Breathing Systems

Question 1

Name 5 important breathing systems?

Answer 1

Insufflation
Draw-over
Mapleson circuit
The circle system
The resuscitation system

Question 2

What is insufflation?

Answer 2

Blowing of anesthetic gases across the patients face.
Good for children that don’t want direkt contact with face mask.

Because insufflation avoids direct contact, there is no rebreathing of exhaled gases if flow is high enough.

Question 3

What is draw-over anesthesia?

Answer 3

A draw-over device is non-breathing circuit that uses ambient air as carrier gas, O2 can be supplemented.
Air is withdrawn through low-resistance vaporizer when the patient inspires.

The U-PAC is a draw-over system.

Question 4

What are the main advantages of draw-over anesthesia?

Answer 4

The main advantages of draw-over anesthesia are the simplicity and mobility. This makes it easier to use in locations where compressed gases are not available, eg. battlefields

Question 5

What are common disadvantages of insufflation and draw-over systems?

Answer 5

1. Poor control of inspired gas concentration, and therefore of depth of anesthesia.
2. Mechanical drawbacks during head and neck surgeries.
3. Pollution of OR with waste gas.

Question 6

What are the components of a Mapleson circuit?

Answer 6

1. Breathing tubes
2. Fresh gas inlet
3. Adjustable pressure-limiting valve
4. Reservoir bag

Question 7

How is the compliance of the breathing tubes defined?

Answer 7

Compliance is the change in volume produced in change of pressure.

Question 8

How does the compliance of breathing tubes affect the ventilation?

Answer 8

Compliance is the change in volume produced by a change in pressure. Long tubes with high compliance increase the difference between the volume of gas delivered to the tubes by a reservoir bag or ventilator and the volume of gas actually delivered to the patient.

For example, if a tube with a compliance of 8 ml gas/cm H20 is pressurized to 20 cmH20 during delivery of a tidal volume, 160 ml gas (8 x 20) will stay in the tubes and not be delivered to the patient.

Question 9

What is the function of the APL-valve?

Answer 9

The pressure in the breathing tubes will rise if the inflow of gases is greater than uptake from the patient and circuit. The adjustable pressure-limiting valve allows release of pressure once it exceeds a preset value.

Question 10

How should the APL-valve be set during spontaneous ventilation?

Answer 10

The APL-valve should be fully open during spontaneous ventilation so that circuit pressure remains negligible during inspiration and expiration.

Question 11

What is the function of reservoir bags?

Answer 11

Reservoir bags function as reservoirs of anesthetic gases and a method of generating positive pressure ventilation.

It is designed in a way so that the compliance increases with increase in volume. After the 3L barrier is reached (phase I) pressure rises rapidly to a peak (phase II). A further increase in volume leads to a plateau and even decrease in pressure. This is a good backup safety if the APL-valve is left completely closed by mistake.

Question 12

How is breathing circuit efficacy measured?

Answer 12

Breathing-circuit effciency is measured by the fresh gas ow required to reduce CO2 rebreathing to a negligible value.

Question 13

How is rebreathing avoided in the Mapleson circuit.

Answer 13

Because there are no unidirectional valves or CO2 absorption in Mapleson circuits, rebreathing is prevented by adequate fresh gas flow into the circuit and venting exhaled gas through the APL valve before inspiration. There is usually some rebreathing in any Mapleson circuit. The total fresh gas flow into the circuit controls the amount. To attenuate rebreathing, high fresh gas flows are required. The APL valve in Mapleson A, B, and C circuits is located near the face mask, and the reservoir bag is located at the opposite end of the circuit.

Question 14

How does Mapleson circuit function during spontaneous ventilation?

Answer 14

During spontaneous ventilation, alveolar gas containing CO2 will be exhaled into the breathing tube or directly vented through an open APL valve. Before inhalation occurs, if the fresh gas flow exceeds alveolar minute ventilation, the in ow of fresh gas will force the alveolar gas remaining in the breathing tube to exit from the APL valve. If the breathing tube volume is equal to or greater than the patient’s tidal volume, the next inspiration will contain only fresh gas. Because a fresh gas ow equal to minute ventilation is sufficient to prevent rebreathing, the Mapleson A design is the most efficient Mapleson circuit for spontaneous ventilation.

Question 15

What is the difference between a Mapleson A and a Mapleson D circuit?

Answer 15

The places of the FGI and the APL-valve are interchanged.

Question 16

Why is the Mapleson D circuit efficient during controlled ventilation?

Answer 16

The Mapleson D circuit is efficient during controlled ventilation because the fresh gas forces alveolar air away from the patient and towards the reservoir bag and the APL-valve. This alters the fresh gas requirements during controlled ventilation (not as high flow needed as in MApleson A)

Question 17

What are the components of a circular breathing system?

Answer 17

1. Fresh gas inlet.
2. CO2 absorbing filter
3. Inspiratory unidirectional valve and breathing tubes
4. Y-connector
5. Expiratory unidirectional valve and breathing tubes
6. APL valve
7. Reservoir

Question 18

What qualities does the circular system has that the Mapleson and insufflation systems lack?

Answer 18

1. Rebreathing of alveolar air
2. Conservation of heat and humidity.
3. Good anesthetic depth control

Question 19

What is the most common CO2 absorber?

Answer 19

Soda lime. Its components are calcium hydroxide (Ca(OH)2), water, and sodium hydroxide (NaOH)

Its reactions are as follows:

CO2 + H2O → H2CO3

H2CO3 + 2NaOH → Na2CO3 + 2H2O + Heat
(a fast reaction)

Na2CO3 + Ca(OH)2 → CaCO3 + 2NaOH (a slow reaction)

Question 20

When should a CO2 absorber be changed?

Answer 20

When 50-70% has changed color.

Question 21

What can be a danger with the degradation of volatile gases in CO2 absorber?

Answer 21

Volatile gases can be degraded to carbon monoxide by dry absorbent to such degree it can cause clinically significant carbon monoxide poisoning. This is highest with sevoflurane and desflurane.

Question 22

What is the main advantage of the CO2 absorber in the circle system?

Answer 22

It allows rebreathing of exhaled gas after it has been filtered of CO2.

With an absorber, the circle system prevents
rebreathing of CO2 at reduced fresh gas flows (≤1 L) or even fresh gas flows equal to the uptake of anesthetic gases and oxygen by the patient and the circuit itself (closed-system anesthesia). At fresh gas flows greater than 5 L/min, rebreathing is so minimal that a CO2 absorber is usually unnecessary.

Question 23

How is a resuscitation breathing circuit different than a Mapleson or circle systems?

Answer 23

It contains nonrebreathing valve. The patient valve opens during spontaneous or controlled ventilation to allow gas to the mask from the ventilation bag. It prevents rebreathing by releasing the exhaled air out to the atmosphere.

Question 24

What are the main disadvantages of a resuscitator?

Answer 24

They require high oxygen flows to achieve high FiO2.

The maximum achievable tidal volumes are less than those that can be achieved with a system that uses a 3-L breathing bag. In fact, most adult resuscitators have a maximum tidal vol- ume of 1000 mL.