I&M: The Anesthesia Machine Flashcards
Four basic functions of the anesthesia machine
- Control composition of inspired gas
- Control/monitor minute ventilation and ventilatory patterns (MV = VT x RR; typical MV = 5L/min, VT = 500ml)
- Monitor physiological function
- Prevent OR contamination from anesthetic waste gases
Wall supply
Primary source of gases for most hospitals.
Diameter Index Safety System (DISS) is a coupling mechanism that ensures the correct tubing is connected to the correct outlet.
Tubing is also color coded:
- White = suctioning
- Purple = Scavenging
- Blue = Nitrous (N2O)
- Green = Oxygen
- Yellow = Air
Gas cylinders
Alternate gas source on the back of the anesthesia machine in case wall supply/pipeline fails. Size E cylinders are the most commonly used and are also colored coded.
Each cylinder has two holes in its valve that correspond to pins on the yoke of the anesthesia machine in the Pin Index Safety System (PISS). Using washers between the yoke and the cylinder can bypass this system.
Oxygen
E Cylinders
Cylinder color - green
Form - gas
Capacity (L) - 660
Pressure (psi) - 1800-2200 (2000)
Air
E Cylinders
Cylinder color - yellow
Form - gas
Capacity (L) - 660
Pressure (psi) - 1800-2200 (2000)
N2O
E Cylinders
Cylinder color - blue Form - liquid Capacity (L) - 1600 Pressure (psi) - 745 When liquid is all gone, about 400L of nitrous remains.
CO2
E Cylinders
Cylinder color - Gray
Form - liquid
Capacity (L) - 1600
Pressure (psi) - 838
Nitrogen
E Cylinders
Cylinder color - Black
Form - gas
Capacity (L) - 650
Pressure (psi) - 1800-2200 (2000)
Helium
E Cylinders
Cylinder color - brown
Form - gas
Capacity (L) - 500
Pressure (psi) - 1600-2000 (1800)
What’s the difference between liquified gas and non-liquified gas?
- When cylinder contains non-liquified gas, cylinder pressure declines proportionally as gas is withdrawn from the cylinder.
- When cylinder contains liquified gas, pressure in cylinder depends on the vapor pressure of liquified gas remaining in the cylinder. Only when all of the liquid is gone does pressure start to fall.
Fail safe valve
Valve shuts off or proportionally decrease flow of all gases when pressure in oxygen delivery line decreases to below 30psi. This helps prevent hypoxic delivery.
Flowmeters
Tells machine L/min of gas to deliver to patient. Flat you read from top; circular, from center.
Oxygen is always green, on the right, larger, protrudes further, and fluted. Ohmedia and Drager machines have mechanisms that prevent delivery of N2O without oxygen (chain and pressure control piston respectively).
APL valve
Adjustable pressure limiting valve AKA pop-off valve limits the amount of pressure that reaches the patient. The excess gas will go into the scavenging system.
During manual ventilation, keep closed to about 10-20cmH2O. Anything between 35-70cmH2O could cause barotrauma. During spontaneous breathing make sure the APL valve is open.
During machine ventilation, APL valve has no effect.
Breathing circuit - Semiclosed circle system
Gas enters through the common gas outlet of anesthetic machine and passes inspiratory limb unidirectional valve through inspiratory breathing tube and Y-piece eventually reaching patient. The expired gas then passes through Y-piece again, through the expiratory breathing tube, pass the expiratory limb unidirectional valve, and in/ out of reservoir bag. Excess gas is vented out through the pop-off/APL valve to the scavenging system, while the rest goes through CO2 absorbent canister and eventually back towards the patient.
Contains:
- Inspiratory limb
- Expiratory limb
- Unidirectional valves
- Reservoir bag
- CO2 absorbant
- O2 analyzer
- Pop-off valve on expiratory limb/ APL valve
Advantages & disadvantages of circle system
Advantages:
- Conserves moisture and heat
- Uses low flows of fresh gas (conservation)
- Ability to scavenge waste gas
Disadvantages:
- Complex design
- Has 10 connections each with the potential for disconnection
