Intermittent Mandatory Ventilation Will Live Forever

To the Editor:

Reading the article by Kacmarek and Branson¹ about whether intermittent mandatory ventilation (IMV) should be abolished, it seemed like arguing whether flip phones should be abolished (eg, in favor of smart phones). Yes, flip phones are still used, but they represent a technological anachronism, and any discussion about their role in everyday communications has become essentially irrelevant. But whether the discussion about IMV is irrelevant depends on how it is defined.

In the introduction of the paper,¹ IMV was defined as “a mode of ventilation where intermittent mandatory breaths are delivered at clinician-defined intervals, and between these mandatory breaths, the patient can breathe spontaneously or with pressure-supported breaths.” In the first place, thinking of IMV as a “mode” (vs the breath sequence component of a complete mode classification²) is a very restricted use of the term. It indicates an outdated paradigm that limits understanding of currently available options for ventilation and prevents a clear vision of future developments.

More importantly, the authors never give definitions for “mandatory” or “spontaneous” breaths, upon which the definition of IMV depends. The result is a certain level of ambiguity that might prevent the reader from appreciating the full scope of the subject. A spontaneous breath is one for which inspiration is both triggered and cycled by the patient (ie, the patient's brain); a mandatory breath is one for which inspiration is either triggered or cycled by the ventilator.² Note that the definition of a spontaneous breath does not imply any particular level of assistance for the work of breathing and, indeed, does not even imply connection to a ventilator. Non-intubated people breathe spontaneously, and intubated patients on CPAP breathe spontaneously, as do those on some type of assistance (eg, pressure support, proportional assist ventilation plus, neurally adjusted ventilatory assist, automatic tube compensation).

Herein lies the problem. The authors completely ignore the fact that there are actually 3 different varieties of IMV. In type 1, preset mandatory breath frequency is always delivered; in type 2, mandatory breaths are delivered only when the spontaneous breath frequency is less than the preset mandatory breath frequency; and in type 3, mandatory breaths are delivered only when the spontaneous minute ventilation (ie, spontaneous breath frequency times average spontaneous breath tidal volume) is less than the preset minute ventilation.²

Type 1 IMV was the original, as described by Kacmarek and Branson.¹ Type 2 IMV exists today as, for example, on the Philips Respironics V60 ventilator in the mode called “spontaneous/timed.”³ Type 3 IMV was originally invented way back in 1977 by Hewlett et al⁴ and called “mandatory minute ventilation.” That mode is still available today on Dräger ventilators (called “mandatory minute volume ventilation”) and on Maquet ventilators (called “AutoMode”⁵) and has evolved further into the mode called “adaptive support ventilation” on Hamilton ventilators.⁶

The important point is that type 2 and 3 IMV both allow spontaneous breaths to suppress mandatory breaths. That feature casts a whole new light on IMV. To put this into a clinical context, it is helpful to categorize patients into 4 basic groups: (1) those incapable of generating spontaneous breaths (eg, organ donors and patients under neuromuscular blockade); (2) those with unreliable spontaneous breath rates (eg, premature infants); (3) those with little risk of apnea (perhaps the majority of ventilated patients); and (4) those with no need of mandatory breaths at all (eg, patients undergoing spontaneous breathing trials). Of course, this spectrum of breathing ability can and often does occur in a single patient, so perhaps it is better to think of the 4 groups as levels of mandatory breath requirement (ie, complete dependence, intermittent dependence, and complete independence). Hence, there is a need for the ventilator, in general, to provide for both spontaneous and mandatory breaths on an intermittent basis. This was the motivation for inventing IMV in the first place, as Kacmarek and Branson mentioned in their paper.¹

How does recognition of only type 1 IMV (as in the article by Kacmarek and Branson¹) impair our understanding of modes? There are only 3 basic goals of mechanical ventilation (safety, comfort, and liberation),^7,8 and the unique benefit of IMV is that can serve all three. All forms of IMV allow presetting of a minimum minute ventilation, serving the goal of safety. Allowing spontaneous breaths to suppress mandatory breaths serves the goal of comfort because spontaneous breaths are invariably more synchronous with patient breathing efforts than mandatory breaths (ie, allowing the patient to control the timing of breaths is better than imposing arbitrary values for frequency and inspiratory time). Finally, elimination of mandatory breaths (through automatic suppression) and automatic reduction in ventilatory support is a safe and effective approach to serving the goal of liberation.^9,10 But if we only perceive the existence of type 1 IMV and its service of the goal of safety, then we fail to recognize how IMV can effectively serve the all 3 goals of ventilation.

Furthermore, perceiving only type 1 IMV, we fail to observe that type 3 IMV is the new paradigm for advanced modes of ventilation (with adaptive, optimal, or intelligent targeting schemes⁸) that will likely become more common in the future¹¹: Over the last 30 years or so, we have seen modes of ventilation evolve from simple volume assist/control, serving only the goal of safety,⁷ to complex modes like Intellivent-ASV¹² that use artificial intelligence tools to serve all 3 goals.⁷ This makes sense in light of the levels of mandatory breath dependence as mentioned above. And if you accept that those levels may occur in any patient at any time, then it follows that the “ultimate mode” of ventilation (yet to be invented) would be able to provide all levels: full support with all mandatory breaths, partial support with IMV, or some level of assistance with all spontaneous breaths, switching between levels automatically according to patient need. It does not take much imagination to see that this ultimate mode of ventilation would be, by definition, some sort of IMV. What remains to be developed are the ultimate targeting schemes⁸ for controlling and coordinating the mandatory and spontaneous breaths. Other modes will not be needed except (perhaps) in rare specialty applications. Hence, I assert that in the not too distant future, virtually all modes will be some form of IMV.

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