Some anesthesia machine ventilators are visible, but others are inside the machine. You may think the ventilator is the “business end” of the machine, and in some ways you are correct. Use and manipulation of a ventilator are parts of acute care at its most acute—delivering oxygen to a patient that will cause the least amount of harm and the greatest amount of good. We will not discuss the physiology of ventilators in this chapter; that can be found in texts that do an infinitely better job of covering that topic. What we will discuss in this chapter is simply the mechanics of anesthesia machine ventilators.

There are a few different ways to ventilate a patient using an anesthesia machine. One is to use the mechanical ventilator that is built into the machine. Another way is to manually ventilate the patient with the reservoir bag (“bagging the patient”). Another method is jet ventilation using the machine’s oxygen supply. Finally, the patient can breathe spontaneously while attached to the anesthesia machine. Each one of these methods has its place in anesthetic management.

Modern anesthesia machines are able to ventilate patients who would have been difficult to ventilate 20 years ago. Improvements in accuracy and power allow us to ventilate patients with stiff lungs that we could not have in the past. Back then an intensive care unit type ventilator may have been brought into the operating room (OR) simply to adequately ventilate such patients. In addition, with increases in accuracy and pressure monitoring and control, we are able to ventilate neonates and infants with a standard anesthesia machine ventilator now that would have been difficult in the past. Special smaller bellows were available for pediatric cases, and clinicians had to remove the standard size bellows and replace them with the smaller type for patients weighing less than around 10 kg.

There are two main types of anesthesia ventilators: bellows type and piston type. We will discuss each one in depth. Keep in mind, however, that machines can also be classified by what power source each one uses (purely pneumatic, pneumatic and electrical, or purely electrical). Although it is important to know how your anesthesia ventilator is powered, we prefer to classify them as to their mechanism of action.

A third type of anesthesia ventilator is the servo type. Formerly made by Siemens, they are now produced and marketed by Maquet. Those clinicians with experience in critical care may be familiar with the servo ventilator. These ventilators were modified to deliver volatile agents, and some even had rebreathing capability from an attached carbon dioxide absorber. Because of a servomechanism and its negative feedback, these ventilators were able to deliver tidal volumes (TVs) much more precisely than bellows-type anesthesia ventilators in the past, especially when dealing with the small TVs of neonates. Thus, the “servo” part of its name referred to its manner of ensuring proper TV delivery instead of how the ventilator itself worked. The means of delivering anesthetic agent is by an injection system into the fresh gas flow (FGF) instead of a traditional flow-over vaporizer. Nowadays, control of TV in modern anesthesia machines, whether bellows or piston type, has improved to the point where it is not difficult to ventilate neonates with a standard anesthesia machine.

BELLOWS VENTILATOR

The bellows-type ventilator may be the only type of anesthesia ventilator that many clinicians have ever used. Until the past decade, they were just about the only type available. Despite inroads made by the piston ventilator, bellows ventilators are ubiquitous, and generations of anesthesiologists and anesthetists have trained on them and use them tens of thousands of times each day.

The classification of bellows ventilators is as follows: pneumatically driven, double circuit, electronically controlled, ascending (or descending) bellows, time cycled, and TV preset. Similar to the classification of vaporizers, we will discuss each part of the description.

Pneumatically Driven

Exactly what makes the bellows move up and down? To the eye, there does not appear to be any mechanism that physically causes the bellows to move during a ventilatory cycle. Did you ever wonder why the bellows are in that clear plastic terrarium-like dome?

The reason is that on inspiration, that clear, airtight dome is pressurized with a gas that pushes the bellows down (or up in a descending bellows; more on that later). The gas is called the driving gas, and it is either oxygen or an oxygen and air mixture, depending on the type and brand of anesthesia machine. That is why the bellows ventilator is called pneumatically driven.

At the onset of inspiration, a valve opens, and the driving gas comes into the dome. The amount of driving gas needed per breath is approximately the same volume as the TV you have chosen to deliver. It is not exactly the same because there are other factors that are involved such as FGF, but for our purposes, we will say that the volume of pressurized driving gas is about the same as TV. This driving gas pushes the bellows, causing the volume of gas inside the bellows to enter the patient. At the end of inspiration, the valve letting the driving gas into the clear dome closes, and another valve opens that allows the driving gas to escape the clear dome. The bellows reexpand with the patient’s passive exhalation because of the compliance of the chest wall. Then the cycle begins again (Figure 8-1).

As you can imagine, there is an incredible amount of driving gas used during an anesthetic. It is not recycled. After each breath, the driving gas is vented into the room. If you are using a cylinder source of oxygen, you will go through oxygen tanks relatively quickly. If the machine is the kind that uses an air–oxygen mix, air cylinders are not needed. The machine entrains air from an orifice in the back of the machine by the Venturi effect.

Because the driving gas is only oxygen or a mixture of oxygen and air, there is no pollution of the room with anesthetic agents. Remember, it is the gas inside the bellows that goes into the patient and has anesthetic in it. The driving gas and the gas inside the bellows do not mix at all. (Actually they can mix, but we will talk about that later in the section Disadvantages and Hazards of Bellows Ventilators.)

Double Circuit

What this means is that there are two compartments of gas involved in a bellows-type anesthesia machine—the gas that is inside the bellows and the gas that is outside the bellows. As explained earlier, the gas outside the bellows is the driving gas that causes the bellows to move, and the gas inside the bellows is what goes into the patient. Under normal conditions, they never intermingle.

Electronically Controlled

Even though the bellows are pneumatically driven, the whole cycle relies on electricity to power the driving gas valves and to control the timing and volume of ventilation.

Ascending or Descending Bellows

By convention, bellows are named for what they do on expiration. They either go up or down. The only bellows ventilators you have ever used more than likely are the ascending type. Descending bellows fell out of favor in the late 1970s and early 1980s. They were believed to be a safety hazard.

Ascending bellows are anchored to the bottom of the clear dome. They move down on inspiration and fill back up again on expiration. Descending bellows, as you can guess, do the opposite of ascending bellows. They are anchored to the top of the clear dome. Imagine a bat hanging upside down. They move up on inspiration and move down as they reexpand. Ascending bellows have a small amount of intrinsic positive end-expiratory pressure (PEEP) as a result of the weight of the bellows being pushed back up on exhalation, somewhere around 1 to 2 cm H₂O.

If there is a disconnect, what happens to ascending bellows? They collapse because of loss of pressure inside them and gravity. The low-pressure alarm goes off, and you see the collapsed bellows. This has been a tremendous visual aid over the years, especially in the days before routine capnography. Experienced clinicians rely on visual cues from the ascending bellows a great deal. Not only do ascending bellows give a visual indication of a disconnect, but they also give a visual cue of a leak. If there is a leak in the circuit, the bellows will not rise all the way to the top of the clear dome after each breath. In addition, ascending bellows give the clinician a visual cue of when the patient “breathes through” the ventilator, initiating spontaneous respirations are returning.

The main problem with descending bellows is what happens during a disconnect. Because of gravity, the descending bellows elongate when there is a loss of pressure and volume. They look like they are full, but actually the opposite is occurring. Remember, when the ascending bellows lose volume and pressure, they collapse, leaving an empty clear dome, which is a good visual alarm to an anesthetist that something is wrong.

In addition to not giving you a visual clue of a disconnect, a descending bellows disconnect might not cause enough of a pressure change to sound the low-pressure alarm.

However, one company, Datascope/Anestar, has produced a machine with descending bellows in the 2000s. The company says the problem of undetected disconnect with a descending bellows is not a factor now because the apnea alarm comes from the capnograph, not the machine itself.

Nevertheless, the two main anesthesia machine companies, Datex-Ohmeda and Draeger, produce ascending bellows machines.

Time Cycled

We control by electric means the number of breaths per minute.

Tidal Volume Preset

We control by electrical means the TV that will be delivered to a patient. Modern bellows machines can also use pressure control, where the peak airway pressure the operator wishes to administer is set and the TV is a result of that pressure.

Disadvantages and Hazards of Bellows Ventilators

Tidal Volume Accuracy As stated, inspiration is initiated because the driving gas enters the clear dome and pressurizes it, and as more driving gas enters, the bellows are pushed down to deliver the set TV to the patient. There is more variability in each breath potentially because of changes in patient lung compliance and airway resistance. The driving gas is compressible, so as an example, the same amount of driving gas that pushed the bellows down to deliver 700 cc TV on one breath and one level of lung compliance and airway resistance may not make the bellows move the same amount on the next breath if compliance and resistance vary from one breath to another. It is the compressibility of the driving gas that is the main problem with TV accuracy with a bellows ventilator.

Use of Oxygen as Driving Gas

As discussed earlier, bellows ventilators use a lot of oxygen. The amount will be around what your minute ventilation is. It is not recyclable. Even with some machine types that use oxygen and entrained room air, quite a bit of oxygen is used. This can be problematic if you are in an anesthetizing location that does not have a wall source for oxygen. The driving gas for the ventilator would have to come from your oxygen cylinders.

Let us go through an example. You are anesthetizing in a location that has no wall or pipeline source of oxygen. You are delivering a TV of 700 cc at a rate of 10 breaths/min. The minute volume therefore is 7 L/min. So you are using approximately 7 L/min just to operate the bellows. After 1 hour of anesthesia, your utilization of oxygen just to drive the bellows is

Your oxygen cylinder has 660 L of oxygen in it.

So you have used two thirds of your E-cylinder of oxygen simply to drive the bellows! This leaves you with 240 L to oxygenate the patient, which averages out to 4 L/min to use for patient oxygenation. It is easy to run low flows during the maintenance of an anesthetic, but most clinicians use higher flows for induction and emergence and to rapidly change inhalational concentration. Don’t forget the oxygen flush button either!

What it boils down to is that you would have to change out cylinders every hour or so. That would be not only a hassle to change them so frequently and lug that many tanks to your location but also potentially dangerous for the patient. Who is going to change the tank? Can they do it quickly? If you change the tank, who will watch the patient while you are behind the machine?

Leaks

As mentioned previously, pediatric-sized bellows were available in the past before the accuracy of bellows ventilators improved. The bellows were easily changed out in only a couple of minutes. However, each time the bellows were changed, it introduced a chance for leaks to happen if the bellows assembly was not exactly fitted properly onto the ventilator. In those days, the bellows attachment was a commonplace to find circuit leaks. Fortunately, we do not have to change bellows anymore. The same bellows ventilator can safely and accurately ventilate an adult or an infant.

Be on guard for bellows assembly leaks if any maintenance was recently done on your machine. The black plastic bellows need to be replaced every once in a while, so improper reassembly can result in a leak.

Another type of leak reported with bellows is when there is a hole in the bellows themselves. Remember we said the bellows assembly is a “double circuit” with one volume of gas inside the bellows that goes into the patient and a separate volume inside the dome assembly that is our driving gas. These two volumes of gas are separate and never intermingle.

With a hole in the bellows, though, the two different volumes of gas can mix. The pressure in the clear dome is around 50 psig when it is pressurized and the bellows are pushed down. That high pressure can potentially be transmitted through the hole in the bellows and cause barotrauma to the patient.

In addition, the fraction of inspired oxygen (FiO₂) can change because of a hole in the bellows. The 100% oxygen that is the driving gas in an Ohmeda machine or the oxygen–air mixture in a Draeger bellows machine can either dilute or enrich your FiO₂, depending on the circumstance if a hole in the bellows allows the two sections of gas to mix.

Barotrauma

In addition to high pressure being transmitted through the bellows from the driving gas in case of a bellows hole, barotrauma can also occur from using the oxygen flush button during inspiration.

PISTON DRIVE

The piston drive ventilator is relatively simpler than the bellows. It may seem a bit more mysterious to you because it is sometimes hidden deep inside the anesthesia machine. See Figures 8-2 and 8-3 as examples of how to access the piston on a Draeger Apollo machine. The piston drive ventilator consists of a cylinder, about the size of a coffee can that contains a soft, flexible plastic insert reminiscent of a flower pot. There is a piston at one end of the cylinder. A motor actuates on inspiration to push the piston to whatever degree it takes to deliver the set TV. The piston pushes the gas volume of the flexible insert into the patient circuit (Figures 8-4 and 8-5).

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