Low-Pressure System in Anesthesia Machine - A Brief Review

Introduction

An anesthesia machine is used to deliver the anesthesia to the patient. Its function is to provide a precise amount of anesthesia gas to the patient under controlled pressure. Anesthesia machine systems have high, intermediate, and low-pressure systems. This topic explains this low-pressure system of the anesthesia machine.

What Is an Anesthesia Machine?

An anesthesia machine is a device that supplies a mixture of oxygen, gas or an anesthetic agent under pressure to the patient. These gases are delivered to the patient by face masks to make them unconscious to perform surgery on them. They are given under pressure to control the flow of each gas individually. It comprises gas supplies, pressure gauges, pressure regulators, flowmeters, vaporizers, and gas outlets.

What Is a Low-Pressure System?

The portion of the machine downstream of the flow meters, known as the "low-pressure system," has a pressure slightly higher than atmospheric pressure. The components present in a low-pressure system are:

• Flowmeters.

• Vaporizers mounted device.

• Common gas outlets.

• One-way valves.

What Are Flowmeters in Low-Pressure Systems?

Flowmeters are installed in anesthesia machines to precisely control and measure gas flow to a common outlet. Separate flowmeters are provided for each gas, and a flow control valve controls the gas flow rate through associated flowmeters.

What Are the Components of a Flowmeter?

Flowmeters consist of a flow control valve and flowmeter sub-assembly.

• Sub-Assembly - Sub-assembly consists of a gas tube and an indicator. The tube comprises chromium-plated metal protected by a plastic window. An indicator made up of aluminum. An indicator is also called a rotameter (bobbin) or float. It is a float or bobbin that moves freely and is not stuck to a tube wall. If the bobbin moves erratically, the reading can be inaccurate. An orifice is present between the outside of the bobbin (ball-shaped float) and inside the tapered glass tube.

• Flow Control Valve - The control valve controls the gas flow rate through the associated flowmeter by adjusting orifices. The valve consists of the control knob and stem. The control knob is color coded for each gas. There are two or three control knobs (one for oxygen, one for nitrous gas, and the other for air). The white color control knob is large and cylindrical, with wide flutes presenting oxygen gas. And other blue color control knob is small, conical in shape, and has narrow flutes presenting nitrous oxide gas. The control knobs are connected to the stem with a pin at the distal end. When the valve is closed, the pin fits on the seat, and no gas flows. When the stem is turned, the opening is formed between the pin and seat, and gas starts flowing.

How Does a Flowmeter Work?

Flowmeters are used to adjust the proportions of gases in an anesthesia machine. They also control the total gas flows delivered to the patient. The flowmeter works as follows:

• When the control valve is turned on.

• Opening between the pin and seat allows the flow of gas through it.

• When the gas flow enters the bottom of the flowmeter glass tube.

• The flow of gas elevates the indicator.

• The indicator starts moving freely on getting downward force.

• The bobbin or float comes to rest when the gravity is balanced by decreased pressure.

• The bobbin increases the area of the orifice.

• The upper end of the bobbin indicates the gas flow.

• The flow is measured in millimeters or liters per minute.

What Affects the Rate of Flow of Gas Through Flowmeters?

There are three factors affecting the gas flow rate through the flowmeter. The factors are:

• Pressure drop across constriction.

• Size of an orifice or annular opening - The larger the opening around the bobbin, the higher the flow will be.

• Physical properties of gas - As the annular space is tubular at low flow rates, the flow is laminar, and viscosity affects it. The high flow rates give a turbulent flow and predominant gas density.

What Happens at the Low and High Flow of Gas?

• Low Gas Flow - When the low flow of gas occurs around the bobbin, it shows a tubular flow. The gas flow is laminar, so viscosity is essential.

• High Gas Flow - When the gas flow is high, gas flowing around the bobbin through an orifice shows a turbulent flow. The flow of gas is turbulent, so density is essential.

As the flow changes from laminar to turbulent within the flowmeter, the gradation on flowmeters is not uniform.

How Are Flow Indicator Tubes Arranged in the Flowmeters?

The arrangements of flow indicator tubes can be:

1. Parallel - When flowmeters are arranged in parallel, they have two control knobs and two flow tubes for single gas. These arrangements are not presently available because of the low flow of oxygen.

2. Series - The series arrangement is safer because of a single control knob. There is one flow control valve for two flow indicator tubes.

3. Single - The safest among the three is a single flowmeter for each gas. The only condition that does not favor low flow techniques.

Where Should the Oxygen Gas Flowmeter Be Placed?

The positioning of the flowmeters of individual gas is essential. Oxygen flowmeters should be placed last on the sequence. The oxygen flowmeter is the last gas to be added to the gas mixture because it is located farthest along the sequence. As a result, the oxygen flowmeter ought to be situated nearest to the common outlet. This flowmeter placement will result in the least hypoxic gas combination. When all nitrous oxide gas is sent to standard gas outlets while oxygen flow is channeled through a leak, a hypoxic mixture results.

What Are the Problems Related to Flowmeters?

The problems related to flowmeters are:

1. Temperature and Pressure Effects:

• Changes in temperature and pressure alter the viscosity and density of gases.

• At high altitudes, pressure decreases and increases the flow.

• The decrease in density will increase the actual flow rate.

• At increased pressure, the actual flow is slightly less.

2. Back Pressure Effects:

• Sometimes, pressure rises above the indicators, making the flowmeters' reading lower than the actual.

3. Static Electricity:

• Static electricity causes the float to stick to the side of the tube, causing an inaccurate reading.

4. Hidden Float:

• Floats can disappear if there is no stop (hurdle provided) present.

• Float can adhere to a stop (hurdle present at the top of the tube so that float does not go outside) at the top of the tube, even if no gas is flowing.

Where Gas Goes After Flowmeters?

Gas flow exits the flowmeters by passing through the manifold (mixing chamber) located at the top of the flowmeters. Further, the gas mixture moves to a mounted vaporizer device.

What Is the Use of a Vaporizer Mounted Device?

The vaporizer mounting device is between a flowmeter device and a common gas outlet. It is permanently mounted. It works by controlling the vaporization of anesthetic gases from liquids and then accurately controlling the concentration of the fresh gas added.

What Is the Use of a Common Gas Outlet?

It receives all vapors and gases from the machine and then delivers the mixture to the patient's breathing system.

Conclusion

The low-pressure system in the anesthesia machine delivers gases under pressure slightly above atmospheric pressure. It uses flowmeters to pass the precise amount of gas to the patient. The flowmeters, vaporizers mounted devices, and a common outlet are the three main components of the low-pressure system. The above topic discusses flowmeters with their components - control valve and sub-assembly. The functioning of flowmeters is also presented, along with the problems related to flowmeters.