Common breathing systems

Question 1

what is normal peak inspiratory flow rate?

Answer 1

30 - 40 L/minute

Question 2

Is it practical to supply a 30 - 40 L/m in flow rate to a patient to account for the possible peak inspiratory flow rate. What is done to avoid this?

Answer 2

NO.

Addition of a reservoir bag –> this prevents the creation of a negative pressure in the breathing system during peak inspiration and allows lower fresh gas flows

Question 3

How is prevention of CO2 rebreathing accomplished?

Answer 3

1. High enough FGF to flush exhaled CO@ from the system (dependent on circuit characteristics)
2. A 1-way system that passes exhaled gases through a canister that removes CO2 chemically (circle systems)

Question 4

What should APL be set at for spontaneous ventilation and manual ventilation

Answer 4

Spontaneous ± 1 -2 cmH2O

Manual ventilation ± 15 - 20 cmH2O

Question 5

At what pressure will the valve always open even if completely closed

Answer 5

60 cm H2O - safety

Question 6

What is the defining feature all Mapelson circuits?

Answer 6

These circuits rely on FGF to wash out CO2

Question 7

What is the Mapelson A circuit?

What is its main advantage and disadvantage

Answer 7

Also known as the Magill system

FGF –>RB –> BT –> APL –> Patient

Advantage: More suited to spontaneous ventilation as low FGF required to prevent CO2 rebreathing. APL must be closed during controlled ventilation which means that higher FGF required to washout CO@ from the patient end of the circuit.

Disadvantage: Cumbersome to connect scavenging system to the exhaust valve near the patient’s airway

Question 8

What system was designed to overcome the cumbersome position of the APL valve in the Mapelson A circuit?

Answer 8

The Lack System is a co-axial Mapelson A system. The expiratory limb of the breathing system extends within the inspiratory limb with two advantages:

1. The APL valve can be connected distal o the patient
2. Counter current heat exchange system is established for warming inhaled air.

Question 9

What is a Mapelson D circuit

Answer 9

RB–>APL–>BT–>FGF–>patient

More suited to controlled ventilation than spontaneous breathing.

Question 10

How is efficiency of a non-circle system breathing circuit determined

Answer 10

The minimum fresh gas flow required to prevent rebreathing of CO2.

Mapelson A - most efficient for spontaneous Ventilation (FGF = 1 x VE)

Mapelson D - most efficient for controlled ventilation
(FGF = 1.5 x VE)

Question 11

What is Mapelson E and a Mapelson F circuit

Answer 11

Otherwise known as the Ayre’s T-piece, this is a lightweight open ended circuit with FGF near the patient.

BT –> FGF –> patient

Mapelson F or the Jackson-Rees modification of the Ayre’s T-piece adds a valveless open ended reservoir bag to the Ayre’s T-Piece

Open ended RB –> BT –> FGF –> PT

Question 12

What is the Humphrey ADE circuit?

Answer 12

This is a system that provides the ability to interchange between Mapelson A, D and E arrangements

Question 13

Describe the Mapelson B circuit

Answer 13

RB –> BT –> FGF –> APL –> PT

Rarely used

Question 14

Describe the Mapelson C circuit and its use and why alternatives are more effective

Answer 14

RB –> FGF –> APL –> PTUsed for resuscitation BUT a self-inflating bag is better because it does not rely on FGF for ventilation

Question 15

For a 70 kg person, define the minimum fresh gas flow required to clear CO2 adequately when breathing SPONTANEOUSLY for Mapelson A, C, D.

Answer 15

A - 5L/minute (VE)

C - >14 L/minute (>2 x VE)

D - 14 L/min (2 x VE)

Question 16

What are the two additional components required in a circle system

Answer 16

Two unidirectional valves

CO2 absorber (soda lime)

Question 17

What does the increased complexity of the circle system cause

Answer 17

More connections which increases the possibility for errors or disconnection

Question 18

What is the theoretical O2 flow rate required in a perfect circle system that is fully equilibrated with anaesthetic gases. how is this different in practice

Answer 18

Matching the patient’s O2 consumption: 250 ml/minute

This is different in practice as equilibration is rarely reachedand the circle system may have a number of small leaks –> a higher flow of 1 L/min O2 is used

Question 19

When during an anaesthetic are high fresh gas flows required

Answer 19

1. At the start –> rapid uptake phase by patient
2. If rapid changes of vapour concentration are required
3. At the end of a case to washout the anaesthetic agents