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 ...