Anesthesia Gas Machine- Table of Contents

The biggest improvement in current ventilators is their flexibility in modes of ventilation, including modes which support assisted and spontaneous ventilation. Various types of pressure controlled ventilation (PCV) allow more efficient and safe ventilation of many patients.

The improvement in ventilator sensors and electronic controls means that switching of circuits (for example, to a non-rebreather for small children) is not as necessary. This is safer because potential misconnections are avoided, and quicker besides. The latest direction the manufacturers have taken is offering modes (such as pressure support) that will support or assist spontaneous ventilation, seen in anesthesia with much greater frequency due to the advent of the laryngeal mask airway and more brief ambulatory procedures. Although ventilators provide more accurate V_T in VCV (due to compliance and leak compensation- see below on this page), this mode is currently used much less frequently than when VCV was the only mode available.

Terminology for modes and controls is not standardized, and can be confusing. The table below gives Dräger and GE terminology for ventilator mode classification.

**Ventilation Modes**; Click on the small image to see a larger image of the pressure & flow waves.
Control Label Drager (GE)	Pressure, Flow waves	Definition
Volume Mode VC-CMV (VCV)		Volume control- Continuous mandatory ventilation: A set number of breaths/minute (f or RR) which are time cycled and machine triggered. Tidal volume (V_T) is controlled, and independent of changes in lung mechanics. Inspiratory flow is constant. Peak inspiratory pressure varies and should be kept as low as possible, certainly < 35 cm water. PEEP may be added to most modes.
Volume Mode VC-SIMV (SIMV-PSV)		Volume control- Synchronized intermittent mandatory ventilation: The mandatory breaths are synchronized with the patient‘s own breathing attempts, if these generate enough negative pressure within a trigger window. If no breathing attempt is detected during the trigger window, the machine-triggered mandatory breath is applied. Thus the minute volume (MV or V_E) remains constant over time. The patient can breathe spontaneously at PEEP level during the expiratory phase, and these breaths can be pressure-supported using PSV.
Volume Mode VC-CMV-AF or Autoflow (PC-VG)		Autoflow applies the set V_T with the minimum pressure required. If Resistance (R) or Compliance (C) changes, the pressure adapts gradually, by automatic adjustment of inspiratory pressure and flow. AF may be applied in any volume controlled mode; it is very similar to pressure control with volume guarantee (PC-VG) which is used in the Aisys and Avance.
Pressure Mode PC-CMV (PCV)		Pressure control- Continuous mandatory ventilation: controls inspiratory pressure (Pinsp), PEEP, and f (breaths/min). The pressures are maintained which may compensate for leaks. Tidal volume (and V_E) vary with changes in patient effort, compliance and airway resistance. The flow generated varies. Flow is high at first to produce the set Pinsp early, and it is less later in inspiration to maintain the set pressure through the inspiratory time (Ti). PEEP, Volume guarantee, SIMV, and PSV may be added to pressure control ventilation.
Pressure Mode PC-SIMV (SIMV-PC)		Pressure control- Synchronized intermittent mandatory ventilation: PC-CMV, with the mandatory breaths delivered in synch with patient efforts.
Pressure Mode PC-CMV-VG (PC-VG + SIMV)		Pressure control with volume guarantee: Volume guarantee may be used with pressure-controlled modes such as PC-SIMV, and PC-CMV. Volume guarantee ensures that for all mandatory breaths the set V_T is applied with the minimum pressure necessary. If the Resistance (R) or Compliance (C) changes, the pressure adapts gradually over a few breaths to restore the set V_T.
Pressure Mode PC-PSV (PSVPro; CPAP/PSV)		Pressure support ventilation: The patient can breathe spontaneously at PEEP level. Every detected inspiratory effort is supported. The absolute level of pressure support is defined by Psupp. The duration of inspiration is flow-cycled and thus depends on the lung mechanics of the patient. The patient determines the number, timing and duration of the pressure-supported breaths. If the breathing frequency of the patient is lower than the set backup frequency (RR) or there is no spontaneous breathing, machine-triggered flow-cycled mandatory breaths with the set pressure Pinsp (PCV) are applied. The backup settings (Pinsp, RR) are controlled by the operator. V_T results from the pressure difference between PEEP and Psupp, the lung mechanics and the breathing effort of the patient. If the Resistance (R) increases, or Compliance (C) of the lung decreases, V_T and V_E will decrease.
Pressure Mode PC-BIPAP (None)		Biphasic positive airway pressure: the patient can breathe spontaneously at any time, but the number of mandatory breaths is specified. Mandatory breaths are synchronized with any breathing attempts of the patient. If no spontaneous breathing attempt is detected during the inspiratory trigger window, the machine-triggered mandatory breath is applied. The V_T results from the pressure difference between PEEP and Pinsp, the lung mechanics and the breathing effort of the patient. If the Resistance (R) or Compliance (C) of the lung changes, V_T and V_E also vary. During spontaneous breathing at PEEP level, the patient can be supported using PS.
Pressure Mode PC-APRV (None)		Airway Pressure Release Ventilation: the patient’s spontaneous breathing takes place at the upper pressure level "P_high". P_high is maintained for the duration of T_high (time at high pressure). To execute an active expiration and support CO₂ elimination, the pressure is reduced (to P_low) for a brief period (T_low). The alternation between the two pressure levels is machine-triggered and time cycled. V_T expired during the relief times (at P_low), results from the pressure difference between P_low and P_high and the lung mechanics. If the Resistance or Compliance of the lung changes, the V_T and thus the V_E also vary.
Spontaneous mode SPN-CPAP (CPAP or PSV)		Spontaneous- Continuous positive airway pressure: In SPN-CPAP, the patient breathes at the PEEP level. Compared to the atmospheric pressure, the airway pressure is increased during the complete breathing cycle.

Manufacturer	Model	Ventilator type	Modes
Dräger Medical	Perseus A500	Turbine	Manual/spontaneous, PC-CMV, PC-BIPAP, VC-CMV, VC-CMV/AF, VC-SIMV/AF. Optional: SPN-CPAP/PS, PC-BIPAP/PS, VC-SIMV/AF/PS, PC-APRV.
Dräger Medical	Apollo	Piston	Manual/spontaneous, VC-CMV, PC-CMV. Optional: PS, VC-CMV/AF.
GE Healthcare	Aisys-CS²	Bellows	Manual/spontaneous, VC-CMV. Optional: PC-CMV, PCV-VG, SIMV (with VC-CMV, PC-CMV, or PC-VG), SPN-CPAP/PS.

Modes not currently implemented in anesthesia ventilators

Volume Controlled -Continuous Mandatory Ventilation (VC-CMV)

The values controlled and kept at the set target value are the V_T and respiratory rate (RR, breaths/min, f). This results in the minute volume (V_E = f x V_T). The duration of the inspiratory phase is defined by the inspiration time (Ti). If the inspiratory flow is so high that the set breathing volume is reached before the set inspiratory time (Ti) has passed, there will be a pause in inspiration.

VC-CMV, once universal, is now used much less frequently, in view of better alternatives such as pressure conytrol modes.

Settings for VCV in an adult

Volume control- Synchronized intermittent mandatory ventilation (VC-SIMV)

Patients may breathe at will between ventilator breaths. The AIsys/Avance ventilators in VCV mode support any "in-between" spontaneous breaths with pressure support ventilation (PSV), resulting in a mode GE calls SIMV-PS.

Pressure Control -Continuous Mandatory Ventilation (PC-CMV)

Target pressure is adjusted to produce a reasonable V_T (reasonable to avoid the extremes of atelectasis and volutrauma). 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.

Pressure controlled modes, especially those which incorporate SIMV, PSV, and Volume guarantee, are often a default mode on new workstations.

Indications

Settings for PC-CMV in an adult

Pressure Control with Volume Guarantee (PC-CMV-VG)

PC-CMV, but with a target V_T set. The ventilator then dynamically adjusts the Pinsp (while staying within the set maximum pressure [Pmax]) to achieve the desired V_T breath-by-breath. Advantages include control of PIP (through the basic pressure-controlled mode) and control of arterial CO₂ (through guarantee of V_T and thus minute ventilation). Volume guarantee may be used with SIMV. Volume guarantee ensures that for all mandatory breaths the set V_T) is applied with the minimum pressure necessary. If the Resistance (R) or Compliance (C) changes, the pressure adapts gradually (over 4 or so breaths) to return delivered V_T to the target V_T.

Pressure control-Synchronized intermittent mandatory ventilation (PC-SIMV)

PC-Biphasic Positive Airway Pressure (PC-BIPAP)

The V_T results from the pressure difference between PEEP and Pinsp, the lung mechanics and the breathing effort of the patient. If the Resistance (R) or Compliance (C) of the lung changes, V_T and V_E also vary. During spontaneous breathing at PEEP level, the patient can be supported using PS.

Modes for spontaneous ventilation-PSV, CPAP, & APRV

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) have now been implemented on various models of anesthesia workstations.

During the spontaneous ventilation modes, the patient carries out the majority of the breathing effort. The pressure level PEEP (CPAP) at which spontaneous breathing takes place, can be adjusted. In all spontaneous ventilation modes, the spontaneous breaths can be supported mechanically. To suit the respective lung mechanics, the speed of the pressure increase for PS (Pressure Support) and VS (Volume Support) can be defined using the slope (also called T_insp)or flow adjustment. Both adjustments, slope and flow, thus define the duration of the pressure increase from the lower to the higher pressure level. With the slope adjustment the time is set in seconds, with the flow adjustment the gas flow is set in liters per minute. This setting directly affects the flow and thus the supplied V_T.

Pressure Support Ventilation (PSV)

Settings for PSV are simple- just P_supp (the inspiratory pressure support level- try 10 to 12 cm H₂O to start) and PEEP (the "expiratory support level" if that makes sense). P_supp should be adjusted to obtain a V_T of 5-7 mL/kg ideal body weight. Note that PSV requires a spontaneously breathing patient as there is no mandatory minimum respiratory rate. PEEP is ordinarily used with PSV to help recruit alveoli.

PSV senses patient inspiratory effort (volume or flow) and delivers pressure support while it is present. This tends to result in larger V_T 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, this mode is called PSV-Pro ("protect"). If no breaths are detected during an adjustable apnea delay period (10-30 sec), the ventilator switches to the backup mode (PCV, at backup settings of Pinsp and RR). If resumption of spontaneous breaths occurs later, the ventilator will return to PSV-Pro mode.

CPAP

Airway Pressure Release Ventilation (PC-APRV)

In PC-APRV, the patient’s spontaneous breathing takes place at the upper pressure level "P_high". P_high is maintained for the duration of T_high (time at high pressure). To execute an active expiration and support CO₂ elimination, the pressure is reduced (to P_low) for the brief period (T_low). The alternation between the two pressure levels is machine-triggered and time cycled. The V_T expired during the relief times (at P_low), results from the pressure difference between P_low and P_high and the lung mechanics. If the Resistance or Compliance of the lung changes, the V_T and thus the V_E also vary.

What is protective ventilation?

Ventilator-associated lung injury is caused by overinflation of some lung areas (volutrauma), excess PIP (barotrauma), underinflation and derecruitment of other areas (atelectrauma), and an increase in inflammatory mediators resulting from alveolar wall stress (biotrauma). [7] , [8]

While no one "recipe" works for all patients or all situations, certain themes have emerged from the research which are probably useful for most patients: [9]

Typical ventilator alarms

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.

New features of modern ventilators

Turbine ventilators

Turbines have been used in the ICU for years, but until recently were not found on anesthesia ventilators. Turbines are an efficient means of generating inspiratory flow quickly, thus they are well-suited to deliver patient-triggered breaths. The TurboVent 2 on Perseus A500 (Dräger) is a compressor/blower which generates the pressure required for ventilation. During inspiration, TurboVent 2 rapidly (in milliseconds) increases the rotation speed to create the set inspiratory pressure (imagine a super powerful hair dryer). It spins at a low rate during expiration also. This maintains PEEP, and also a flow of 0.6 L/min though the breathing circuit, in order to improve mixing of gases in the circuit and lessen the work of breathing of any small spontaneous breaths.

The maximum inspiratory flow is 180 L/min (higher than any current anesthesia ventilator except Servo types). Operation is controlled electronically, based on data from the pressure and flow sensors contained in the breathing system. The ventilation pressure and flow during inspiration and expiration are attained by means of the turbine and the Pmax/PEEP flow valve in the breathing system.

Piston ventilators

Piston ventilators have 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 a piston ventilator system (Apollo & Fabius GS):

Flexibility

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 which are capable of supporting the patient with spontaneous respirations (like PSV, SIMV, CPAP, and Volume Guarantee) extends our capabilities further.

Accuracy at lower tidal volumes

Compliance and leak testing

An electronic leak and compliance test must be repeated every time the circuit type is changed (e.g. adult circle to pediatric circle, or adult to long circuit). This test is part of the electronic self-test portion of the morning checklist.

The placement of the sensor used to compensate tidal volumes for compliance losses and leaks has some interesting consequences. All flow and volume sensors are somewhat sensitive to moisture accumulation, which is more likely due to rebreathing as fresh gas flows decrease. Apollo, Perseus, and Aisys test compliance and leaks of all components to the Y-piece via pressure transducer and flow sensors within the internal circuitry near the bellows. Here the sensors are somewhat protected from moisture. Aisys relies on an optional (but recommended) condenser to keep the breathing circuit dry and protect the flow sensors; Perseus uses a heated absorber head.

Fresh gas decoupling versus compensation

Modern ventilators compensate delivered tidal volume for changes in fresh gas flow (FGF). In older ventilators 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.

The visual appearance is unusual as compared to a bellows ventilator. The action of the piston closes a one-way (decoupling) valve, diverting FGF to the manual breathing bag during the inspiratory cycle.

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, and Aespire. 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.

A similar approach is taken in the Dräger Perseus. The breathing circuit is like Apollo, but has no fresh gas decoupling valve. The placement of the fresh gas flow (FGF) inlet between turbine and inspiratory unidirectional valve means that FGF is not diverted to the reservoir bag during inspiration (as in Apollo or Fabius) but is added to V_T during inspiration. Data from flow and pressure sensors is used to control turbine speed. The Pmax/PEEP valve, which stops inspiration when the desired V_T and inspiratory time (Ti) are achieved. In Perseus (opposite to Appollo or Fabius), the bag behaves like a standing bellows- deflating during inspiration and inflating during expiration.

Suitability for low flows

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 FGF in modern ventilators include:

Current models

Dräger Perseus A500

Dräger Apollo

GE Aisys

Aisys. Click on the link to see the larger version.

Aisys controls. Click on the link to see the larger version.

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).

End tidal Control. Click on the link to see the larger version.

The Aisys CS2 features End Tidal Control, by which the users sets targets for expired agent, oxygen,and minimum fresh gas flows, and the machine manipulates FGF and agent concentration to achieve and maintain these targets more quickly and with less control interactions with the operator.

GE Avance

Avance. Click on the link to see the larger version.

The Avance uses the same 7900 ventilator and breathing system as the Aisys. The difference in the machines is that the Avance uses conventional vaporizers.

GE Aespire

Aespire. Click on the link to see the larger version.

Aespire ventilator controls. Click on the link to see the larger version.

Aespire (and Aestiva) share the current GE 7100 ventilator. Aespire shares the Advanced Breathing System used by Aisys and Avance. Aespire uses traditional flowmeters for FGF (needle valve, glass flowtube). The 7100 ventilator features VCV, PCV, and electronic PEEP. V_T range is 45-1500 mL. Maximum inspired pressure is 50 cm water.

Mindray A9

Mindray A9. Click on the link to see the larger version.

Modern servo type ventilator with a wide selection of modes (VCV, SIMV-VC, PCV, PCV-VG, SIMV-PC, SIMV-VG, CPAP/PSV, APRV). Maximum inspiratory flow 180 L/min; V_T 10-2000 mL (VCV).

Features include lung recruitment & protective ventilation toolkits, high-flow nasal cannula, agent usage calculation, volume exchanger (reflector) in breathing system, low flow optimizer, injector vaporizers, electronic gas mixer for FGF. Automatic Controlled Anesthesia automatically adjusts the FGF & vaporizer output to quickly achieve the preset target end-tidal agent and inspiratory oxygen concentration).

Maquet Flow-e

Maquet Flow-e. Click on the link to see the larger version.

Maquet Flow-e & Flow-I use a modern servo type ventilator with VCV, PCV, PSV, SIMV, & PRVC. Maximum inspiratory flow 200 L/min; V_T 20-2000 mL (VCV).

Maquet suggests that patients should be ventilated using Pressure Regulated Volume Control (PRVC) unless specified otherwise. PRVC delivers a set tidal volume but automatically adjusts the inspiratory pressure on a breath-to-breath basis to achieve it with the lowest pressure possible. This adaptive strategy provides lung-protective ventilation, especially for patients with fluctuating compliance or resistance, while ensuring consistent gas exchange.

Features include active hypoxic guard, lung recruitment, agent usage calculation, volume reflector in breathing system, low flow optimizer, injector vaporizers, electronic gas mixer for FGF. Automatic Gas Control (AGC)- users sets the targeted inspired oxygen, end-tidal anesthetic agent levels and required speed, AGC Controlled Anesthesia automatically adjusts the FGF & vaporizer output to quickly achieve the preset targets.

Older or obsolete models

How do you determine that a machine needs replacement? [14]

GE Aestiva

Aestiva/5 with 7100 ventilator. Click on the link to see the larger version.

Paragon

Paragon. Click on the link to see the larger version.

AV-S ventilator. Click on the link to see the larger version.

The breathing circuit is latex free, like most modern machines. 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 automated 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-S

Anestar. Click on the link to see the larger version.

The Anestar ventilator is a hanging bellows, gas-driven, electronically controlled ventilator. It offers VCV, PCV, and PSV modes. V_T is 10-9999 mL. Insp/Exp ratio is 3:1 to 1:5. The breathing circuit is warmed and V_T compensation is achieved through fresh-gas decoupling. Flow sensors are hot-wire anemometers (like most Draeger machines). The internal volume of the breathing system is 2.5 L (of which 1.4 L is absorbent).

Fabius GS ventilator

Fabius GS ventilator controls and piston window (left of flowmeters). Click on the link to see the larger version.

Fabius GS Ventilator controls. Click on the link to see the larger version.

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

V_T 20-1400 mL
Freq 4-60 bpm
I:E ratio 4:1 to 1:4
Inspiratory Pause 0-50%
PEEP to 20 cm H₂O
Adjustable Peak flow, plateau time, insp flow (max 75 L/min)
Pressure limit 15-70 cm H₂O
Inspiratory pressure (pressure control mode) 5-60 cm H₂O
Inspiratory flow (pressure control mode) 10-75 L/min

ADU ventilator

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 link to see the larger version.

D-Lite sensor. Click on the link to see the larger version.

The ADU ventilator has a suite of useful and unique features, many of which it shares with other modern anesthesia ventilators. Single switch activation (setting the Bag/Vent switch to "Auto") is all that is needed to start mechanical ventilation (all new ventilators are activated in this way). Entering the patient’s weight will suggest appropriate ventilator settings. V_T 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 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.

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.

Dräger Divan ventilator

Divan Controls (front panel). Click on the link to see the larger version.

The Dräger Divan ventilator is a modern piston ventilator, offering features such as: pressure control mode, SIMV, correction for compliance losses, and integrated electronic PEEP. Unlike the ADU, 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 it cannot ventilate patients in VCV 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 6000/6400.

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.

Dräger AV-E and AV-2

AV2 controls. Click on the link to see the larger version.

Classification: pneumatically and electrically powered, double circuit, pneumatically driven, ascending bellows, time cycled, electronically controlled, V_T 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

Ohmeda 7000 controls. Click on the link to see the larger version.

Same classification as Dräger AV-E except it is minute-volume preset (unique among anesthesia ventilators). V_T cannot be set directly, it is calculated from settings of V_E and respiratory rate (V_E = RR x V_T). Inspiratory flow stops when set V_T 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.

Ohmeda 7800

Ohmeda 7800 controls. Click on the link to see the larger version.

This ventilator or the older Ohmeda 7900 Smart-Vent were standard on newer Excel or Modulus machines. Same classification as Dräger AV2 ventilator; V_T preset. Tidal volume, respiratory rate, inspiratory flow and pressure limit controls are present.

Older Ohmeda 7900 "SmartVent"

Ohmeda 7900 controls. Click on the link to see the larger version.

Same classification as Dräger AV ventilator, V_T preset. Microprocessor control delivers set V_T, 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, and PCV. 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. [15], [16]

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 "V_T" 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.

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[1] Thille AW, Lyazidi A, Richard JC, Galia F, Brochard L. A bench study of intensive-care-unit ventilators: new versus old and turbine-based versus compressed gas-based ventilators. Intensive Care Med. 2009 Aug;35(8):1368-76. doi: 10.1007/s00134-009-1467-7.

[2] Wallon G, Bonnet A, Guérin C. Delivery of tidal volume from four anaesthesia ventilators during volume-controlled ventilation: a bench study. Br J Anaesth. 2013 Jun;110(6):1045-51. doi: 10.1093/bja/aes594.

[3] Bahns E. The Breathing Book- Spontaneous breathing during artificial ventilation (2015) https://www.draeger.com/Content/Documents/Products/breathingbook-bk-9066665-en.pdf, Accessed Feb 1, 2026; Drager Ventilation Mini Manual (2015) https://www.draeger.com/Content/Documents/Content/Draeger-Ventilation-Mini-Manual.pdf Accessed Feb 1, 2026; Deden K. Ventilation modes in intensive care. (2010) https://www.draeger.com/Content/Documents/Content/ventilation-modes-bk-9066587-en-us.pdf Accessed Feb 1, 2026

[4] Operator's Manuals (Perseus, Apollo, Aisys); Maquet Critical Care AB. 2015. Getinge Maquet Flow-I 4.2 User Manual; Mindray. 2024 Operating Instructions A9 Anesthesia System; Ball L, Dameri M, Pelosi P. Modes of mechanical ventilation for the operating room. Best Pract Res Clin Anaesthesiol. 2015 Sep;29(3):285-99. doi: 10.1016/j.bpa.2015.08.003.

[5] Ferrando C, García M, Gutierrez A, Carbonell JA, Aguilar G, Soro M, Belda FJ. Effects of different flow patterns and end-inspiratory pause on oxygenation and ventilation in newborn piglets: an experimental study. BMC Anesthesiol. 2014 Oct 22;14:96. doi: 10.1186/1471-2253-14-96.

[6] Brimacombe J, Keller C, Hörmann C. Pressure support ventilation versus continuous positive airway pressure with the laryngeal mask airway: a randomized crossover study of anesthetized adult patients. Anesthesiology. 2000 Jun;92(6):1621-3. doi: 10.1097/00000542-200006000-00019.

[7] Severgnini P, Selmo G, Lanza C, Chiesa A, Frigerio A, Bacuzzi A, Dionigi G, Novario R, Gregoretti C, de Abreu MG, Schultz MJ, Jaber S, Futier E, Chiaranda M, Pelosi P. Protective mechanical ventilation during general anesthesia for open abdominal surgery improves postoperative pulmonary function. Anesthesiology. 2013 Jun;118(6):1307-21. doi: 10.1097/ALN.0b013e31829102de.

[8] Melo MF, Eikermann M. Protect the lungs during abdominal surgery: it may change the postoperative outcome. Anesthesiology. 2013 Jun;118(6):1254-7. doi: 10.1097/ALN.0b013e3182910309.

[9] Young CC, Harris EM, Vacchiano C, Bodnar S, Bukowy B, Elliott RRD, Migliarese J, Ragains C, Trethewey B, Woodward A, Gama de Abreu M, Girard M, Futier E, Mulier JP, Pelosi P, Sprung J. Lung-protective ventilation for the surgical patient: international expert panel-based consensus recommendations. Br J Anaesth. 2019 Dec;123(6):898-913. doi: 10.1016/j.bja.2019.08.017.

[10]Wallon G, Bonnet A, Guérin C. Delivery of tidal volume from four anaesthesia ventilators during volume-controlled ventilation: a bench study. Br J Anaesth. 2013 Jun;110(6):1045-51. doi: 10.1093/bja/aes594.

[11] Thille AW, Lyazidi A, Richard JC, Galia F, Brochard L. A bench study of intensive-care-unit ventilators: new versus old and turbine-based versus compressed gas-based ventilators. Intensive Care Med. 2009 Aug;35(8):1368-76. doi: 10.1007/s00134-009-1467-7.

[12] Feldman JM, Smith J. Compliance compensation of the Narkomed 6000 explained. Anesthesiology. 2001 Mar;94(3):543-4. doi: 10.1097/00000542-200103000-00038.

[13] Bachiller PR, McDonough JM, Feldman JM. Do new anesthesia ventilators deliver small tidal volumes accurately during volume-controlled ventilation? Anesth Analg. 2008 May;106(5):1392-400, table of contents. doi: 10.1213/ane.0b013e31816a68c6.

[14] Dorsch JA. Guidelines for Determining Anesthesia Machine Obsolescence (2004) https://www.apsf.org/article/guidelines-published-for-determining-anesthesia-machine-obsolescence/ Accessed Feb 2, 2026

[15] Krock JL, Padda GS, Moore JD. Ohmeda 7900 Ventilator Flow Sensor failure. Anesth Analg. 1998 Jul;87(1):231-2. doi: 10.1097/00000539-199807000-00052.

[16] Rothschiller JL, Uejima T, Dsida RM, Coté CJ. Evaluation of a new operating room ventilator with volume-controlled ventilation: the Ohmeda 7900. Anesth Analg. 1999 Jan;88(1):39-42. doi: 10.1097/00000539-199901000-00008.

8. Ventilators

Classification

Bellows classification

Ventilator modes and settings

Modes not currently implemented in anesthesia ventilators