The Anaesthesia Machine- 2

Presented by: Dr. Vineet Chowdhary

Moderated by: Dr. Avnish Bharadwaj

LOW PRESSURE SYSTEMThe low-pressure system is downstream of the flow control devices. Pressure in this section is only slightly above atmospheric and variable.

Consists of:1. Flowmeters2. Hypoxia prevention safety devices3. Unidirectional valve4. Pressure relief devices5. The common gas outlet6. Vaporizers and their mounting devices

FLOWMETER

Flowmeters (flow indicators, flow tubes, rotameters) indicate the rate of flow of a gas passing through them.

They may be mechanical or electronic.

Electronic flowmeters usually have a representation of a mechanical flowmeter on a screen or a number representing the flow.

Mechanical Flowmeters Measuring gas flow in a mechanical flowmeter is

based on the principle that flow past a resistance is proportional to pressure.

Mechanical flowmeters measure the drop in pressure that occurs when a gas passes through a resistance.

• Flowmeters on modern anaesthetic machines consist of a tapered glass tube containing a bobbin or ball, which floats on stream of moving gas

• As gas flow rate increases,float is carried further up tube, thereby indicating flow rate

• Flowmeters are specifically constructed for each gas, since flow rate depends on both viscosity and density of gas

• Only correct tube and bobbin or ball can be used to repair broken flowmeters

• Since bobbin floats in gas stream, flowmeters will only function correctly if tube is vertical

•   Flowmeters will not function correctly if the tube cracked

Physical Principle of Gas Flow Traditional mechanical flow indicators used in

anesthesia machines have been of the variable orifice (variable area, Thorpe tube) type.

A vertical glass tube is internally tapered with its smallest diameter at the bottom. It contains an indicator that is free to move up and down inside the tube.

When there is no gas flow, the indicator rests at the bottom of the tube.

When the flow control valve is opened, gas enters at

the bottom and flows up the tube, elevating the indicator.

Gas passes through the annular opening between the indicator and the tube and on to the outlet at the top of the tube.

The indicator floats freely at a position where the downward force caused by gravity equals the upward force caused by the gas pressure on the bottom of the indicator.

Because the tube is tapered, the size of the annular opening around the indicator increases with height, and more gas flows.

A scale marked on or beside the tube indicates the gas flowrate

The rate of flow through the tube will depend on three factors:

Pressure drop across the constriction

Physical properties of the gas.

Size of the annular opening

Pressure Drop across the Constriction As gas flows around the indicator, it encounters

frictional resistance between the indicator and the tube wall. There is a resultant loss of energy reflected in a pressure drop.

This pressure drop is constant for all positions in the tube and is equal to the weight of the float divided by its cross-sectional area.

Size of the Annular OpeningAnnular cross-sectional area varies while the pressure drop across the indicator remains constant for all positions in the tube.

Flowmeters are often called constant-pressure flowmeters.

Increasing the flow does not increase the pressure drop but causes the indicator to rise to a higher position in the tube, thereby providing greater flow area for the gas.

The elevation of the indicator is a measure of the annular area for flow and, therefore, of the flow itself

Physical Characteristics of the Gas At (low flows) flow is laminar and is a function of the viscosity of the gas (Hagen-Poiseuille equation).

When the constriction is shorter and wider (high flows), flow is more turbulent and depends on gas density (Graham's law).

Q is flow; n is viscosity; p/l is the pressure gradient; r is the radius of the tube

Temperature and Pressure Effects Flowmeters are calibrated at atmospheric pressure

(760 torr) and room temperature 20°C Temperature and pressure changes will affect both

the viscosity and the density of a gas and so influence the accuracy Variations in temperature- no significant effectHyperbaric( high pressure)- less gas delivered than intendedLow pressure(increased altitude)- More gas delivered

Flow indicator indicators. The plumb bob and skirted floats are kept centered in the tube by constant rotation. The reading is taken at the top. The ball indicator is kept centered by rib guides. The reading is taken at the center. The nonrotating float does not rotate and is kept centered by gas flow

Stop

At the top of the flowmeter tube. Prevents the indicator from plugging the outlet which

could lead to damage to the tube Prevents the indicator from ascending to a point in

the tube where it cannot be seen. Flowmeter with the indicator hidden at the top looks

much like one that is turned OFF Scale Flow indicator scale either be marked on or immediately adjacent to the tube

Flowmeters are calibrated in liters per minute. For flows below 1 L/minute, the flow may be expressed

either in milliliters or in decimal fractions of a liter per minute with a zero before the decimal point. Lights

• Flow indicator lights are offered as an option on most modern anesthesia machines. These are useful when the machine is used in a darkened room.

Flowmeter Tube ArrangementFlowmeter tubes for different gases are grouped side by side. The various gas flows meet at the common manifold (mixing chamber) at the top.Sometimes, there are two flowmeters for the same gas: One for low and one for high flows. The tubes are arranged in series (tandem) There is one flow control valve for the both flowmeter tubes. Gas from the flow control valve first passes through a tube calibrated up to 1 L/minute, then passes to a second tube that is calibrated for higher flows.

Series flowmeter tubes offer increased accuracy .

Flow indicator sequenceA,B: Potentially dangerous arrangements, with the oxygen flow indicator upstream. If a leak occurs, oxygen will be selectively lost.

C,D: Oxygen is downstream from other gases, which is a safer situation because anesthetic gas rather than oxygen will be lost.Arrows represent flows of gases.

Auxiliary Oxygen FlowmeterIt is a self-contained flowmeter with its own flow control valve, flow indicator, and outlet. It usually has a short tube with a maximum flow of 10 L/minute and a barbed fitting on the outlet. It is usually mounted on the left side of the machine.

This can be used to supply oxygen to the patient

without turning ON the anesthesia machine.

Problems with Flowmeters

InaccuracyThe flowmeter scale, tube, and indicator must be regarded as an inseparable unit.Should any one of the components need replacement, a complete new set must be obtained. The tube assembly calibrated for one gas cannot be used for a different gas. The degree of inaccuracy that will occur if flowmeters are transposed depends on the gases for which the flowmeters were intendedThe greatest accuracy in flowmeter tubes is in the middle half of the tube.

Having one tube in the series arrangement for low flows (less than 1 L/minute) and one for high flows (1 to 10 L/minute) will result in greater accuracy.

Indicator ProblemsDamage to the flow indicator can result from a sudden projection to the top of the tube due to a sudden rise in pressureFlow indicators can become worn or distorted by handling The stop at the top of the flowmeter tube can become dislodged and rest on top of the indicator.

LeaksA leak in a flowmeter downstream of the indicator but upstream of the common manifold will result in a lower than expected concentration of that gas in the fresh gas

A leak may occur if a flow control valve is left open and there is no cylinder or yoke plug in the yoke

Using the Wrong FlowmeterThe problem is most likely to occur between air and nitrous oxide, as the position of oxygen is generally fixed by national custom or standards.

Care of flowmeters

 includes ensuring that:Floats spin freely,Qualified service personnel regularly maintain gas machines,An oxygen analyzer used always One never adjusts a flowmeter without looking at it,One includes flowmeters in visual monitoring sweepsOne turns flowmeters off before opening cylinders, connecting pipelines, or turning machine "on".

HYPOXIA PREVENTION SAFETY DEVICE

1.MANDATORY MINIMUM OXYGEN FLOW

2. MINIMUM OXYGEN RATIO• OHMEDA LINK 25 PROPTIONING SYSTEM• DRAGER-SENSITIVE OXYGEN RATIO

CONTROLLER

3.UNIDIRECTIONAL CHECK VALVES

4. ALARMS

Mandatory Minimum Oxygen

Anesthesia machines require a minimum (50 to 250 mL/minute) flow of oxygen before other gases will flow.

This is preset by the manufacturer (sometimes to the customer's specification).

Some machines activate an alarm if the oxygen flow goes below a certain level even if no gases are being used.

The minimum flow is activated when the master switch is turned ON.

On some machines, it is disabled when air is used. The minimum oxygen flow does not in itself prevent a hypoxic gas concentration from being delivered. A hypoxic gas mixture can be delivered with only modest anesthetic gas flows

MINIMUM OXYGEN RATIOThe anesthesia workstation standard requires that an anesthesia machine be provided with a device to protect against an operator-selected delivery of a mixture of oxygen and nitrous oxide having an oxygen concentration below 21% oxygen (V/V) in the fresh gas or the inspiratory gas

Mechanical Linkage • There is a 14-tooth sprocket on the nitrous oxide

flow control valve and a 28-tooth sprocket on the oxygen flow control valve.

• If the flow control valves are adjusted so that a 25% concentration of oxygen is reached, a pin on the oxygen sprocket engages a pin on the oxygen flow control knob.

• This causes the oxygen and nitrous oxide flow control valves to turn together to maintain a minimum of 25% oxygen.

• This minimum oxygen ratio device (proportioning system) permits independent control of each gas as long as the percentage of oxygen is above the minimum.

If the operator attempts to increase the nitrous oxide flow beyond that ratio, the oxygen flow is automatically increased. If the operator attempts to lower the oxygen flow too much, the nitrous oxide flow is lowered proportionallyIt should be noted that these devices only link two gases, normally nitrous oxide and oxygen. Administering a third gas such as helium can result in a hypoxic mixture.

 In contrast to the Datex-Ohmeda Link-25 system, which actively increases oxygen flow to maintain a fresh gas-oxygen concentration greater than 25%, the Dräger ORMC and S-ORC systems limit nitrous oxide flow to prevent delivery of a fresh gas mixture with an oxygen concentration of less than 25%

Limitations of Proportioning System

Anti hypoxic device may also deliver hypoxic mixture under following conditions-Wrong supply of gas.Defective pneumatics/mechanics.Inert gas administration(3rd gas-He,N2,CO2).Leaks downstream.

UNIDIRECTIONAL CHECK VALVESWhen ventilation is controlled or assisted, positive pressure from the breathing system can be transmitted back into the machine.Using the oxygen flush valve may also create a positive back pressure.This pressure can affect flowmeter readings and the concentration of volatile anesthetic agents delivered from the vaporizers on the machine.Some machines have a unidirectional (check) valve to minimize these effects. This valve is located between the vaporizers and the common gas outlet, upstream of where the oxygen flush flow joins the fresh gas flow.

This valve will lessen the pressure increase but not prevent it, because gas will be continually flowing from the flowmeters.These check valves are of great importance when checking the machine for leaks. Testing the breathing system for leaks will not detect a leak upstream of the check valve in a machine equipped with a check valve.

Common (Fresh) Gas Outlet

Some new anesthesia machines provide two common gas outlets. A hazard with this arrangement is that the fresh gas may be directed to the wrong outlet The anesthesia workstation standard states that there shall be only one functional common gas outlet at a timeMany new anesthesia machines have internal connections to the breathing system. These machines may not have the conventional common gas outlet described previously.

CAUTION!!The common gas outlet should not be used to administer supplemental oxygen to a patient This will delay use of the breathing system if an emergency arises.Another potential problem is that a vaporizer may be accidentally left ON, leading to undesired administration of inhalational agent. Either the auxiliary oxygen flowmeter or a separate flowmeter should be used to supply oxygen.

Alternative Oxygen ControlWhen using an anesthesia machine, there is always the possibility that the electronics will fail. Some machines provide an alternative means to administer oxygen. This is separate from the auxiliary (courtesy) flowmeter. If there is a mechanical total flow flowmeter, it can be used to measure the delivered oxygen.

Thank You