How Do Anesthesia Machines Work?

Anesthesia

By: Mike Dosch

Published: 10/11/2011

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A look under the hood of these OR mainstays.


Anesthesia machines keep patients asleep and breathing during procedures. That much you know. But do you know how they perform anesthesia's essential tasks? Let's explore the inner workings of surgery's often-overlooked workhorses.

1. Vaporizers add precise amounts of volatile anesthetics to the fresh gas flow. Many agent-specific vaporizers are now electronically controlled. On most machines, total fresh gas flow enters variable bypass vaporizers and splits into carrier gas — which flows over the liquid anesthetic agent — and bypass gas — which does not enter the vaporizer's chamber — and meet up again at the vaporizer's outlet for delivery into the patient breathing circuit. Providers control the splitting ratio to increase or decrease the gas flow and concentration, and in turn how much anesthetic the patient receives.

2. Flowmeters let providers set levels of nitrous oxide, oxygen and air delivery, the desired percentage of inspired oxygen and total fresh gas flow (in liters per minute). Flowmeters on newer machines have minimum oxygen flow settings of 50ml/minute compared to 200 to 300 ml/minute on slightly older models. Low-flow settings let providers use less volatile anesthetics, which reduces the cost and environmental impact of anesthesia care. In addition, low-flow delivery of anesthetics helps maintain a patient's core body temperature. The latest electronic flowmeters let computerized anesthesia records chart fresh gas flow, and are said to be more accurate in gas flow delivery than conventional flowtubes.

3. Ventilators help maintain close to normal respiratory rates and normal blood chemistry in a wide variety of patients. They are the features on anesthesia machines that have changed the most in recent years. Older machines housed ventilators with 2 settings: on or off. The ventilators were set to breathe for deeply anesthetized patients. Pretty basic. Newer machines boast up to 7 different modes of ventilation in order to match the flexibility needed for keeping older, sicker and heavier patients breathing spontaneously in the OR.

4. Breathing circuits deliver oxygen and anesthetic gases to patients and eliminate the CO2 they exhale. Current machines offer breathing circuits adaptable to a wide variety of patients, from newborns weighing less than 10 lbs. to morbidly obese patients weighing 300 lbs. or more. There are 2 types of breathing circuits: non-rebreathing and circle. Circle breathing circuits are by far the most popular systems in use today. The circuits cleanse CO2 expelled from the patient, allowing for the rebreathing of exhaled anesthetic gases. They let patients breathe in constant concentrations of the anesthetic; the systems conserve respiratory heat and humidity; and they are suited for low-flow anesthetic delivery.

5. Scavenging systems (in back) collect and remove expelled anesthetic gases from the OR. Systems are active (suction is applied to remove gases) or passive (gas escapes through tubing to a room ventilation exhaust grill). When active systems are used, the patient's airway must be protected from the suction device and positive-pressure buildup of the waste gases; passive systems require only the monitoring of positive pressure. Most new scavenging systems are open to the atmosphere, which is safer for the patient than antiquated closed systems that released waste gas into the atmosphere through valves.

Anesthesia machines continue to evolve
As more and more procedures migrate to the outpatient setting, providers are taking care of sicker patients during more sophisticated and quicker procedures. Efficiency-minded surgeons and administrators want to keep patients — and cases — moving through their facilities. To maintain clinical efficiencies, providers aren't anesthetizing patients as deeply as they used to and aim to keep them breathing on their own as much as possible, which leads to quicker emergence in PACU and faster discharges. Today's anesthesia machines have evolved to keep up with this shift in patient care philosophy.

  • Current and future needs. Make sure the machines you purchase have ventilators that include volume ventilation and a pressure control mode that are able to ventilate difficult patients. The capability to export fresh gas values to electronic health records is quickly becoming a must-have feature.

The disposables you used with previous machines (the breathing circuits, for instance) must be compatible with the new units you purchase. Should you buy integrated monitors? They are compact and work well with the anesthesia machine as a whole, but machines can last as long as 15 years. Will you be satisfied with a company's monitoring technology for that long? Can it be upgraded?

  • Safety measures. The latest machines feature more electronically driven components than ever before. During my anesthesia training in the 1980s, power outages would only impact a machine's ventilation capabilities, but there's little on today's newer machines that does not require electric power.

While your facility's main power source will rarely, if ever, fail, and you likely have emergency generators in place, be sure your anesthesia machines contain enough battery backup power to last 30 minutes — more than enough time for the surgeon to close and for your provider to bring the patient back to consciousness. A note of caution: Never plug external monitoring devices into outlets located in the back of an anesthesia machine. That could overload the machine's circuits and jeopardize the functioning of the monitors. Instead, always plug monitors into wall sockets.

Nitrous oxide is a greenhouse gas, but it's used in such small quantities in the OR that its impact on the environment is negligible. (Nitrous oxide is released into the environment when fertilizer breaks down. The amount of N2O generated during the delivery of anesthesia is less than 5% of the amount generated by our nation's agriculture.) The collection of waste anesthetic is still important, however, especially in limiting the surgical team's exposure to high concentrations of expelled gases.

  • Cost containment. The cost of disposables needed to operate newer anesthesia machines are on the rise, so limiting the expense of anesthetic delivery in other ways is paramount. Change the breathing circuit's high-efficiency filters, along with the mask and elbow joint, for every new patient, but limit exchanging the flexible corrugated breathing circuit hoses, spirometry tubing and D-lite sensor to once daily. Use low-flow settings on flowmeters to limit usage of expensive volatile agents. Opt for loose carbon dioxide granules instead of single-use canisters to capture expelled CO2.
  • Provider training. Don't skimp on educating providers on the use of new anesthesia machines. The latest, largely automated units are more complex than the knob and tube models on which many providers trained.

15 Anesthesia Machine Safety Checks

Perform this entire checklist daily. Repeat items 2, 4, 7 and 11 to 15 before each case.

1 Verify auxiliary oxygen cylinder and self-inflating manual ventilation device are available and functioning.
Failure to ventilate a patient is a major cause of morbidity and mortality related to anesthesia care. Make sure a self-inflating and properly functioning manual ventilation device (an Ambu bag, for example) is present at every anesthetizing location for every case.

2 Verify that patient suction is adequate to clear the airway.
Safe anesthetic care requires the immediate availability of suction to clear the airway if needed.

3 Confirm that AC and battery power is available.
Many anesthesia machines have visual indicators of the power source showing the presence of both AC and battery power. Check these indicators and confirm connection of the power cord to a functional AC power source.

4 Verify availability of required monitors and check alarms.
Visually confirm that appropriate monitoring supplies are available. Turn on all monitors and complete power-up self-tests to confirm they are in working order. Check that all audible alarms are on and working.

5 Verify that pressure is adequate on the spare oxygen cylinder.
Make sure oxygen cylinders have acceptable minimum pressures, which depends on the intended use, the design of the anesthesia delivery system and the availability of piped oxygen. Close the oxygen cylinder valve after verifying that adequate pressure is present, unless the cylinder is to be the primary source of oxygen.

6 Verify that piped gas pressures are = 50 psig.
Gas supplied from a central source can fail for a variety of reasons. Check the psig (pound-force per square inch gauge) pressure in the piped gas supply at least once daily.

7 Verify that vaporizers are adequately filled and filler ports are closed.
Partially open filler ports are a common cause of leaks that may not be detected if the vaporizer control dial is not open when a leak test is performed. This leak source can be minimized by tightly closing filler ports. Newer vaporizer designs have filling systems that automatically close the filler port when filling is completed. Use high and low anesthetic agent alarms to help prevent over- or under-dosage of anesthetic vapor and set them to the appropriate limits.

8 Verify that there are no leaks.
Turn off each vaporizer to check for leaks in the vaporizer mount and chamber. Also perform a negative pressure test to check for leaks between flowmeters and the common gas outlet. When relying upon automated testing to evaluate the system for leaks, repeat the automated test for each vaporizer and whenever a vaporizer is changed.

9 Test scavenging system function.
A properly functioning scavenging system prevents anesthetic gases from contaminating the OR. Perform daily checks of the system to ensure positive- and negative-pressure relief valves are functioning properly.

10 Calibrate or verify calibration of the oxygen monitor and check the low oxygen alarm.
Continuous monitoring of the inspired oxygen concentration is the last line of defense against delivering hypoxic gas concentrations to the patient. Check once daily that self-calibrating oxygen monitors read 21% when sampling room air. Ensure that the low-oxygen-concentration alarm is functioning and audible.

11 Verify carbon dioxide absorbent is functioning.
Proper function of a circle anesthesia breathing system relies on the absorbent to remove carbon dioxide from rebreathed gas. Replace exhausted absorbent, as indicated by the characteristic color change.

12 Test breathing system for proper pressure and leaks.
Perform pressure and leak tests with the circuit configuration to be used during anesthetic delivery. If any of the circuit's components are changed, complete the test again.

13 Verify that gas flows properly through the breathing circuit.
Pressure and leak testing does not identify all obstructions in the breathing circuit or confirm proper function of the inspiratory and expiratory unidirectional valves. Use a test lung or second reservoir bag to confirm unimpeded flow through the circuit during inspiration and exhalation.

14 Document completion of checkout procedures.
Document completion of these procedures, which can be helpful if an adverse event occurs. Some automated systems maintain an audit trail of completed checkout procedures that are dated and timed.

15 Confirm ventilator settings and evaluate readiness to deliver anesthesia care.
Answer these questions during the anesthesia time out. Are monitors functional? Is a capnogram present? Will oxygen saturation by pulse oximetry be measured? Are flowmeters and ventilator settings correct? Is the ventilator switch set to manual? Are vaporizers adequately filled?

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