Pressures in the Anaesthetic Nachine From Oxygen Cylinder

Pressures in the Anaesthetic Machine From Oxygen Cylinder

Anesthesia Machine Pressure System

Once the cylinders are turned on, compressed gases may pass from their respective high-pressure cylinder sources into the anesthesia machine The pressure contained within gas cylinders is initially very high and falls as the contents are used. The function of the regulator, or reducing valve, is to convert this variable, high-pressure gas supply to a constant, low-pressure gas supply of around 50 psi (4atm). Atm=atmospheric pressure, psi=pound square inch

• High pressure system: The pressure in full oxygen cylinder is 137 atm (2000 psi. Cylinder supply this high pressure to pressure regulator or pressure regulator receives gasses from the high pressure H or E cylinders attached to the back of the anesthesia machine at 2000 psig for O2, it is too high for anesthetic machine. Pressure reducing devices reduces the high and variable pressures found in a cylinder to a lower and more constant pressure found in the anesthesia machine. Some anesthetic machine may not have this device, for safe use of pressurized oxygen the pressure gauge as in the image above should routinely used because it reduces the cylinder pressure to 4 atm (50 psi) which can not damage the anesthetic machine although it is still harmful to the patient (Figure 2.4)
• Intermediate pressure system: Receives gasses from the regulator or the hospital pipeline at pressures of 4 atm (40-55 psig). This pressure is safe for anesthesia machine but would harm or kill patient (Figure 2.4).
• Low pressure system (Flow meters to common gas outlet): Second-stage oxygen pressure regulator located just upstream of the flow control valve set at a specific value ranging from 12 to 19 psig. Output from the oxygen flow meter is constant when the oxygen supply pressure exceeds the threshold (minimal) value (Figure 2.4).

Calculating the Amount of Oxygen

The amount of time that an anesthesia machine can operate from the cylinder supplies is an important knowledge. For oxygen, the volume of gas remaining in the cylinder is proportional to the cylinder pressure. One author has proposed the following equation to help estimate the remaining time.

If you measure the oxygen pressure in psig (pound square inch gauge):

• Approximate remaining time (Hrs) = oxygen cylinder pressure (psig)/ 200 X oxygen flow rate (L/min).

If you measure the oxygen pressure in atmosphere (we commonly use):

• Step 1. Cylinder content in liter = Original Volume in liter X pressure
• Step 2. Cylinder content in liter / oxygen flow rate (L/M)

Example: If your cylinder original volume is 50 liter which is written on the shoulder of the cylinder and you read a cylinder pressure 120 atm while constantly administering oxygen at a rate of 4L/min your oxygen will be sufficient for 1500 minutes.

• Step 1- 50 X 120= 6000 liter of oxygen
• Step 2 - 6000 liter / 4 liter = 1500 minute = 25 hrs

Note: It should be noted that this calculation will provide only a gross estimate of remaining time and may not be exact. Furthermore, users should be cautioned that use of a pneumatically driven mechanical ventilator will dramatically increase oxygen utilization rates and decrease the remaining time until cylinder depletion.

Fail Safe Valve

Anesthesia machines are equipped with a fail-safe valve designed to prevent the delivery of hypoxic gas mixtures from the machine in the event of failure of the oxygen supply. The fail-safe valve prevents flow of nitrous oxide if the pressure in the oxygen supply circuit decreases to less than 30 psi. This valve shuts off or proportionally decreases the flow of all gases when the pressure in the oxygen delivery line decreases to less than 30 psi. This safety measure will protect against unrecognized exhaustion of oxygen delivery from a cylinder attached to the anesthesia machine or from a central source.