Chris Thompson 2003
Gas mixing systems
OVERVIEW
Both flow and composition of the gases entering the breathing system must be controllable by the anaesthetist. The primary pupose of the gas mixing system is to allow the anaesthetist to set:
- inspired oxygen composition (fresh gas O2 percentage, gas flows, nitrous uptake, O2 consumption)
- the amount of inhaled agent in the patient (end-tidal agent composition)
- the rate of change of composition of the circuit (total fresh gas flow).
On most older machines, these adjustments are made mechanically. The operator turns knobs (one for each gas) that directly open or close or open needle valves; the resulting gas flows are displayed on rotameter banks. To prevent inadvertent selection of hypoxic mixtures, an 'anti-hypoxia' device is fitted to all new machines. The complete solution is mechanically fairly complex (requiring second stage regulators as well as the mechanics of the anti-hypoxia system), the knobs can be bumped, in the dark the display can be hard to read, rotatmeters may stick or crack and misread if tilted or at altitude and are affected by back-pressure, and the whole system not self-monitoring in the event of internal failure. It is not possible to feedback-control such a system to a predetermined inspiratory O2.
On some newer electronic machines, gas mixing is performed by electronically controlled valves (either pulsed on/off solenoid type or proportional flow type). Display is by means of a colour screen, and 'anti-hypoxia' prevention incorporated into the controller algorithm. Mechanically these systems are very simple, but they are dependent on electronics to function normally. Most of them will maintain the set flows over a wide range of inlet pressures, so second stage regulators are not required. It is normal on these systems to directly set fresh gas O2% and fresh gas flow independently. Soon it will be possible to set the patient's inspired O2 and end-tidal agent directly, leaving the machine to automatically drop the FGF to the lowest permitted value. Full self-checking and monitoring is built in, and the gas flows will read true regardless of tilting, altitude etc.
It is likely that lectronic gas mixing systems will replace mechanical systems over the next 10-15 years so we all must know about them.
An auxiliary (emergency) O2 outlet is alsways provided. This consists of a shrouded button that delivers approx. 35-70 l/min O2 (but no vapour) to the fresh gas outlet / breathing circuit when depressed.
ROTAMETER BANKS
The needle valve itself has a flat seat so that a complete seal can be made when turning off without damaging any tapered part of the valve and without requiring much force. It is a "constant pressure variable orifice' flow controller. The upstream pressure equals the internal gas supply working pressure, and the downstream pressure is a little above the circuit pressure (basically atmospheric pressure).
Mechanical knobs turn clockwise to increase flow and electronic knobs anticlockwise.
The oxygen knob is a specialised 8-sided ribbed knob 2mm longer than the others - so that it can be identified by feel alone.
Flowmeters are constant pressure variable resistance devices. The tube is made of tapered conductive (stannous chloride coated) glass, individually calibrated to 20C at amospheric pressure (+/1 0.5% of full scale reading or 5% of indicated flow). They may be dual-conical taperered single tubes (with a more gradual taper at the bottom for fine control of low flows) or two tubes may be provided for each gas (one for low flows, the output of which enters the high-flow tube. O-rings seal the top and bottom, and they are protected from fracturing by a perspex sheet. In Australian machines, the Oxygen knob is leftmost.
The bobbin is pushed up by the force exerted by friction of the gas on it; it's weight pushes down. With higher gas flows the equilibrium point becomes higher up the tube, where the gap between bobbin and tube is greater. Bobbins are fluted to spin so that sticking (which may cause incorrect readings) can be detected.
Low flows (below 4-6 l/min) are laminar, so bobbin position tends to be proportional to viscosity of the gas according to the the Hagen-Poiseuille equation. Viscosity increases with increasing temperature, causing the flowmeter to read slightly high.
Higher flows are turbulent, so bobbin position is proportional to the square of the density of the gas. Density decreases with increasing temperature, causing the flowmeter to read slightly low.
Anti-Hypoxia Devices
These have been 'added-on' to the basic rotameter blocks to prevent inadvertent selection of a hypoxic gas mixture when oxygen is used with nitrous (ie, all nitrous, for example).
They have no effect when air is used as a carrier gas for oxygen.
Additionally some of them ensure a minimum absolute oxygen flow rate at low flows to compensate for oxygen-consumption-induced falls in inspired oxygen at low flows.
The Ulco system uses a system of levers and relies on second stage regulators to maintain equal inlet pressures, and assumes a linear relationship between physical needle valve position and flow.
Neither the Oxygen nor the Nitrous knobs are directly connected to their respective needle valves, instead the needles always tend to open and are held closed by levers.
If both knobs start closed, and the Nitrous lever is opened, the oxygen lever limits how far the nitrous needle valve can open and therefore the maximum amount of nitrous that can be delivered. The mechanics of the bars and pivots are such that a hypoxic gas mixture is unlikely. If the Oxygen knob is off, turning on the nitrous knob will deliver no gas.
The system may be inaccurate at low flows or as parts become worn over time.
The Datex-Ohmeda Safety-Link system uses chain-linked gears and differential incoming gas pressures to interlink the nitrous and oxygen knobs. If both are initially closed, turning on the nitrous will turn the oxygen knob for you. If both are open, and you turn down the oxygen knob, at some point the inner screw thread will touch the gear block on the oxygen spindle, and it will then close the nitrous for you - in the right proportion. At 25% settings, the chain links the two knobs such that reducing oxygen turns down the nitrous, while increasing the nitrous increases the oxygen.
The system has the same issues with wear and tear as the Ulco system. The chain can break. It can lead to 'bad habits such as routinely turning only the nitrous knob on a the start, knowing you'll get 25% oxygen...
Oxygen Failure Devices
Most mechanical machines are fitted with an 'oxygen failure device' that provides a gas-powered auditory alarm, some brief emergency oxygen supply and that disconnects the nitrous oxide from the circuit when the oxygen supply fails. Current devices are variants of the Howison type:
Nitrous disconnected if O2 falls below 275kPa; 800ml reservoir opens at 150kPa .
Oxygen-powered whistle.
Sources of Error
1. Atmospheric Pressure
True Flow = Indicated Flow / Sq.root of atmospheric pressure diff
In a hyperbaric chamber, flowmeters over-read; gas in the tube will be 'thicker', so the bobbin is pushed higher up the tube for any given flow. eg at 2 ATA, an indicated flow of 2.8 l/min is actually 2.0 l/min (2.8 / 1.4).
Conversely, at altitude, flowmeters under-read; he gas is 'thinner', so the bobbin is not pushd as far up for a given flow. Hence at 0.5 ATA, an indicated flow of 2.0 l/min is actually 2.8 l/min
2. Wrong gas
Heliox (80% He, 20% O2) is 1/3 the density of O2. Under-reads by a factor of about 1.7 times at high flows, but reads approximately true at low flows, since viscosities are nearly identical.
N2O and CO2 are almost interchangeable.
CO2 has twice the viscosity of cylcopropane, so at low flows, cyclo through a CO2 rotameter under-reads by half.
3. Leaks
A failure of an upper o-ring will not be detected by the flowmeter. If O2 is the last to enter the gas stream, a leak in the air or N2O seals will not cause hypoxia. Hence O2 functionally enters last into the common outlet tube. In the US this means that the O2 knob is at the right, whereas in Australia it is at the
4. Sticking
5. Float unnoticed at top of tube
6. Change in supply pressure / dirt in needle (unintended flow change; reads true)
7. Knob bumped accidentally etc
8. Gas flow fails totally if oxygen supply fails
ELECTRONIC GAS MIXING SYSTEMS
Electronic systems feedback control the internal flow control unit, making frequent minor adjustments so that the delivered flow is whatever you've asked for. Gas supply pressure changes, atmospheric pressure changes or partial valve occlusions have no effect on delivered flow. They cannot be bumped accidentally, are not affected by tipping, sticking or leaks, and the display can be easily read in dim light, as well as being set with a high degree of precision. Usually they are self-calibrating, and a second redunant computer system monitors the primary system to check for software errors. On ending the case, all flows are turned off, and at start-up the machine always enters a known state -no more accidentally leaving the gases on all weekend or starting a case with a flow tube at the top.
Internal flow sensor failures are usually detected the instant they happen by comparing the sum of the inflows of each individual gas to the total outflow of the combined mixture; they should add up properly. If not, the system will let you know. Finally, if Oxygen and/or Nitrous fails, they can automatically switch to air at the same flow you had.
A backup system is desirable in case the electronics fails. It should permit manual metering of a 100% O2 gas supply via the vapouriser.
In practice, the operator just sets the Fresh Gas Oxygen as % and the total Fresh Gas flow in l/min as indepenent variables. They are accurate to very low flows. You can select either air or nitrous as carrier gases. A software algorithm performs 'anti-hypoxia' actions such as preventing the selection of hypoxic fresh gas mixes as well as setting a limit on the minimum oxygen flow rate at very low flows (to compensate for the effect of O2 uptake at low flows.
General schematic of an electronic gas mixing system
The Drager Julian and Primus machines use fuel-injector type on/off valves to inject small pulses of gas direct from each of the the 400kPa internal lines into a 500ml container. The volume of each pulse is known, and they are feedback controlled to (a) provide gas in the correct proportions and (b) to ensure that the container pressure is maintained constant at about 200kPa. Another type valve has only to deliver the correct total fresh gas flow (again in very small pulses) from this reservoir. If the operator needs a rapid change in gas composition, the chamber is flushed, but occasionally there can be a slight delay in seeing a change in composition at the fresh gas outlet.
The Datex Avance mixer uses fully proportional flow valves to directly meter the gas flow. - just like having someone twiddling the knobs inside the machine for you. There is no reservoir. The actual position of the mechanical parts of the valve is irrelevant; it is continually adjusted to ensure the delivered flow is what you set.
Problems ?
Unfamiliar user interface - potential for abrupt failure of gas delivery. Otherwise they generally just do exactly what they are supposed to do.
Fresh Gas Failure in electronic machines
Usually annunciated by specific electronic alarms. May or may not have small backup / reserve gas supplies. Should automatically switch to the next most appropriate of the available gases.
