The Anesthesia Gas Machine

Michael P. Dosch CRNA PhD
University of Detroit Mercy Graduate Program in Nurse Anesthesiology
This site is http://www.udmercy.edu/crna/agm/.

Revised July 2012

COMPONENTS & SYSTEMS> DELIVERY> USING BREATHING CIRCUITS AND VENTILATORS

Using Breathing Circuits and Ventilators

Humidification

Dry gas supplied by the gas machine may cause clinically significant dessication 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 patientís 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 patientís exhaled water- it canít add heat or moisture- and it is less efficient with longer cases or higher flows. But itís easy to use, inexpensive, silent, wonít overheat or overhydrate the patient.

Heated airway humidifiers provide perfect conditions- 100% RH at body temperature. Types: Cascade, flow-over (Fisher-PaykellTM, MarquestTM), heated wet wick (AnamedTM). However, these are rarely 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 volatiles. 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 changes delivered tidal volume, unless fresh gas decoupling or compensation are employed. A decrease to low flows on older machines will cause delivered VT to decrease, and end tidal carbon dioxide to increase to an extent.

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, 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 near MAC). 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. So we must turn the vaporizer dial well beyond what we might have to 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 3 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.

Treatment of acute episodes in OR High fresh gas flow (10 L/min), hyperventilation, 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.

Management of known susceptible patients- To prepare the gas machine:

  1. Remove or at least drain all vaporizers and tape over the dial.
  2. Change breathing circuit disposables and soda lime.
  3. Flush by ventilating with high (10 L/min) fresh gas flow for at least 10 minutes (20 min if you cannot remove and replace breathing circuit and granules).
    • Note that this guideline does NOT apply to all machines. One study showed that a modern machine (Fabius GS) required 105 minutes of flushing before it was agent-free (Kim TW, Nemergut ME. Preparation of modern anesthesia workstations for malignant hyperthermiaĖsusceptible patients: A review of past and present practice. Anesthesiology. 2011;114(1):205-212).
    • The Siemens Kion requires flushing before use on a susceptible patient for 25 minutes- four times as long as required by an Excel 210 (Anesthesiology 2002;96:941-6). Any new equipment has the potential to contain internal components that absorb or sequester volatile agent, so each model should be tested before it can be accepted that the standard procedures apply.
  4. Monitor pETCO2 and core temperature in all.
  5. Avoid triggers
  6. Use rocuronium, particularly if rapid sequence induction is indicated.
    • Sixty (60) sec after rocuronium 0.6-1.2 mg/kg, intubating conditions indistinguishable from succinylcholine can be produced (at the price of a clinical duration of 30-40 min).

You can contact the Malignant Hyperthermia Association of the United States for further information. The "frequently-asked questions" is very helpful.

Ventilator and Breathing Circuit: Problems and Hazards

Disconnection

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 donít 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:

Misconnection

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, 2012).

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.

Infection

Clean the bellows after any patient with diseases which may be spread through airborne droplets, or donít 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.
  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.
  4. If #3 is not possible, then visualize the hypopharynx and cords, or reintubate (?).
Donít delay reestablishing ventilation to diagnose a problem. Proceed expeditiously from one approach to another.

Difficult Airway Algorithm Anesthesiology 2003;98:1269–77

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).

Malfunctioning unidirectional valves can cause serious problems. If the inspiratory valve is incompetent, the patient exhales into both limbs. The capnogram may show a slanted downstroke inspiratory phase (as the patient inhales carbon dioxide-containing gas from the inspiratory limb) and increased end-tidal carbon dioxide (as in the bottom capnogram in the figure).

If the expiratory valve is incompetent, increased inhaled and exhaled carbon dioxide levels may appear with a normal appearing capnogram. The cardinal sign in either is an elevated baseline- a non-zero inspired CO2. Failure of granules or valves has been defined as inspired CO2 of 2 (or more) mm Hg (Anesth Analg 2001;93:221-5). Both situations result in respiratory acidosis unresponsive to increased ventilation. If the valves stick closed, all gas flow within the circle system ceases, and one cannot ventilate the patient.

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.

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.

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. Change the granules at the end of the case, or as soon as practical and safe.
  2. Second, inspect the unidirectional valves (if increased FGF was not effective in reducing increased inspired CO2).
    • If the increased fresh gas flow doesn't solve the problem, the granules can not be at fault.
    • The expiratory valve is more prone to trouble because of the higher humidity on that side of the circle.
    • Clean or replace the expiratory valve while bagging the patient. But do NOT dissassemble the breathing circuit while caring for a patient without practicing your ability to do so quickly ahead of time, or without assuring that a backup means or ventilation is readily available. Otherwise, you will be doing mouth to tube ventilation (!).

 


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