"Hanging" bellows. Click on the thumbnail, or on the underlined text, to see the larger version (101 KB).
The Anesthesia Gas Machine
Michael P. Dosch CRNA MS
University of Detroit Mercy Graduate Program in Nurse Anesthesiology
This site is http://www.udmercy.edu/crna/agm/.
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- exceptions are the new Dräger Divan ventilator, which has a horizontal piston, and the Julian hanging bellows). The ventilator relief valve (spill valve) allows scavenging ONLY during expiratory phase.
The hanging design was chosen for the Julian for compactness and ease of sterilization of the entire breathing circuit. The Julian 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) detection, make piston or hanging bellows designs safe. 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 bellows (lessening tidal volume and creating an infection risk), but this tendency should be opposed by the heated 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 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 next direction the manufacturers will take 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.
Waves- CMV.
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All ventilators offer controlled mandatory volume (CMV) ventilation. In this mode, the volume is kept constant, and it 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. Rate and volume are adjusted for reasonable end-tidal carbon dioxide and peak inspiratory pressure.
Waves- PCV.
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Pressure control ventilation (PCV) controls inspiratory pressure, and allows inspired volume to vary (with changes in compliance and airway resistance). The flow generated varies; high at first to produce the set pressure early in inspiration, and less later in inspiration to maintain this pressure through the inspiratory time. Target pressure and rate are adjusted to a reasonable end-tidal carbon dioxide, and tidal volume is monitored. The result is increased tidal volume at a lower PIP, in many instances where peak inspiratory pressure (PIP) had been high when employing CMV (for example laparoscopy).
How is it possible to get greater tidal volumes at a lower PIP? The answer is that 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.
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 could support a spontaneously breathing patient would be useful to provide normocapnia without bucking. Modes which might be useful include synchronized intermittent mandatory ventilation (SIMV), pressure support ventilation (PSV), continuous positive airway pressure (CPAP), and airway pressure release ventilation (APRV).
Of these modes, SIMV is currently available only on the S/5 ADU and the Narkomed 6000. Pressure control ventilation (PCV) may be used with an LMA as well (Anesthesiology 2000;92:1621-3). It is available on the Datex-Ohmeda 7900 SmartVent, S/5 ADU, and the Dräger 6000, and Fabius GS.
Piston ventilators use an electric motor to compress gas in the breathing circuit, creating the motive force for mechanical ventilator inspiration to proceed. Thus they use no driving gas, and may be used without depleting the oxygen cylinder in case of oxygen pipeline failure.
NM 6000 piston bellows.
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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 pressure and capnography waveforms to guard against disconnects or other problems.
Piston ventilator window Fabius GS.
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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 Divan piston ventilator system (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 control (CMV) mode, such as patients with ARDS or morbid obesity. PCV also allows safety in ventilating patients in whom excessive pressure must be strictly avoided; such as neonates and infants, and emphysematous patients. The future appearance of modes capable of supporting the patient with spontaneous respirations will extend 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. 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) with the NM 6000 and the Fabius GS. Smaller filters, a pediatric DLite sensor, and less- compliant circle breathing systems must be used on the S/5 ADU as well.
| Ventilator | Tidal volume mL |
| Divan- Narkomed 6000 | 10-1400 |
| Fabius GS | 20-1400 |
| Julian | 50-1400 |
| Aestiva | 20-1500 |
| S/5 ADU | 20-1400 |
| Kion | unavailable |
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 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 Datex-Ohmeda D-Lite sensor is placed just distal to the Y-piece. 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 the fresh gas flow. In traditional ventilators, which are not fresh gas decoupled, the delivered tidal volume is the sum of the volume delivered from the ventilator and the fresh gas volume. 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 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). So 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). So 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). So 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.
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 Julian, Narkomed 6000 and Fabius GS use fresh gas decoupling. The fresh gas is 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 (check) 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 the piston retraction draws gas from it.
The second approach is fresh gas compensation, which is utilized in the Aestiva, and S/5 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 the total fresh gas flow, or in case of changes in 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).
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NM 6000 Monitor screen.
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The Dräger Divan ventilator is a modern ventilator, offering features such as: pressure control mode, SIMV, correction for compliance losses, and integrated electronic PEEP. Unlike the S/5, newer Dräger absorber heads warm the gases in the breathing circuit. Also unique is that fresh gas flow does not add to delivered tidal volume ("fresh gas decoupling"- see New features above on this page). The Divan is limited to a pressure of 70 cm water- so like the Ohmeda 7000, it cannot ventilate patients in CMV mode beyond this pressure (although, again, it is possible and even perhaps preferable to ventilate the ARDS patient with pressure control mode). It is installed on the Narkomed 6000. The Fabius GS has a piston ventilator as well, but the piston is mounted vertically to the left of the flowmeters and visible through a window.
Divan Controls (front panel).
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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).
Julian Ventilator controls. Click on the thumbnail, or on the underlined text, to see the larger version (68 KB).
The Julian ventilator is electronically controlled, gas-driven, hanging bellows ventilator which may be used in Manual/Spontaneous, CMV, or pressure control modes. With a small absorber volume and heated head, it is well-suited for low flow anesthesia. Parameters include:
Fabius GS Ventilator controls. Click on the thumbnail, or on the underlined text, to see the larger version (57 KB).
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).
The Fabius GS ventilator is an electronically controlled, electrically driven piston ventilator. It consumes no drive gas. The piston is continuously visible. One button is unassigned on the controls, for the next mode to be developed. Operating parameters include
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. See instructions for using the AV2+ in volume or pressure mode at the Dräger web site.
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 current 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. Though this ventilator is still commercially available, there is little reason to purchase it, in view of the useful features on newer designs. Read Datex-Ohmeda's description and specifications
Aestiva/5 with 7100 ventilator. Click on the thumbnail, or on the underlined text, to see the larger version (31 KB).
Same as the Smart Vent 7900 (see below), except that the optional pressure control mode is not as strong as the 7900. Features tidal volume compensation.
Read Datex-Ohmeda's description.
Ohmeda 7800 controls. Click on the thumbnail, or on the underlined text, to see the larger version (34 KB).
This ventilator or the 7900 Smart-VentTM are 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 offers 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:
Read Datex-Ohmeda's description and specifications
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.
Photograph of the S/5 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).
Ohmeda's newest ventilator has a suite of useful and unique features not previously seen in ventilators meant for use during anesthesia. Single switch activation (setting the Bag/Vent switch to "Auto") is all that is needed to start mechanical ventilation. Entering the patient’s weight will suggest appropriate ventilator settings. Delivered VT is adjusted to compensate for changes in fresh gas flow, and total (absorber head and corrugated limbs) breathing circuit compliance losses through the D-Lite sensor at the elbow.
The ventilator can utilize either oxygen or air as a driving gas, and will switch automatically from one to the other if pipeline pressure is lost. Volume-control, pressure control, and synchronized intermittent mandatory ventilation (SIMV) modes are offered, along with integrated electronic PEEP. Overpressure release valve at 80 cm water (in spontaneous or mechanical ventilation) 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). SIMV is unique to this machine (and the Divan) among anesthesia ventilators, and may prove useful during emergence. Flow-volume (resistance) or pressure-volume (compliance) loops may be displayed breath-by-breath.
Siemens Kion. Click on the thumbnail, or on the underlined text, to see the larger version (64 KB).
The Kion ventilator operates in Manual, Volume control, and Pressure control modes. Pressure support is implemented on some machines. Very few Kion machines have yet been sold in the US.