Principles of Anesthesia

C02 absorber area

Rise in ETC02, low expired vT, bellows fail to expand.

Between breathing circuit and ETT connector

hypoventilation, failure of C02 absorber. Increase ventilation (unless that’s the problem)
Increase Fresh Gas Flows until you can change absorber
With some anesthesia machines, you cannot do this mid-case because you breach system integrity. Some newer machines allow you to change mid-case.

Could see hypercapnea. Inspiratory and expiratory valves can become stuck (normally free-floating with gas flowing by) from moisture. Review your specific capnography waveforms for these problems.
May have a reverse flow alarm
Capnography elevated from baseline with abnormal morphology

Reservoir bag is limited to 50 cm H20 pressure
Required that the workstation pressure is never greater than 125 cmH20 (not what the patient sees)

Oxygen flush valve stuck
Oxygen flush valve pushed while the patient is inspiring – don’t do this. APL valve malfunction. Unintentional PEEP
Detection? Peak airway pressures Treatment?
Immediately disconnect the patient at the tracheal tube connector and manually resuscitate if the problem is not readily identified

Tipped Vaporizer (whole module on vaporizers coming up)
Incorrect vaporizer filling
Improper vaporizer instillation
Vaporizer or N20 left on
Vaporizer interlock system failure

Hanger Yoke (reserve gas cylinder holder)
Check valve (prevent reverse flow of gas)
Cylinder Pressure Indicator (Gauge)
Pressure Reducing Device (Regulator)

1570 L

Index pins: Pin Index Safety System (PISS) is gas specific-prevents accidental rearrangement of cylinders (e.g.. switching O2 and N2O) on the hanger yoke

Every 5 years. 10 years by special permit.

Reduces the high and variable pressures found in a cylinder to a lower and more constant pressure found in the anesthesia machine (45 psig)
Reducing devices are preset so that the machine uses only gas from the pipeline (wall gas), when the pipeline inlet pressure is 50 psig. This prevents gas use from the cylinder even if the cylinder is left open (i.e. saves the cylinder for backup if the wall gas pipeline fails)
Cylinders should be kept closed routinely. Otherwise, if the wall gas fails, the machine will automatically switch to the cylinder supply without the anesthetist being aware that the wall supply has failed (until the cylinder is empty too).

Receives gasses from the regulator or the hospital pipeline at pressures of 40-55 psig
Consists of:
Pipeline inlet connections
Pipeline pressure indicators, Piping, Gas power outlet, Master switch, Oxygen pressure failure devices, Oxygen flush, Additional reducing devices, Flow control valves

Inlets are non-interchangeable due to specific threading as per the Diameter Index Safety System (DISS)
Each inlet must contain a check valve to prevent reverse flow (similar to the cylinder yolk

Machine standard requires that an anesthesia machine be designed so that whenever the oxygen supply pressure is reduced below normal, the oxygen concentration at the common gas outlet does not fall below 19%
A Fail-Safe valve is present in the gas line supplying each of the flowmeters except O2. This valve is controlled by the O2 supply pressure and shuts off or proportionately decreases the supply pressure of all other gasses as the O2 supply pressure decreases

Pressure sensor shut-off valve (Ohmeda)
Oxygen failure protection device (Drager)

The Ohmeda pressure sensor shut-off valve is a threshold valve which is either open or closed.
Oxygen supply pressure opens the valve as long as it is above a pre-set minimum value (e.g.. 20 psig).
If the oxygen supply pressure falls below the threshold value the valve closes and the gas in that limb (e.g.. N2O), does not advance to its flow-control valve.

Based on a proportioning principle rather than a shut-off principle
The pressure of all gases controlled by the OFPD will decrease proportionately with the oxygen pressure

The machine standard specifies that whenever the oxygen supply pressure falls below a manufacturer-specified threshold (usually 30 psig) a medium priority alarm shall blow within 5 seconds.
Electronic alarms: A pressure operated electric switch operates this alarm
Ohmeda: 28 psig
Drager: 30-37 psig
Pneumatic alarms (aka Bowman’s Whistle): Uses a pressurized canister that is filled with oxygen when the anesthesia machine is turned on. When the oxygen pressure falls below a certain value, the alarm directs a stream of oxygen through a whistle

Fail-safe valves do not prevent administration of a hypoxic mixture because they depend on pressure and not flow.
These devices prevent hypoxia from some problems occurring upstream in the machine circuitry (disconnected oxygen hose, low oxygen pressure in the pipeline and depletion of the oxygen cylinder)
These devices do not prevent hypoxia from accidents such as pipeline crossovers or a cylinder containing the wrong gas
Equipment problems that occur downstream (for example leaks or partial closure of the oxygen flow control valve) are not prevented by these devices.

Located just upstream of the flow control valves
Receives gas from the pipeline inlet or the cylinder reducing device and reduces it further to 26 psig for N2O and 14 psig for O2
Purpose is to eliminate fluctuations in pressure supplied to the flow indicators caused by fluctuations in pipeline pressure

Extends from the flow control valves to the common gas outlet
Consists of:
Flow meters
Vaporizer mounting device
Check valve
Common gas outlet

Note that a leak in the oxygen flowmeter tube can cause a hypoxic mixture, even when oxygen is located in the downstream position

On the Ohmeda: Mechanical integration of the N2O and O2 flow-control valves
Automatically intercedes to maintain a minimum 25% concentration of oxygen with a maximum N2O:O2 ratio of 3:1

The Oxygen Ratio Monitor Controller maintains a minimum oxygen concentration of at least 25% +/- 3%
Back pressure from resistors located in the N20 and O2 flowmeters provides pneumatic input to the N2O slave control valve
The value of the O2 resistor is 3-4 times that of the N2O resistor, therefore if O2 flow falls below 25%, the N2O slave control valve reduces the flow of N2O

Datex-Ohmeda AS/3 Anesthetic Delivery Unit (ADU) now uses a linear, electrically driven solenoid valve controlled by the CPU.
The CPU receives input from the O2 and N2O flow measurement devices and sends and electric current to the solenoid valve
The valve limits the N2O flow to a maximum of 75% of the total flow (assuring a minimum of 25% oxygen)

Machines equipped with proportioning systems can still deliver a hypoxic mixture under the following conditions:
Wrong supply gas
Defective pneumatics or mechanics (e.g.. The Link-25 depends on a properly functioning second stage regulator)
Leak downstream (e.g.. Broken oxygen flow tube)
Inert gas administration: Proportioning systems generally link only N2O and O2
In general, an oxygen analyzer is the only machine safety device that can detect these problems (gas sampling done at the Y-piece of the patient circuit)

Built into Ohmeda machines, but not Drager machines, between the vaporizer and the common gas outlet to prevent positive pressure from the patient circuit from being transmitted back into the vaporizer and affecting the amount of volatile issued from the vaporizer. Necessary to permit jet ventilation from the common gas outlet using the O2+ flush valve. Significant when checking for leaks (see later)