Heater fan and Heat mix controls. Click on the thumbnail, or on the underlined text, to see the larger version (60 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/.
Revised June 2009
COMPONENTS & SYSTEMS> DELIVERY> USING BREATHING CIRCUITS AND VENTILATORS
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 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. 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.
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 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. 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.
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.
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:
Please note that the 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 agent, so each model should be tested before it can be accepted that the standard procedures apply.
You can contact the Malignant Hyperthermia Association of the United States for further information. The "frequently-asked questions" and "ABCs of managing MH" are very helpful.
Most common site is Y piece. The most common preventable equipment-related cause of mishaps. Direct your vigilance here by:
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
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.
Protocol for mechanical ventilator failure
Difficult Airway Algorithm Anesthesiology 2003;98:1269–77
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.
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.
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: