Anesthesia Machine Operation, Maintenance, and Troubleshooting

Andrew Oken

Scott Richins

▪ INTRODUCTION

The anesthesia machine began quite humbly as a basic device to deliver anesthetic gases. It has evolved to a sophisticated integrated computer-assisted physiologic monitoring system and anesthesia delivery system. Fundamentally, it continues to serve principally to facilitate gas exchange in the anesthetized patient; however, a number of functions have been integrated to improve the machine’s safety profile. Some of these improvements include agent-specific vaporizers, oxygen proportioning systems, oxygen analyzers, oxygen failure safety valves, breathing circuit pressure monitors, and the pin index safety system.

The anesthesia machine consists of various components managing gas delivery and elimination, including a ventilator, gas inflows from a variety of sources, anesthetic vaporizers, scavenging system, breathing circuit, and CO₂ absorption system. All these systems have appropriate check mechanisms and associated alarms or notifications to alert the medical providers to potential problems. While there are a number of machines available on the market, they principally share some very similar fundamental components and functionalities. It is therefore critical to have a basic understanding of the principles of the workings of the machine. Such a core base of knowledge is absolutely necessary to safe medical practice and the maintenance and evaluation/troubleshooting of anesthesia machines. Despite the similarities between anesthesia machines, it is important to recognize that machines have distinct differences. Anesthesia technicians should be thoroughly familiar with the unique properties of machines in use at their institutions.

The American Society of Anesthesiologists (ASA) and the Food and Drug Administration (FDA) have collaborated to develop recommendations for checking out an anesthesia machine prior to administering an anesthetic (see Chapter 27). Each manufacturer provides maintenance schedules, troubleshooting instructions, and guidelines for checkout of its specific machines. Internal machine designs vary and hence the need for specific manufacturer recommendations. In addition to testing machine components, the checkout will test associated alerts/alarms. Anesthesia technicians should have detailed knowledge of the anesthesia machine checkout procedures as this can be the first indication that there is a problem with a machine. Anesthesia technicians will also be asked to troubleshoot machine problems between cases or even during a case. This chapter assists the anesthesia technician with the operation, maintenance, and troubleshooting of anesthesia machines.

▪ BRIEF OVERVIEW OF THE ANESTHESIA MACHINE

Although anesthesia machines can include several functions, the main function will always be to provide a controlled supply of oxygen and other anesthetic gases to the patient during surgery. The details of gas supply to the machine have been discussed previously; however, a short review is presented here to facilitate a troubleshooting discussion (see Fig. 31.1).

The machine circuit can be broken down into high- and low-pressure circuits. The high-pressure circuit starts where the gas enters the machine and ends at the flow control valves.
Gas enters the machine at a pressure near 50 psi from the pipeline. Gas can also be supplied from E cylinders, which supply a pressure of 2,200 psi for oxygen and 745 psi for nitrous oxide when tanks are full. This pressure is then further regulated to 45 psi downstream. Traveling toward the patient the next device is the fail-safe valve, which is downstream from the nitrous oxide source and serves to decrease the supply of nitrous oxide if the oxygen pressure drops. Most Datex-Ohmeda machines also have an oxygen supply alarm that sounds when the pressure drops below a safe level. Both these devices serve to decrease the potential for delivering a hypoxic mixture of gas. Gas then enters the flow control valves. The oxygen and nitrous oxide valves are coupled together to limit the percent of nitrous oxide that can be given as another means to reduce the risk of giving the patient a hypoxic mixture of gas.

▪ FIGURE 31.1 Diagram of a generic gas anesthesia machine. (With permission from Barash P G, Cullen BF, Stoelting RK, et al. Clinical Anesthesia. 6th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2005.)

The low-pressure system of the anesthesia machine begins downstream of the flow valves. Gas travels through the flowmeters into a common manifold and then into one of the calibrated vaporizers. Between the vaporizers and the common gas outlet, which connects to the patient circuit, there is a check valve that prevents gas from flowing backward through the vaporizers. Exhaled gas passes through the carbon dioxide (CO₂)-absorbing canister and rejoins the fresh gas here as well. The combined gas fills the ventilator or the bag depending on which manual ventilation or the ventilator is selected. If excess gas is present at this juncture, it overflows to the scavenging system past the adjustable pop-off lever (APL), or through the ventilator relief
valve. Gas then flows out through the inspiratory flow device past the oxygen analyzer to the patient through the inspiratory limb of the circuit. Expired gas is recycled as it flows back though the expiratory limb of the circuit, through the flow sensor, and then through the CO₂-absorbing canister to ultimately rejoin the fresh gas from the common gas outlet prior to entering the ventilator or bag (see Fig. 31.2).

▪ Figure 31.2 Picture of circle system with ventilator. (From Barash PG, Cullen BF, Stoelting RK, et al. Clinical Anesthesia. 6th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2005.)

▪ GENERAL AND DAILY MAINTENANCE

Regular service should be performed by a machine representative according to the manufacturer’s recommendations. Daily maintenance by an anesthesia technician is crucial for proper function. Previous chapters have described sterilization techniques and daily machine checkout. These will not be covered in this chapter, but we will refer to the checkout process frequently because many of the problems that require troubleshooting can be prevented or later identified by the simple tests performed in a daily machine checkout.

At the end of the day, further routine maintenance should be done. This includes removing the flow transducer to dry unwanted moisture, exchanging the CO₂ absorber if it is exhausted, ensuring the gas flowmeters are off, and powering down the machine.

▪ TROUBLESHOOTING COMMON PROBLEMS

Low Pressure

If low pressure in the anesthesia machine is detected, the following items should be examined:

Check for low pressure leak.
Check for a circuit disconnect starting with the patient and working back to the machine.
Inspect bellows.
Inspect CO₂ canister.
Check lids/coupling of vaporizers.
Inspect the flow sensor.
Check oxygen pressure from pipeline/cylinder.
Check for incompetent ventilator relief valve.

The most common cause for a low-pressure alarm is a leak in the low-pressure circuit, and the most common cause of a leak is a disconnection or partial disconnection in the patient circuit. This is evaluated quickly starting at the patient and moving back to the machine evaluating the placement of the endotracheal tube (ETT), the ETT cuff pressure, the connection between the ETT and the circuit, the gas sample line port, and the connection of the inspiratory and expiratory tubing distal and proximal to the machine. If the leak is not resolved, continue to move upstream. Ensure the flow sensor is installed properly and tightly engaged. Check that the oxygen sensor is properly installed and all fittings are tight. Another common place for a leak is in the canister holding the CO₂ absorber (see Fig. 31.3). Check to see that the latch is tight and there are no granules in the way of the seal with the gasket. Also, look and listen for cracks in the canister and ensure the condensation drain is closed. When the ventilator is turned off, if the pressure in the circuit returns to normal with hand ventilation, the leak can be isolated to the ventilator. The housing can be removed to look for cracks. This, along with poor seating of the plastic housing, can result in inappropriately low tidal volumes delivered to the patient because the driving gas is leaking into the room rather than compressing the bellows. Next, inspect the vaporizer lids to ensure they are on tight and inserted into the coupling system correctly. With the use of electronic flowmeters in newer machines there are no fluted tubes that can crack, but this is another place a leak can occur and should be examined when mechanical flowmeters exist.

▪ FIGURE 31.3 Carbon dioxide absorption canister in open/unlocked position (see space between plastic housing and blue plastic gasket); also condensation drain open at bottom.

It is uncommon for a low-pressure alarm to be due to problems in the high-pressure circuit, but they can occur. Pressure can be lost from inadvertent disconnects from the main supply, loss of the main supply pressure, or unrecognized depletion of a cylinder. Other less common causes may include a malfunctioning scavenging system or the vacuum is set too high such that the negative pressure from the scavenging system is able to lower the pressure in the circuit. Occasionally, high negative pressure in the scavenging system can also cause high pressure in the breathing circuit. In some machines, high negative pressure from the scavenging system can close the ventilator relief valve blocking excess gas from exiting the circuit, leading to increased airway pressures. Lastly, an incompetent ventilator relief valve can lead to hypoventilation because gas is delivered to the scavenging system instead of to the patient when the bellows collapse.

Hypoxic Gas Mixture

Turn on 100% oxygen and check for proper reading from the oxygen sensor.
Recalibrate the O₂ sensor.
Replace O₂ electrode if it does not appropriately recalibrate.
Check for main pipeline crossover.

A common cause for a hypoxic gas mixture alarm is improper calibration of the oxygen sensor at the time of machine checkout. If the alarm sounds while a patient is connected to the circuit, the patient should be put on 100% oxygen (the patient may need to be ventilated with an alternative oxygen source during troubleshooting, e.g., bag-mask or replacement anesthesia machine). If the sensor does not read 100% after stabilizing for several minutes, the sensor should be recalibrated. If it will not recalibrate to the proper setting, the oxygen cell should be replaced. If there is a hypoxic reading, the high-pressure circuit should be evaluated. If a central pipeline crossover is suspected, the oxygen tank should be opened and the pipeline supply disconnected. If oxygen levels are restored by this change, it is critical to alert the appropriate hospital staff to avoid further serious adverse events.

Flowmeter Problems

Check for a leak.
Check for a stuck bobbin.

Flowmeter problems can often be detected during machine checkout. If the flowmeters are set to specific values and the oxygen or gas sensors are not reading the correct percentages of gas, either the sensors are malfunctioning or there is a problem with the circuit or flowmeters. Newer models of anesthesia machines are equipped with electronic flowmeters rather than the standard fluted glass tubes with a floating bobbin. Troubleshooting of electronic flowmeters should be done by the machine representative. The most common problem with fluted tube flowmeters is a float that is stuck from debris, leading to inaccurate readings. The quick fix is to gently flick the flow tube in an attempt to free the float from the debris. Ultimately, this should be brought to the attention of the machine representative for definitive cleaning and repair. The presence of a crack in the glass flow tube can cause a leak and thus a faulty reading. This can usually be detected during machine checkout when the low-pressure leak test is performed. Cracks that cause only a very small leak may be hard to detect. In some machines, a check valve, located between the common gas outlet and the flowmeters, prevents gas from flowing back through the vaporizer and into the flow tubes. In these machines, a positive-pressure leak test of the low-pressure circuit will not detect a leak in the flowmeters. A negative-pressure leak test should be performed on machines with a check valve in this location.

▪ VENTILATORS

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