“Current nomenclature relevant to ventilator modes is hopelessly confused and outdated”, wrote Robert Chatburn in his 2007 article proposing to update the methods of classifying mechanical ventilation. “Perhaps no other word in the mechanical ventilation lexicon is more used and less understood than ‘mode’.” That remains an accurate assessment at the time of writing. Each manufacturer has their own naming schema and the control algorithms of mechanical ventilators are proprietary, giving rise to a proliferation of trademarked modes of ventilation with confusing abbreviated names. Some are so ubiquitous that they may seem standard (eg. “PCV”, “CMV”, “Pressure support”) but in fact are not, and there is no agreement on what those terms actually mean, even though everybody seems to use them.
Fortunately, there is no reason for an ICU trainee to have any level of detailed familiarity with every possible mode of ventilation. It is would suffice to understand the consequences of choosing a control variable, the effects of spontaneous and mandatory modes, and the influence of the modes’ targeting schema on the way the objectives of mechanical ventilation are achieved.
- A “mode” of ventilation is a pre-set combination of settings designed to achieve specific objectives in mechanical ventilation
- Modes are generally classified according to their major characteristics
- The control variable: pressure vs. volume
- Breath sequence: spontaneous vs. mandatory
- Targeting scheme: Set-point, dual, servo, adaptive, etc.
- Manufacturer abbreviations of modes are proprietary and inconsistent, making it difficult to compare the effects of similar modes.
This topic does not appear as one of the 2017 CICM primary syllabus, nor is there much in the WCA competency “Ventilation” which focuses on more pragmatic matters (like actually setting the ventilator). As such, the entire chapter can be omitted from primary exam revision. For further reading on modes and mode classifications one can pay for the UpToDate article, or read this free post by Chatburn from 2014, or go to Ball et al (2015) for the anaesthetist’s perspective.
Looking for a formal definition of “mode” seems to lead nowhere fun. Wikipedia and UpToDate both define it with an identical phrase, without actually defining anything:
“The mode refers to the method of inspiratory support“
That of course totally ignores the expiratory support (which arguably does most of the work) and the possibility that during inspiration, no support of any sort is offered. Fortunately, most textbooks of mechanical ventilation tend to do a better job. For example, in Tobin Chatburn first describes the goals of mechanical ventilation (safety, comfort, adequate gas exchange etc) and then defines a “mode” in terms of them:
“The preset pattern of patient-ventilator interaction designed to achieve these objectives is referred to as a mode of ventilation.”
Similarly, Egan’s Fundamentals of Respiratory Care explains a mode as “the manner in which a ventilator achieves this objective”. In general, it appears everybody is on the same page, and many textbooks (eg. Pilbeam’s) don’t even feel the need define what a “mode” is because most people who routinely deal with ventilators have some sort of intuitive grasp of this concept, coded in the language of thought. Pragmatically, for the purposes of exams one should probably borrow Chatburn’s definition. From the point of view of working with a ventilator, one might also define a mode as “a pre-set combination of ventilator settings”, because that’s how modes are presented in the interface.
There are several different ways to classify modes of ventilation. To borrow from Tobins, as it is something of a gold standard:
The targeting scheme is best defined as the method of feedback control used to deliver a particular pattern of ventilation. There are several types of such feedback control mechanisms: