylinders is high and variable, with makes control difficult and dangerous. A full cylinder of =2 at room temp has a pressure of 1900 psi.

The machine utilizes a pressure regulator to reduce pressure of cylinder gas to 45-47 psig. This pressure is slightly lower than that of pipeline supply, which allows use of pipeline supply if the cylinder is left open.

Question 7

Describe the oxygen supply failure protection devices.

Answer 7

Oxygen can pass directly to its flow control valve, whereas other gases cannot. The devices permits administration of other gases only in the presence of oxygen. This is to prevent delivery of hypoxic gas mixture via a cut-off valve.

Most modern machines use proportionating safety device instead of cut-off valves. This reduces the pressure of other gases when O2 pressure drops, but not for air. They completely shut off pass of gases after O2 drops below a set minimum pressure point)

Question 8

What is the function of the flow valves?

Answer 8

Once the pressure has been reduced to a safe level, each gas must pass through flow control valves and is measured by flowmeters before mixing with other gases.

Gas lines proximal to flow valves are considered to be in the high- pressure circuit whereas those between the flow valves and the common gas outlet are considered part of the low-pressure circuit of the machine.

Question 9

What is the oxygen/nitrous oxide ratio controller?

Answer 9

It is a safety devise that ensures minimum oxygen concentration och 25%.

Question 10

Where in the anesthesia machine are vaporizers located?

Answer 10

Between the flow meters and the common gas outlet.

Question 11

Define boiling point of liquids

Answer 11

It is the temperature at which a liquids vapor pressure is equal to the atmospheric pressure.

As the atmospheric pressure decreases (as in higher altitudes), the boiling point also decreases. Anesthetic agents with low boiling points are more susceptible to variations in barometric pressure than agents with higher boiling points. Among the com- monly used agents, des urane has the lowest boiling point (22.8°C at 760 mm Hg).

Question 12

What are electronic vaporizers used for?

Answer 12

Desflurane. It has a relatively high vapor pressure, which makes it hard to control at room temperatures.

Some machines use electronic vaporizers for all gases.

Question 13

What is the function of the oxygen flush valve?

Answer 13

It delivers oxygen at high flow rates (35-75 L/min) directly to the common gas outlet, bypassing the flow meters and vaporizers.

It is used to rapidly refill or flush the breathing circuit.

The oxygen has same pressure as the source (45-55 psi) and should therefore be used carefully if patient is connected to the circuit due to risk of barotrauma.

Question 14

What are spirometers?

Answer 14

Devices that measures the tidal volumes of expiratory gas, typically near the expiratory valve,

Question 15

Where can the most accurate inspiratory and expiratory pressures of the circuit be measured?

Answer 15

At the Y-connector. The pressure in the breathing circuit reflects the airway pressure if it is close enough to the patient.

Question 16

What can a rise in airway pressure indicate?

Answer 16

Worsening pulmonary compliance, rise in tidal volume, or obstruction at the level of breathing circuit, tracheal tube och patients airway.

Question 17

What can a fall in airway pressure indicate?

Answer 17

Increase pulmonary compliance, decrease in tidal volume, or leak in the circuit.

Question 18

How should the APL valve be adjusted during spontaneous ventilation?

Answer 18

It should be fully open.

During manual or assisted bag ventilation it must be partially closed.

Question 19

How are absolute and relative humidity defined?

Answer 19

Absolute humidity is the weight of water in 1 L of gas.

Relative humid- ity is the ratio of the actual mass of water present in a volume of gas to the maximum amount of water possible at a particular temperature.

Question 20

Why is humidification of inspiratory gases important?

Answer 20

Usually air is warmed and humidified in the upper respiratory tract. When administering gases through tracheal tube directly to lower respiratory tract, the humidification och the gases can result in dehydration och the mucosa and altered ciliary function.

Body heat is also lost while heating cold gases.

Question 21

What patient populations will benefit most of humidifying and heating of inspiratory gases?

Answer 21

Pediatric patients and older patients with underlying lung diseases.

Question 22

How does heat moisture exchanger work?

Answer 22

The HME is a passive humidifying device. It works by trapping water and heat from exhaled gases and releases it again to inhaled gases.

Heat and moisture exchanger (HME) functions as an “artificial nose” that attaches between the tracheal tube and the right-angle connector of the breathing circuit.

Question 23

How do active humidifiers work?

Answer 23

Active humidi ers are more e ective than passive ones in preserving moisture and heat. Active humid- i ers add water to gas by passing the gas over a water chamber (passover humidi er) or through a satu- rated wick (wick humidi er), bubbling it through water (bubble-through humidi er), or mixing it with vaporized water (vapor-phase humidi er). Because increasing temperature increases the capacity of a gas to hold water vapor, heated humidi ers with ther- mostatically controlled elements are most e ective.

Question 24

What are the hazards of active humidifiers?

Answer 24

e hazards of heated humidi ers include ther- mal lung injury (inhaled gas temperature should be monitored and should not exceed 41°C), noso- comial infection, increased airway resistance from excess water condensation in the breathing cir- cuit, interference with owmeter function, and an increased likelihood of circuit disconnection

Question 25

How do ventilators work?

Answer 25

Ventilators generate gas ow by creating a pres- sure gradient between the proximal airway and the alveoli.

Question 26

What types of inspiratory generators are there in ventilators?

Answer 26

1. Constant pressure generators
2. Constant gas flow generators
3. Non-constant generators.

Question 27

How does a ventilator generate expiration?

Answer 27

Expiration is a passive movement. The ventilator reduces the airway pressure to atmospheric pressure or a preset PEEP.

Question 28

What measurement can be indication of dynamic compliance?

Answer 28

Peak inspiratory pressure. It is the highest circuit pressure generated during an inspiration.

Question 29

What measurement can be an indication of static compliance?

Answer 29

Plateau pressure. It is the pressure measured during inspiratory pause (the time of no gas flow) aka breath hold.

Question 30

What can cause increase in peak inspiratory pressure with an increase in plateau pressure?

Answer 30

Changes in both dynamic (circuit) and static (lung) compliances.

Increased tidal volume. (circuit/dynamic)

Decrease pulmonary (static) compliance:
Pulmonary edema
Trendelenburg position
Pleural effusion
Ascites
Abdominal packing Peritoneal gas insufflation
Tension pneumothorax
Endobronchial intubation

Question 31

What can cause increase in peak inspiratory pressure without increase in plateau pressure?

Answer 31

Basically changes in the dynamic compliance, which lies in the circuit.

Increased gas flow rate

Increased airway resistance:
Kinked endotracheal tube
Bronchospasm
Secretions
Foreign body aspiration
Airway compression
Endotracheal tube cuff herniation

Question 32

When should the oxygen flush valve NOT be used and why?

Answer 32

During the inspiratory cycle of the ventilation.

Because the ventilator spill valve will be closed and the APL valve is excluded; the surge of oxygen (600–1200 mL/s) and circuit pressure will be transferred to the patient’s
lungs.