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Central supply of O2 & N2O

  1. O2 & N2O are supplied at 60 psi through central supply.
  2. Colour of Cylinders Pressure Filled as
    a. O2 – Black body with white shoulders 2000 psi gas
    b. N2O – Blue 760 psi liquid
    c. CO2 – Grey 750 psi liquid
    d. Cyclopropane – orange 75 psi liquid
    e. Helium – Brown 1400 psi gas
    f. Air – Grey body with black & white shoulders 2000 psi gas
    g. Entonox - Blue body with blue & white shoulders
    (50% O2 + 50% N2O)
    2000 psi gas
  3. Note: Gas which are there in liquid form in cylinders: N2O, CO2, Cyclopropane (N.C.C.)
    For agents in liquid form contents does not correspond with pressure
  4. Pin index system
    Machines are usually equipped with one or two E type cylinders that hang on specific hanger yokes. The medical gas pin-index safety system ensures that the correct medical gas cylinder is hung in the correct yoke. The system consists of two pins that are fixed in the yoke, and which fit into two corresponding holes in the cylinder valve. The two pins are in a unique configuration for each gas and should never be removed from the hanger yoke. Specific pin configurations exist for each of the medical gases supplied in small cylinders in order to prevent erroneous misconnections of gas supplies

i. Oxygen 2,5
ii. N2O 3,5
iii. Air 1,5
iv. Nitrogen 1,4
v. Cyclopropane 3,6
vi. Entonox 7
vii. Carbon dioxide 2,6

Anesthesia Machine

The anaesthesia machine can be conveniently divided into three parts: (a) The high pressure system, which receives gases at cylinder pressure, reduces the pressure and makes it more constant, (b) the intermediate pressure system, which receives gases from the regulator or hospital pipeline and delivers them to the flow meters or O2 flush valve and (c) the low pressure system, which takes gases from the flow meters to the machine outlet and also contains the vapourisers.
Anaesthesia Machine
High pressure system Intermediate pressure system Low pressure system
1. Cylinder 1. From Yoke to flow control valve 1. Downstream to flow control valve
2. Pressure regulator 2. O2 failure alarms  
3. Yoke assembly 3. Pressure reducing valve  

Low pressure system

  1. Flow control valves colour coded
    1. O2 – white
    2. N2O – Blue
    3. CO2 – Grey
Extra Edge
  1. Rotameter is used to measure flow of gases
  2. Flow meters (also k/a as Thorpe tube) are of variable orifice with smallest diameter at base. It contains an indicator K/a BOBIN which is made of aluminum and the upper level of bobbin indicates the flow. The position of the flow meters of individual gases is also important. O2 flow meter should be downstream to all other gases to prevent hypoxic mixture delivered to the patient
  3. (a) Arrangement of flow meters with nitrous oxide (N2O) upstream, leak in the middle tube resulting in N2O flowing out but oxygen flow intact.
    (b) A wedge in the manifold creating oxygen to be downstream
  1. Vaporizers:
    To deliver to inhalational agents. Made of copper colour codes are red – halothane, purple -isoflurane, yellow-sevoflurane, orange-enflurane
  2. Breathing System
    1. Open Circuit:
      By putting mask or gauge piece directly over nose & mouth previously used for ether, chloroform. A special mask Schimmelbusch mask is used for ether.
    2. Semiclosed Circuit
      1. Described by mapleson
Description: circuits


Description: circuits


  1. Mapleson A system à also K/a Magill circuit
  2. Best for spontaneous respiration
  3. Fresh gas flow should be Equal to minute volume to prevent rebreathing when patient is on spontaneous respiration.
  4. It is > 3 times the minute volume (and in spite rebreathing may not be prevented) when patient is on controlled ventilation.

Lack circuit – modification of Type A system


Type B – Obsolete circuit, no more used


Type C – Obsolete circuit, no more used (functionally B. C. – equally effective for spontaneous and CV)


Type D

  • Most commonly used circuit is Bain’s (modification of mapleson D system)
  • Length of tubing is 1.5 meters
  • Bain’s circuit is best for controlled ventilation
  • Fresh gas flow should be 1.6 times of minute volume or 70-100 ml/kg if RR is high to prevent rebreathing in Bain’s circuit on controlled ventilation. It is 2.5 times of minutes volume when patient is on spontaneous ventilation.

Mapleson E – AYRES T - Piece

Mapleson F
  • It is Jackson Rees modification of T piece (mapleson E) system
  • Used for children usually < 6 yrs. of age or <20 kg
Extra Edge
FGF = 2-3 x MV for CV, 3-4 x MV for spontaneous in pediatric patient on Mapleson F
Best circuit for children both for spontaneous and controlled ventilation


Other Circuits: Humpry ADE, Penlon, Mera-F, Lacks (all these are co-axial circuits)

Anaesthesia Gas Delivery System

  1. Closed Circuit:
    It is the circle system in which CO2 is absorbed by soda lime from exhaled gases & exhaled gases can be reused.

Soda lime composition

  1. Soda lime
    1. NaOH (5%)
    2. KOH (1%)
    3. Ca (OH)2(94%)
    4. Water (14-19%)
    5. Silica and Kieselguhr
    6. Indicator
    7. Size (4-8 mesh)
  2. Baralyme  
    1. Ca(OH)2(80%)
    2. Ba(OH)2 .8H2O(20%)
    3. KOH(1%)
    4. Water(11-14%)
    5. Indicator
    6. Size (4-8 mesh
  1. Indicator:
    Silica is added to make it hard so that minimum powdered dust is formed.
    1. Chemistry 
      1. CO2 + H2O = H2CO3
      2. H2CO3 + NaOH = Na2CO3 + H2O + Heat
      3. H2CO3 + KOH = K2CO3 + H2O + Head
      4. Na2CO3 + Ca (OH)2 = CaCO3 + NaOH (Regenerated)
      5. K2CO3 + Ca (OH)2 = CaCO3 + KOH (Regenerated)
Indicator of colour changes
  Fresh Exhausted
i. Phenolphthalein White Pink
ii. Ethyl violet White Purple
iii. Mimosa Z Red White
iv. Durasorb Pink Red
  1. Signs of exhaustion of soda lime (MCQ):
    1. Rise in blood pressure (followed by fall)
    2. Deepening of spontaneous respiration
    3. Rise in pulse rate
    4. Increased bleeding from wounds
    5. Sweating
    6. Change in colour of soda lime granules
  2. Properties of Soda Lime
    1. Size of soda lime granules – 4-8 mesh size
    2. Air space in canister – 53%
    3. Hardness >75
    4. Humidity should be >505
    5. Max amount of CO2 that can be absorbed by 100 gms of soda lime is 26L
    6. 13,700 calories are produced for 1 mole of CO2 absorber.
  3. Comparison between semiclosed & Closed system

Semiclosed (Mapleson System)



Closed (circle system)



a. Light weight easy to carry

a. Heavy weight difficult to carry

b. Due to light weight chances of accidental extubation is not high

b. Chances are high

c. Work of breathing is less

c. More

d. Danger of hypercapnia is less

d. More

e. No such accumulations

e. Accumulations of toxic metabolites may occur (of inhaled anaesthetics)

f. Can be used safely neurotoxic phosgene

ARDS sevoflurane DESFLURANE with

f. Use with trielene C/I- reaction with soda lime produces Dicholoro acetylene which is (encephalitis) & which causes produces compound (is and enf. Also) produces dessicated sodalime


a. High flows are required economical

b. Scavenging is difficult; high theatre pollution

c. Not useful in malignant hyperthermia

d. Not well preserved


a. Low fresh gas flow required so

b. Low pollution

c. Can be managed

d. Humidity is preserved 40-100% can be achieved


Important: To & fro system (modification of type C) – Type of closed circuit useful for paediatric anaesthesia

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