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A good ventilator classification scheme describes how ventilators work in general terms, but with enough detail so that one particular model can be distinguished from others. It facilitates description by focusing on key attributes in a logical and consistent manner. A clear description allows us to quickly assess new facts in relation to our previous knowledge. Learning the operation of a new ventilator or describing it to others then becomes much easier. Understanding how the ventilator operates, we can then anticipate appropriate ventilator management strategies for particular clinical situations. The classification system described in this chapter is based on previously published work.17

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A ventilator is simply a machine, a system of related elements designed to alter, transmit, and direct energy in a predetermined manner to perform useful work. We put energy into the ventilator in the form of electricity (energy = volts × amps × time) or compressed gas (energy = pressure × volume). That energy is transmitted or transformed (by the ventilator’s drive mechanism) in a predetermined manner (by the control circuit) to augment or replace the patient’s muscles in performing the work of breathing. Thus to understand mechanical ventilators in general, we must first understand their basic functions: (a) power input, (b) power transmission or conversion, (c) control scheme, and (d) output. This simple format can be expanded to add as much detail as desired (Table 2-1).

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Table 2-1: Outline of Ventilator Classification System
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A discussion of input power sources and power conversion and transmission is beyond the scope of this chapter; these topics have been treated elsewhere.7,8 The chapter does, however, explore in detail control schemes and ventilator modes because these directly affect patient management.

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Models of Patient–Ventilator Interaction

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