The Anesthesia Gas Machine

Michael P. Dosch CRNA PhD, Darin Tharp CRNA MS
University of Detroit Mercy - Nurse Anesthesia
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Revised April 2021


Using Breathing Circuits and Ventilators


Dry gas supplied by the gas machine may cause clinically significant desiccation of mucus and an impaired mucociliary elevator. This may contribute to retention of secretions, blocking of conducting airways, atelectasis, bacterial colonization, and pneumonia.

Absolute humidity is the maximum mass of water vapor which can be carried by a given volume of air (mg/L). This quantity is strongly determined by temperature (warm air can carry much more moisture). Relative humidity (RH) is the amount present in a sample, as compared to the absolute humidity possible at the sample temperature (expressed as a %).

Some examples:

It is ideal to provide gases at body temperature and 100% RH to the patient’s airway. For cases lasting longer than 1 hour, humidification measures are often employed including:

The heat and moisture exchanger has large thermal capacity, hygroscopic, and (sometimes) bacterial filtration. It can do no more than return the patient’s exhaled water- it can’t add heat or moisture- and it is less efficient with longer cases or higher flows. But it’s easy to use, inexpensive, silent, won’t overheat or overhydrate the patient.

Heated airway humidifiers provide perfect conditions- 100% RH at body temperature. However, these are no longer used because of various problems: overhydration, overheating (burns), require higher flows (flow-over type), melted circuits, aspiration.

How is the "best" fresh gas flow (FGF) determined?

The fresh gas flow used determines not just FIO2, but also the speed with which you can change the composition of gases in the breathing circuit.

Low flows

Low flows are used to decrease the usage, cost, and pollution of volatile anesthetics. A 50% reduction in FGF translates to a 50% savings, without placing the patient at risk or lessening the quality of their care. Tracheal heat and humidity, and patient core body temperature are preserved better than at higher flows.

Since the fresh gas flowing during the inspiratory phase of each breath augments delivered tidal volume (VT), changing FGF on older machines changes delivered tidal volume. Newer machines employ fresh gas decoupling or compensation to eliminate this problem. A decrease to low flows on older machines will cause delivered VT to decrease, and end tidal carbon dioxide to increase to an extent, particularly with low compliance or low VT.

The composition of gases in the breathing circuit may change as lower flows are employed, since a greater fraction of the gas inspired by the patient will be rebreathed.

Heater fan and Heat mix controls Heater fan and Heat mix controls. Click on the thumbnail, or on the underlined text, to see the larger version (60 KB).

Large discrepancies between dialed and inspired agent concentration can be unsettling, raising apprehensions about vaporizer or breathing circuit malfunction. An analogy may help to clarify why this is an expected result of low flows. Imagine you are entering an automobile in the winter. You turn the heater on at maximum heat level and fan speed. After the car is warmed to a comfortable temperature, you can't turn the heater entirely off. Heat must still be supplied, since it is always dribbling out (the car is not airtight). To keep the car at equilibrium, you may either flow a moderate to high fan speed, but decrease the heat mix to nearly room temperature air, or you may leave the heat mix level high, and slowly blow in a small amount of very hot air. It makes no difference- in either case the car stays at the desired temperature.

Similarly, we begin cases with higher flows. Since there is little rebreathing at 4 L/min FGF and above, the dialed and inspired agent concentration are very similar. We induce with overpressure until the patient is saturated (reflected in an end-tidal agent concentration which approaches the dialed agent concentration). Then we may either leave the flows high with a moderate agent concentration near MAC, or turn the flows to low flow. But if we use low flows, we must still provide the same number of molecules of agent in order to replace that lost due to dilution, leaks, and uptake to fat and muscle. So we must turn the vaporizer dial well beyond what we might have at higher flows.

Advantages of low flows

Disadvantages of low flows

Contraindications for low flows- Absolute and relative

How to denitrogenate ("preoxygenate")

You can "preoxygenate" with a nasal cannula. We need to do more- denitrogenate (cleanse the functional residual capacity of nitrogen)- to help our patients tolerate a potential 2 or more minutes of apnea if we have difficulties with intubation.

  1. Fresh gas flow 6-8 L/min
  2. APL valve open fully
  3. Tight mask fit
  4. Every time you place a mask on a patient's face, look back at the breathing bag (to ensure it is fluctuating with respirations) and the oxygen flowmeter (to ensure it is on).

Malignant hyperthermia: Implications for equipment

Clinical presentation The cause of the tachycardia, tachypnea, and elevated end-tidal CO2 seen in malignant hyperthermia (MH) must be distinguished from ventilator or unidirectional valve malfunctions (producing respiratory acidosis), as well as hyperthyroidism, cocaine intoxication, pheochromocytoma, and sepsis.

Triggers Succinylcholine and all inhaled agents are the only anesthetic agents that will trigger MH.

Safe anesthetics Barbiturates, propofol, etomidate, ketamine, opioids, local anesthetics, catecholamines, nitrous oxide, and all non-depolarizing muscle relaxants are presently considered safe. MHAUS list of safe/unsafe (MHAUS= Malignant Hyperthermia Association of the United States)

Treatment of acute episodes in OR High fresh gas flow (10 L/min), notify surgeon, hyperventilation, use activated charcoal filters on inspiratory and expiratory limbs of breathing circuit, stop inhaled agents and remove vaporizers, stop succinylcholine, and as time permits change soda lime granules & breathing circuit. The mainstay of treatment is dantrolene 2.5 mg/kg (up to 10 mg/kg). Cooling by any and all means, NaHCO3, treatment of hyperkalemia, and other measures are also important. MHAUS Recommendations for management of acute crisis.

Management of known susceptible patients- MHAUS recommends four alternatives to prepare the gas machine:

  1. "Flush and prepare workstation according to manufacturer’s recommendations or published studies; this may take 10 to >90 minutes.
    • Most studies also physically disconnect vaporizers from the workstation;
    • use a new, disposable breathing circuit;
    • and replace the carbon dioxide absorbent.
    • During the case, fresh gas flow should be kept at 10 liters per minute to avoid “rebound phenomenon” (increased release of residual volatile anesthetic agent when fresh gas flow is reduced after a set period of flushing).
  2. OR Use commercially available activated charcoal filters that have been shown to remove trace levels of volatile anesthetic agents following a 90 second flush with high fresh gas flows. These filters have been demonstrated in one in vitro study to be effective for 12 hours.
  3. OR If available, use a dedicated “vapor free” machine for MH-susceptible patients. The machine must be regularly maintained and safety-checked.
  4. OR If appropriate to the institution, use an ICU ventilator that has never been exposed to volatile anesthetic agents."

Ventilator and Breathing Circuit: Problems and Hazards


Most common site is Y piece. The most common preventable equipment-related cause of mishaps. Direct your vigilance here by:

  1. precordial ALWAYS
  2. if you turn the vent off, keep your finger on the switch
  3. use apnea alarms and don’t silence them.

Monitors for disconnection

Occlusion/obstruction of breathing circuit

Beside inability to ventilate, obstruction may also lead to barotrauma. Obstruction may be related to:


Much less of a problem since breathing circuit and scavenger tubing sizes have been standardized. However, breathing systems ARE reconfigured for preventive maintenance and other reasons. One such incident resulting in apnea, inability to ventilate, and asystole in 2007 (US FDA. Adverse event report MW5003097, Manufacturer and User Facility Device Experience (MAUDE) database. 2007. Available here. Accessed February 16, 2016).

Failure of emergency oxygen supply

May be due to failure to check cylinder contents, or driving a ventilator with cylinders when the pipeline is unavailable. This leads to their rapid depletion, perhaps in as little as an hour, since you need approximately a VT of driving gas per breath, substantially more if airway resistance (RAW) is increased. Anesth Analg 2002;95:148


Clean the bellows after any patient with diseases which may be spread through airborne droplets, or don’t use the mechanical ventilator, or use bacterial filters, or use disposable soda lime assembly, or use a Bain.

Mechanical ventilator failure

Protocol for mechanical ventilator failure

  1. If the ventilator fails, manually ventilate with the circle system. (Why? Allows you to continue using vaporizers.)
  2. If #1 is not possible, then bag with oxygen (if a portable cylinder is available) or room air.
  3. If #2 is not possible, then try to pass suction catheter through the tracheal tube. (When in doubt, pull it out is not the best advice in all circumstances.)
  4. If #3 is not possible, then visualize the hypopharynx and cords with the tube in place, or reintubate (?).
Don’t delay reestablishing ventilation to diagnose a problem. Proceed expeditiously from one approach to another.

Look under Respiratory Care tab at Difficult Airway Algorithm which is Anesthesiology Feb 2013;118:251. (A new revision is in comments stage in 2021.)

Increased inspired carbon dioxide

Inspired unidirectional valve problem- bottom capnogram Inspired unidirectional valve problem- bottom capnogram. Click on the thumbnail, or on the underlined text, to see the larger version (18 KB).

The causes of increased inspired carbon dioxide are almost exclusively either malfunctioning unidirectional valves, or exhausted absorbent.

Increased inspired carbon dioxide has other potential causes, but these are rare

Treatment must be accurately directed at the cause or it will be ineffective. Many approaches are useless: increasing minute ventilation, seeking signs of malignant hyperthermia, checking for leaks in the circuit, obtaining arterial blood gases, bronchoscopy for mucous plugs, central line insertion, recalibrating or replacing ventilator, capnograph, or entire gas machine.

Malfunctioning unidirectional valves can cause serious problems.

Differential diagnosis: Machine malfunction? Altered patient physiology?

Increased carbon dioxide production will not result in increased inspired carbon dioxide. The capacity of the soda lime granules is sufficient to cleanse each breath entirely, even if carbon dioxide production is increased. Further, respiratory acidosis will not cause visibly dark blood, or desaturation on the pulse oximeter.

Diagnosis and treatment

If the granules are not exhausted, and the inspiratory and expiratory unidirectional valves are forcing all exhaled gas through the granules, there can be no increase in inspired carbon dioxide. So, if it is detected:

  1. First, increase fresh gas flow (FGF) to much greater than minute ventilation.
    • A fresh gas flow of 8-10 L/min creates a semi-open system, with essentially no rebreathing, since the amount of fresh gas is sufficient to dilute any exhaled carbon dioxide to very low levels (and send it to the scavenging system).
    • If the granules are exhausted, inspired CO2 will return to normal when FGF is increased.
    • Either leave FGF high and change granules at end of case, or change the granules as soon as practical and safe during the case and return to low flow.
  2. If the increased fresh gas flow didn't decrease inspired CO2, an incompetent valve is likely.
    • Modern breathing circuits do not allow cleaning, drying, or replacing the valve while a patient is attached to the breathing circuit.
    • The entire workstation should be replaced while bagging the patient, and assuring that anesthesia depth continues with TIVA.

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