Anaesthesia Machine Demystified

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Anaesthesia machine is used to deliver a precise amount of oxygen and volatile anesthetic under controlled conditions to patients undergoing general anesthesia during surgery. so the important points with respect to anesthesia machine is to deliver anaesthetic gas mixtures:

  • At safe pressure and flows
  • At safe concentration of oxygen
  • And with safe concentration of inhaled agents

It can also be used to delivery of Oxygen without any anesthetic agent. Anesthesia machine is often combined with the following:

  • Breathing systems and ventilation
  • Suction and scavenging
  • Patient monitoring systems

What are the components of anesthesia machine ?

Gas Cylinder:

It contains carrier gas that is compressed in a metal cylinder and held under pressure. Gas lines may also be used to bring in the gas from another room. Generally there are three tanks and are color coded for safety and recognition

  • Oxygen tank
  • Nitrous oxide (Inhalation anesthetic used as an anesthetic in dentistry and surgery) tank
  • Carbon dioxide tank

Small cylinders attach to the anesthetic machine via a YOKE. In addition to color-coded tanks, the yokes are gas-specific. A nitrous oxide tank cannot be attached to an oxygen yoke. The pin indexing system that is used to attach it is also specific to its specific gas. Large cylinders are attached via hoses or pipes.

Oxygen cannot flow out of the tank unless the outlet valve has been turned to the left. (Righty tighty, lefty loosey)

Pressure guage:

  • Measure pressure in the cylinder or pipeline.
  • Consists of a robust flexible and coiled tube ,sealed at its inner end and connected to a needle pointer which moves over a dial.
  • Other end is exposed to gas supply.
  • It is calibrated for a particular gas or vapour.

Pressure Reducing Valve (Regulator):

  • Regulates the pressure of the gas leaving the tank and going into the anesthesia machine
  • Allows a constant flow of gas into the machine, despite pressure changes within the tank
  • Reduces the pressure of oxygen that leaves the tank at 2200 psi to a safer 50 psi (around 345 kp).
  • In cases of small tanks and gas lines, the line pressure is preset at 50 psi. Only the tank pressure gauge is visible.

Flowmeter:

Now that the gas is in the machine, it’s job is to mix with the anesthetic and be delivered to the patient/

  • Measure flow rate of a gas passing through them (Calculated in Liters per minute)
  • Separate flow meters for each gas. Each flowmeter knobs can be distinguished from one another via labeling, touch, or color.
  • Carrier Gas travels through the flowmeter

 

Vaporizer:

Adds controlled amount of inhalation agent to the fresh gas flow

  • Converts the liquid anesthetic agent (usually isofluorane or sevofluorane) into a vapor.
  • Adds controlled amount of these vapors to the carrier gas.Temperature sensitive valves controls the splitting ratio.
  • Interlocking extension rods prevents more than one vaporizer being used at a time and one should use the correct anesthetic with the correct vaporizer (Don’t put sevo in the iso machine).
  • The vaporizer must be on to deliver any anesthesia to the patient. Otherwise, only the carrier gas will be delivered

BREATHING CIRCUIT:

  • The mixture of the anesthetic gas and carrier gas is known as FRESH GAS.
  • The system that brings the fresh gas from the vaporizer to the patient and takes the expired gases from the patient.
  • It May contain:
  1. Unidirectional valves: One-way valves that allow the flow of fresh gas to enter the inhalation valve and exit the exhalation valve and hence exit valve prevents the expired gases from traveling back to the patient before the CO2 is removed.
  2. Reservoir bag: It Stores gas and helps in determining correct endotracheal tube placement. Movement of bag with breaths means that the tube is in trachea. It also Allows assessment of respiratory rate and depth
  3. pop off valve (pressure relief )
  4. CO2 canister: CO2 canister usually contains soda lime that removes CO2 from other gases breathed out.
  5. O2 flush: When activated, O2 bypasses the flowmeter and vaporizer and enters the circuit.
  6. Negative pressure relief valve: ( if negative pressure is detected in the circuit, this valve opens and allows room air in as its Better for the patient to breathe room air than no air)
  7. Pressure manometer /gauge: It measures the pressures of gas within the circuit, usually in CM H2O.

References and Further reading:

  1. The Basic Anaesthesia Machine by CL Gurudatt

  2. http://medi.moph.go.th/education/ANESTHESIA_GAS_MACHINE.pdf

  3. http://en.wikipedia.org/wiki/Anaesthetic_machine

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