Abstract
The diversity of operation of volume-preset ventilators often makes it difficult for students and others to understand the operation of differing models. Poor under-standing of how a ventilator is controlled can result in less-than-proficient operation. All volume-preset ventilators have three characteristics in common: the ability to 1) produce volume exchange in the lungs, 2) deliver relatively constant volumes, and 3) mimic basic breathing patterns. Major factors that affect these three functions are inspiratory time, expiratory time, ratio of inspiratory time to expiratory time, respiratory time or period, respiratory frequency, tidal volume, minute volume, and inspiratory flow rate. These eight factors were incorporated in a simplified diagram to comprehensively explain the operation of volume-preset ventilators. Every model of volume-preset ventilator is controlled by three of the eight factors, constituting an operational triad. When the operational triad of a volume-preset ventilator is known and applied to the diagram, the method of operation can be more clearly understood. Twenty equations are given to quanti-tatively explain the interrelationships of the eight factors and to be used to sub-stantiate how an operational triad controls the unit. The diagram has limited clinical applications because of clinical conditions that also affect the operation of ventilators, and therefore it should be used primarily as an instructional tool.
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