Modes and targeting schemes of mechanical ventilation

“Current nomenclature relevant to ventilator modes is hopelessly confused and outdated”, wrote Robert Chatburn in his 2007 article proposing to update the methods of classifying mechanical ventilation.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.

In summary:

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.

What is a “mode” of ventiation

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.

Classification of modes of ventilation

There are several different ways to classify modes of ventilation. To borrow from Tobins, as it is something of a gold standard:

• Control variable: pressure vs. volume. Realistically, there are some modes for which the target schema changes back and forth between control variables, or where the control variable is subject to some sort of proportional adjustment or feedback from ventilator sensors, making this a difficult characteristic to apply to all cases. It plays the greatest role in classifying the “classical” modes, which generally apply a set-point or adaptive target schema.
• Breath sequence: spontaneous vs. mandatory. This is determined partly by the trigger variable and partly by the cycling variable.
• Targeting scheme: Set-point, dual, servo, adaptive, etc. This is sufficiently complex to merit further discussion here.

Targeting scheme

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:

• Set point:  where you pick a parameter, and the ventilator tries to achieve it. For example, this is the pressure level in a pressure-control mode of ventilation. The ventilator’s pressure sensors will feed back to the flow regulator, adjusting the flow rate to maintain the pressure set-point. Classical set-point schema include pressure control (PCV) and volume control (VCV).
• Dual targeting: this is where your ventilator has two rabbits to chase. In a dual targeted schema, the ventilator will switch from one control variable to another in the middle of the breath. In this fashion, a breath may start with a pressure control variable using a decelerating flow waveform, then reach the pressure limit mid-breath and change to volume control until the targeted volume is reached. There are probably no advantages to this method as compared to an adaptive targeting scheme.
• Servo control: a targeting scheme which converts some signal from the patient into some proportional level of ventilator support. Examples of this include the automated tube compensation (ATF), “proportional assist” ventilation (PAV) or NAVA.  The patient’s signal might therefore be volume, flow, or diaphragmatic EMG. The term apparently refers to the original definition of what a “servo” is, which is a device for amplifying mechanical power.
• Adaptive targeting scheme specifically refers to a control system that converts a small mechanical motion into one requiring much greater power, using a feedback mechanism. A good example is the PRVC (pressure regulated volume control) mode, where both pressure and volume are variables of interest. The inspiratory pressure is automatically adjusted to achieve an average tidal volume target, and this varies from breath to breath – adapting to the changing compliance. The only disadvantage of such a targeting scheme is the variability of mean airway pressure; one would potentially achieve a more stable pressure profile with a set-point target.
• Optimal targeting scheme is essentially the same as the adaptive scheme, but more complex. Instead of choosing a parameter like pressure or volume for each breath, an operator instructs the ventilator with patient parameters (eg. height and weight) and then leaves the optimal targeting scheme to achieve the weight-appropriate minute volume with whatever the best pressure and volume settings would be.