Intermittent Mandatory Ventilation: What's in a Name?

In Reply

That which we call a rose, By any other name would smell as sweet.

—William Shakespeare

Chatburn finds fault in our description of intermittent mandatory ventilation (IMV),¹ as we primarily discuss IMV as first described by Downs.² That is, a preset mandatory rate with spontaneous breaths allowed between mandatory breaths. And although we appreciate the work Chatburn has done³ in further refining the work of Mushin et al⁴ in classifying ventilators and ventilator operation, in his letter he also does not use his classification system when making reference to modes. He refers to IMV as IMV and pressure support and proportional assist ventilation and neurally adjusted ventilatory assist and automatic tube compensation. None of these terms are part of his classification system. So why has he used them in his letter? The only reason we can conceive for this is “clarity of communication.” One problem with the classification system he has devised is that it speaks more to engineers than to clinicians and is rarely used in day-to-day clinical practice. In addition, as far as we can determine, no ventilator manufactured anywhere in the world is using this classification system to identify the modes of ventilation available on the manufacturer's ventilator.

He goes on to provide trade names for his 3-level classification of IMV, including ASV (Hamilton Medical) and the S/T setting on Respironics devices. These modifications of IMV operate by allowing an increased spontaneous breathing frequency to alter the mandatory rate. And although both of these modes have been available for more than 20 years in the United States, he can muster only 2 references that marginally support ASV, and none to support the S/T version of IMV. In both cases, spontaneous breaths are pressure supported, obfuscating the only real advantage of IMV, maintenance of the thoracic pump.

But to the point, Chatburn lists the goals of mechanical ventilation as safety, comfort, and liberation; all of which he believes can be addressed by IMV. The evidence proves him incorrect. Comfort, best assessed by evaluating synchrony, patient appearance, and measures of work, have all demonstrated that IMV routinely fails to unload the respiratory muscles, increases asynchrony and in some cases promotes fatigue.^5–7 Liberation has only been facilitated by daily spontaneous breathing trials, and the sharing of mandatory breaths with spontaneous breaths, if anything, has been shown to delay ventilator discontinuation.^8–10 Similarly, the gradual withdrawal of ventilatory support, heralded by IMV supporters, has no basis in fact.¹¹ Finally, how can safety be assured in the context of prolonged ventilation times and the failure to meet patient needs?

Chatburn continues work on ventilator classification, often with increasing complexity and contradiction. But these are paper exercises that fail to consider the bedside physiology. So what's in a name? As kids, we all learned about the plant-eating dinosaur with the long neck featured on the Sinclair gasoline station signs, which we knew as Brontosaurus. Turns out paleontologists made a mistake in classification, and we now know this dinosaur as Apatosaurus. Regardless of the taxonomy, that dinosaur is extinct. Chatburn can ponder such issues from his desk, change the name of IMV, or modify it, but in the end, physiology is a cruel taskmaster and the evidence predicts a similar fate for IMV.

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