The Anesthesia Gas Machine
Revised July 2012
ANESTHESIA GAS MACHINE> COMPONENTS & SYSTEMS> DELIVERY> VENTILATORS
"Hanging" bellows. Click on the thumbnail, or on the underlined text, to see the larger version (101 KB).
To remember the classification: "ascend" and "descend" have "e" in them - so look at them during expiration. Ascending bellows ("standing") ascend during expiration (modern type - preferred by many) and descending bellows ("hanging") descend during expiration. Ventilator relief valve gives 2 - 3 cm water pressure positive end-expiratory pressure (PEEP) (true for almost all mechanical ventilators with standing bellows- exceptions are the new Dräger piston ventilators, and any double-circuit ventilator which has a hanging bellows such as the Anestar). The ventilator relief valve (spill valve) allows scavenging ONLY during the expiratory phase.
"Hanging" bellows on the Anestar. Click on the thumbnail, or on the underlined text, to see the larger version (367 KB).
The hanging design is chosen for compactness and ease of sterilization of the entire breathing circuit. The newer hanging bellows housing, unlike older designs, lacks an internal weight, and senses when the bellows do not return to the full "down" position. These factors, plus integration of disconnect alarms based on chemical (capnograph), and mechanical (pressure, volume, and flow sensors) apnea detection, make piston or hanging bellows designs safe. However, the placement of the hanging bellows below the writing surface makes visual detection of disconnects difficult. Also it is less easy to determine if the patient is breathing spontaneously in addition to the rate set on the mechanical ventilator. The user must rely more on the pressure and capnography waveforms as opposed to the bellows. Water may gather in the descending bellows (lessening tidal volume and creating an infection risk) but this tendency should be counteracted by heating the absorber head.
Besides increased accuracy (due to compliance and leak compensation- see below on this page), the biggest improvement in current ventilators is their flexibility in modes of ventilation. Offering pressure controlled ventilation (PCV) allows more efficient and safe ventilation of certain types of patients. The improvement in tidal volume accuracy afforded by modern ventilators means that switching of circuits (for example, to a non-rebreather for small children) is not as necessary. (This is safer because potential misconnects are avoided, and quicker besides.) The latest direction the manufacturers have taken is offering modes (such as pressure support) that will support spontaneous ventilation, seen in anesthesia with much greater frequency due to the advent of the laryngeal mask airway and more frequent ambulatory procedures.
The Aisys offers the following modes of ventilation:
Waves- CMV. Click on the thumbnail, or on the underlined text, to see the larger version (40 KB).
All ventilators offer volumed controlled ventilation (VCV). In this mode, the set volume is delivered at a constant flow. The peak inspiratory pressure is allowed to vary, and it does, according to the patient's compliance and airway resistance. Volume is adjusted to avoid atelectasis, and rate is adjusted for reasonable end-tidal carbon dioxide while monitoring the peak inspiratory pressure.
SIMV-Volume is VCV which detects spontaneous breaths (if any) and delivers volume-controlled breaths in synch with the patient's inspiratory efforts. This helps maintain minute ventilation, while avoiding breath-stacking or bucking. If too many (or too few) synchronized breaths are delivered, adjust trigger window and sensitivity.
Patients may breathe at will between ventilator breaths. Some ventilators support these spontaneous breaths with pressure support ventilation (PSV), resulting in a mode called SIMV-PS.
Waves- PCV. Click on the thumbnail, or on the underlined text, to see the larger version (35 KB).
Pressure control ventilation (PCV) controls inspiratory pressure, and allows inspired volume to vary (and it does, with changes in compliance and airway resistance). The flow generated varies. Flow is high at first to produce the set pressure early in inspiration, and it is less later in inspiration to maintain the set pressure through the inspiratory time. Target pressure is adjusted to produce a reasonable VT (reasonable to avoid the extremes of atelectasis and volu-trauma). Rate is adjusted to a reasonable end-tidal carbon dioxide. The result (in many instances where peak inspiratory pressure [PIP] had been high when employing VCV [e.g. laparoscopy]) is often that PCV delivers increased tidal volume at a lower PIP.
How is it possible to get greater tidal volumes at a lower PIP? The answer is that the flow of gas is greater early in inspiration (see waveforms above). Overall this may result in greater delivered volume with the same (or lower) pressure.
If there is a danger of high PIP, use PCV to limit pressure within the airway and lungs.
If compliance is low, use PCV to obtain a higher tidal volume.
PCV is also used to compensate for leaks
New mode in which the ventilator operates as PCV, but a tidal volume target is also set. The ventilator then dynamically adjusts the inspiratory pressure (while staying within the set maximum pressure [Pmax]) to achieve the desired VT breath-by-breath. Advantages include control of PIP (through the basic pressure-controlled mode) and control of arterial CO2 (through guarantee of VT and thus minute ventilation).
SIMV can be pressure controlled, resulting in a mode called SIMV-PC.
With the advent of the LMA, spontaneous (unassisted) breathing is much more common during general anesthesia. But it is difficult to maintain a light enough plane of anesthesia to permit spontaneous ventilation, while retaining sufficient depth for surgery to proceed. Too deep, and respiratory acidosis will occur; too light, and bucking and awareness are risks. Ventilation modes which support the spontaneously breathing patient are useful to provide normocapnia without bucking. Many ventilators currently incorporate pressure support ventilation (PSV). Continuous positive airway pressure (CPAP), and airway pressure release ventilation (APRV) are probably on the horizon, but are not yet implemented in current equipment.
Settings for PSV are simple- just pressure support level (12 cm H2O is the default on the ADU). Note that PSV requires a spontaneously breathing patient as there is no (or very low) default backup respiratory rate.
PSV senses patient inspiratory effort (volume or flow) and delivers pressure support while it is present. This tends to result in larger VT than the patient would produce on their own. PSV is useful to support minute ventilation and control arterial carbon dioxide for spontaneously-breathing patients during maintenance or emergence.
In the Aisys, PSV-Pro ("protect") is found. If no breaths are detected during an adjustable apnea delay period (10-30 sec), the ventilator switches to the backup mode (PCV, at whatever settings are chosen when PSV-Pro is selected). If resumption of spontaneous breaths occurs later, the veniltor will return to PSV-Pro mode.
All current gas machines have VPO (volume, pressure, oxygen) monitoring built in the breathing circuit. Most have agent monitoring as well. Some have spirometry and capnography.
Piston ventilators use an electric motor to compress gas in the breathing circuit, creating the motive force for mechanical ventilator inspiration to proceed. The motor's force compresses the gas within the piston, raising the pressure within it, which causes gas to flow into the patient's lungs. Thus a piston ventilator uses no driving gas, and may be used without depleting the oxygen cylinder in case of oxygen pipeline failure.
NM 6000 piston bellows. Click on the thumbnail, or on the underlined text, to see the larger version (44 KB).
In the Narkomed 6000, the bellows are occult, being placed horizontally under the writing surface. Although they can be viewed by lifting the writing surface, their to-and-fro movement is not normally visible during mechanical ventilation. The anesthetist relies on flow, pressure, and capnography waveforms (and the movement of the breathing bag during mechanical ventilation as a result of fresh-gas decoupling[see section on fresh gas decoupling below on this page]) to guard against disconnects or other problems.
Apollo waveforms - ETCO2, Flow-time, Pressure. Click on the thumbnail, or on the underlined text, to see the larger version (105 KB).
The Apollo is similar, in that the bellows are hidden.
Piston ventilator window Fabius GS. Click on the thumbnail, or on the underlined text, to see the larger version (35 KB).
The Fabius GS has a piston ventilator similar to the Divan, but the bellows travel vertically, and their movement is continuously visible through a window to the left of the flowmeter bank.
The piston ventilator has positive and negative pressure relief valves built in. If the pressure within the piston reaches 75 + 5 cm H2O, the positive pressure relief valve opens. If the pressure within the piston declines to -8 cm H2O, the negative pressure relief valve opens, and room air is drawn into the piston, protecting the patient from NEEP (negative end-expiratory pressure).
There are several advantages to the piston ventilator system (Apollo, NM 6000 & Fabius GS):
The appearance of pressure control ventilation is a major advantage, allowing patients to be ventilated efficiently who were very difficult with volume control mode, such as patients with ARDS or morbid obesity. PCV also allows safe ventilation when excessive pressure must be strictly avoided; such as neonates and infants, and emphysematous patients. The appearance of modes like PSV which are capable of supporting the patient with spontaneous respirations extends our capabilities further.
Factors contributing to a discrepancy between set and delivered tidal volumes are especially acute in pediatrics and include
Because of the greatly increased accuracy in tidal volume delivery achieved through compliance and leak testing and compensation, modern ventilators have an unprecedented tidal volume range. They are able to ventilate smaller patients much more accurately than any previous anesthesia ventilator could (Anesth Analg 2008;106:1392-1400). This will undoubtedly lessen the need for non-rebreathing (Mapleson & Bain) circuits, and make care safer, since anesthetists will no longer have to disassemble and reconfigure to a non-rebreathing circuit for a child in the middle of several adult cases. However, it is mandatory to substitute a pediatric circuit for tidal volumes less than 200 mL (Anesthesiology 2001;94:543-4) on all ventilators. Smaller filters and a pediatric D-Lite sensor must be used on the ADU for VT < 150 mL.
Tidal volume mL
The accuracy comes with a price. An electronic leak and compliance test must be repeated every time the circuit is changed, particularly if changing to a circuit with a different configuration (adult circle to pediatric circle, or adult to long circuit). This test is part of the electronic morning checklist.
Photograph of the Aestiva flow sensor. Click on the thumbnail, or on the underlined text, to see the larger version (25 KB).
The placement of the sensor used to compensate tidal volumes for compliance losses and leaks has some interesting consequences. The Aestiva flow sensors are placed between the disposable corrugated breathing circuit limbs and the absorber head. Here they are able to compensate tidal volumes for fresh gas flow, compliance losses and leaks internal to the machine and absorber head- but not in the breathing hoses.
Photograph of the D-Lite sensor. Click on the thumbnail, or on the underlined text, to see the larger version (46 KB).
The GE D-Lite sensor is placed just distal to the Y-piece on the ADU. In this position, it can compensate for all leaks and compliance losses out to the Y piece (thus including the breathing circuit hoses). However, at this point it adds appreciable and perhaps objectionable bulk and weight close to the patient's face. This may make mask ventilation a bit more cumbersome. Further, a sensor closer to the patient is exposed to more exhaled moisture, but the impact can be lessened with a heat and moisture exchanger between patient and sensor. Unfortunately, this adds further bulk and weight.
The Narkomed 6000 tests compliance and leaks of all components to the Y-piece via a pressure transducer within the internal circuitry near the bellows. Here the sensor is relatively protected from moisture.
A final factor adding to modern ventilator accuracy is that they compensate delivered tidal volume for changes in fresh gas flow (FGF). In traditional ventilators, which are not fresh gas decoupled, the delivered tidal volume is the sum of the volume delivered from the ventilator bellows, and the fresh gas flow delivered during the inspiratory phase of each breath. Thus, delivered tidal volume may change as FGF is changed.
For example, consider a patient with a FGF of 4 L/min, a respiratory rate of 10, inspiratory:expiratory (I:E) ratio of 1:2, and a tidal volume of 700 mL. During each minute, the ventilator spends 20 seconds in inspiratory time and 40 seconds in expiratory time (1:2 ratio). During this 20 seconds, the fresh gas flow is 1,320 mL (4000 mL/min FGF times 1/3). Each of the 10 breaths of 700 mL is augmented by 132 mL of fresh gas flowing while the breath is being delivered, so the total delivered tidal volume is 832 mL/breath. This 19% increase is reasonably unimportant.
But what happens if we decrease to lower fresh gas flow? Assume the same parameters, but a FGF of 1,000 mL/min. During each minute, the ventilator spends 20 seconds in inspiratory time and 40 seconds in expiratory time (1:2 ratio). During this 20 seconds, the fresh gas flow is 330 mL (1000 mL/min FGF times 1/3). Each of the 10 breaths of 700 mL is augmented by 33 mL of fresh gas flowing while the breath is being delivered, so the total delivered tidal volume is 733 mL/breath. This means that changing FGF from 4,000 mL/min to 1,000 mL/min, without changing ventilator settings, has resulted in a 14% decrease in delivered tidal volume (832 to 733 mL). It would not be surprising if the end tidal carbon dioxide rose as a result.
The situation is more acute with a traditional anesthesia ventilator in children. Assume a 20 kg patient with a FGF of 4 L/min, a respiratory rate of 20, inspiratory:expiratory ratio of 1:2, and a tidal volume of 200 mL. During each minute, the ventilator spends 20 seconds in inspiratory time and 40 seconds in expiratory time (1:2 ratio). During this 20 seconds, the fresh gas flow is 1,320 mL (4000 mL/min FGF times 1/3). Each of the 20 breaths of 200 mL is augmented by 66 mL of fresh gas flowing while the breath is being delivered, so the total delivered tidal volume is 266 mL/breath. This is a 33% increase above what is set on the ventilator.
We now decrease the FGF from 4 to 1 L/min for the same 20 kg child, (RR 20, I:E ratio of 1:2, and VT 200 mL). During each minute, the ventilator spends 20 seconds in inspiratory time and 40 seconds in expiratory time (1:2 ratio). During this 20 seconds, the fresh gas flow is 333 mL (1000 mL/min FGF times 1/3). Each of the 20 breaths of 200 mL is augmented by 16.5 mL of fresh gas flowing while the breath is being delivered, so the total delivered tidal volume is 216 mL/breath. This is a 23% decrease in VT (266 to 216 mL/breath) caused solely by changing FGF, and without altering vent settings.
Fabius GS ventilator schematic. Click on the thumbnail, or on the underlined text, to see the larger version (39 KB).
There are two approaches to dealing with the problem. The Dräger Apollo, Narkomed 6000 and Fabius GS use fresh gas decoupling. Fresh gas is diverted by a decoupling valve to the manual breathing bag, and is thus not added to the delivered tidal volume. Thus, fresh gas decoupling helps ensure that the set and delivered tidal volumes are equal. This is most clearly visualized by visiting the Virtual Fabius GS Simulation.
The action of the piston closes a one-way (decoupling) valve, diverting FGF to the manual breathing bag during the inspiratory cycle. The visual appearance is unusual:
With fresh gas decoupling, if there is a disconnect, the manual breathing bag rapidly deflates, since piston retraction draws gas from it.
The second approach is fresh gas compensation, which is utilized in the Aisys, Aestiva, Avance, Aespire, and ADU. The volume and flow sensors provide feedback which allows the ventilator to adjust the delivered tidal volume so that it matches the set tidal volume, in spite of changes in the total fresh gas flow.
Low fresh gas flow is desirable to reduce pollution and cost of volatile agents and nitrous oxide, preserve tracheal heat and moisture, prevent soda lime granules from drying, and preserve patient body temperature. Factors which enhance the safety and efficiency of low flows in modern ventilators include:
NM 6000 piston bellows and ventilator controls (front panel). Click on the thumbnail, or on the underlined text, to see the larger version (44 KB).
Divan Controls (front panel). Click on the thumbnail, or on the underlined text, to see the larger version (78 KB).
Unlike most other anesthesia ventilators, there are no visible bellows on the NM6000 Divan ventilator. It is unique among current models in having a horizontal piston which is hidden within the writing surface of the gas machine. To provide a visible indication of lung inflation, fresh gas is diverted to the manual breathing bag, which inflates during mechanical ventilator inspiration, and deflates during expiration. A disconnect will cause the manual breathing bag to gradually lose volume (in addition to activating other apnea alarms). A pressure transducer within the ventilator measures compliance losses and leaks in the total breathing circuit (absorber head and corrugated limbs).
The Fabius GS has a piston ventilator as well, but the piston is mounted vertically to the left of the flowmeters and is visible through a window.
|Fabius GS ventilator controls and piston window (left of flowmeters). Click on the thumbnail, or on the underlined text, to see the larger version (36 KB).|
Fabius GS Ventilator controls. Click on the thumbnail, or on the underlined text, to see the larger version (57 KB).
The Fabius GS ventilator is an electronically controlled, electrically driven piston ventilator. It consumes no drive gas. The piston is continuously visible. Operating parameters include
Apollo. Click on the thumbnail, or on the underlined text, to see the larger version (96 KB).
Apollo controls. Click on the thumbnail, or on the underlined text, to see the larger version (105 KB).
Photograph of the ADU ventilator controls. The left arrow shows the Bag/Auto and APL valve location. The right arrow shows the location of the thumbwheel and buttons by which ventilator settings are changed. Click on the thumbnail, or on the underlined text, to see the larger version (92 KB).
Photograph of the D-Lite sensor. Click on the thumbnail, or on the underlined text, to see the larger version (46 KB).
The ventilator can utilize either oxygen or air as a driving gas, and will switch automatically if oxygen pipeline pressure is lost. Volume-control, pressure control, synchronized intermittent mandatory ventilation (SIMV-Vol),and PSV modes are offered, along with integrated electronic PEEP. Overpressure release valve at 80 cm water means that patients requiring higher peak inspiratory pressure cannot be ventilated in volume control mode (but they may be ventilated successfully in pressure control mode).
The pressure control mode should be very useful to increase delivered tidal volume when lung compliance is low (laparoscopic procedures, obesity, pregnancy) or when high peak inspiratory pressures must be avoided (pediatric patients, laryngeal mask ventilation, emphysema). Flow-volume (resistance) or pressure-volume (compliance) loops may be displayed breath-by-breath.
Aisys. Click on the thumbnail, or on the underlined text, to see the larger version (202 KB).
Aisys controls. Click on the thumbnail, or on the underlined text, to see the larger version (185 KB).
The Aisys has a dual circuit, ascending bellows 7900 ventilator. Modes include VCV, PCV, PC-VG, SIMV-Vol, SIMV-Press, and PSV-Pro. Maximum flow is 120 L/min. Maximum pressure is 60 cm H2O. PEEP 0-30 cm H2O. Inspiratory/expiratory ratio can be selected from 2:1 to 1:8. The vent features tidal volume compensation, one switch activation from manual to mechanical ventilation,two key presses to total standby (end case), and a cardiac bypass case mode.
The Advanced Breathing System (ABS™) has a minimal number of parts and tube connections, which greatly reduces the potential for leaks and misconnects. It is easy to disassemble (no tools), fully autoclavable, and latex-free (as are most modern anesthesia machines).
Avance. Click on the thumbnail, or on the underlined text, to see the larger version (151 KB).
Aespire. Click on the thumbnail, or on the underlined text, to see the larger version (414 KB).
|7100 controls . Click on the thumbnail, or on the underlined text, to see the larger version (43 KB).|
|Aestiva/5 with 7100 ventilator. Click on the thumbnail, or on the underlined text, to see the larger version (31 KB).|
Paragon breathing circuit. Click on the thumbnail, or on the underlined text, to see the larger version (157 KB).
AV-S ventilator. Click on the thumbnail, or on the underlined text, to see the larger version (48 KB).
The breathing circuit is latex free. Absorbent capacity is 1.3 kg of loose or pre-packed absorbent. The standing bellows may be driven by oxygen or air.
The AV-S ventilator on the Paragon Platinum SC430 offers integrated FGF compensation, VPO (volume, pressure, oxygen) and spirometry monitoring, and auomated compliance and leak testing. VCV (20-1600 mL/breath), PCV (to 70 cm H2O), and PSV modes are available with integrated electronic PEEP. Insp/Exp ratio is selectable from 1:0.3 to 1:8. Pmax 80 cm H2O.
|Anestar. Click on the thumbnail, or on the underlined text, to see the larger version (367 KB).
The Anestar ventilator is a hanging bellows, gas-driven, electronically controlled ventilator. It offers VCV, PCV, and PSV modes. VT is 10-9999 mL. Insp/Exp ratio is 3:1 to 1:5.
The breathing circuit is warmed and VT compensation is achieved through fresh-gas decoupling. Flow sensors are hot-wire anemometers. The internal volume of the breathing system is 2.5 L (of which 1.4 L is absorbent).
A 2004 document called "Guidelines for determining anesthesia machine obsolescence" is available. The link is under the heading "Anesthesia Machine" at ASA Standards, Guidelines and Statements
AV2 controls. Click on the thumbnail, or on the underlined text, to see the larger version (46 KB).
Classification: pneumatically and electrically powered, double circuit, pneumatically driven, ascending bellows, time cycled, electronically controlled, VT-preset vent. Incorporates Pressure Limit Controller (PLC) which allows maximum peak inspiratory pressure (PIP) adjustment from 10-110 cm water. Inspiratory flow control must be set properly (like the Ohmeda 7800), so that driving gas flow does not create an inspiratory pause. Standard on Narkomed 2A, 2B, 2C, 3, 4, and Narkomed (not Fabius) GS.
Ohmeda 7000 controls. Click on the thumbnail, or on the underlined text, to see the larger version (43 KB).
Same classification as Dräger AV-E except it is minute-volume preset (unique among anesthesia ventilators). VT cannot be set directly, it is calculated from settings of VE and respiratory rate (VE = RR x VT). Inspiratory flow stops when set VT worth of driving gas has been delivered to the driving circuit side of the bellows chamber or if pressure greater than 65 cm water is attained. Thus, a patient requiring peak inspiratory pressure > 65 cm water cannot be mechanically ventilated with this ventilator.
Ohmeda 7800 controls. Click on the thumbnail, or on the underlined text, to see the larger version (34 KB).
This ventilator or the older Ohmeda 7900 Smart-VentTM were standard on newer Excel or Modulus machines. Same classification as Dräger AV2 ventilator; VT preset. Tidal volume, respiratory rate, inspiratory flow and pressure limit controls are present.
Ohmeda 7900 controls. Click on the thumbnail, or on the underlined text, to see the larger version (34 KB).
Same classification as Dräger AV ventilator, VT preset. Microprocessor control delivers set VT, in spite of changes in fresh gas flow, small leaks, and absorber or bellows compliance losses proximal to the sensors. These flow sensors are placed between corrugated plastic breathing circuit and the absorber head, in both limbs. These are connected to pressure transducers in the ventilator. Compliance losses in the breathing circuit corrugated hoses are not corrected, but these are a relatively small portion of compliance losses.
The first "modern" ventilator- it offered such desirable features as integrated electronic PEEP control, and pressure-controlled ventilation (PCV) mode. It has been reported that the sensors can be quite sensitive to humidity, causing ventilator inaccuracy or outright failure. The problem may be more likely when active airway humidifiers are used- read more at:
Controls are similar to the 7800. Users should be vigilant for cracked tubing in the flow sensors, which are located where the breathing circuit corrugated hoses attach to the absorber head. Leaks here have been reported to cause inability to ventilate, either mechanically or manually. When these failures occur, the ventilator may indicate alarm messages like "VT" or "Apnea", rather than "Check sensor". Flow sensor tubing must be vertical, must be changed regularly, and sensors must be in the proper side (inspiratory or expiratory). Although the sensor plugs are keyed by size and shape, if both sensors come off the absorber head when the circuit is changed they can be inadvertently replaced on the wrong side.