Neurally Adjusted Ventilatory Assist: Insufficient Evidence of Broad Clinical Outcomes

To the Editor:

This is a reply to the editorial by Moss¹ on the study by Delisle et al,² entitled “Effect of Ventilatory Variability on Occurrence of Central Apneas.” In their prospective crossover study, Delisle et al² compared neurally adjusted ventilatory assist (NAVA) and pressure support ventilation in intubated adult patients during sleep and wakefulness. The results showed increased variability in tidal volume and elimination of central apneas (> 10 s duration) during non-rapid-eye-movement sleep during NAVA. Delisle et al stated that their findings were due to over-assistance during pressure support ventilation, and that this was avoided during NAVA due to adaptation of the assist. This study carries several important and novel findings of physiological interest.

Physiology is one of the pillars of medicine, especially in mechanical ventilation, where the knowledge gained in respiratory and circulatory physiology has formed the clinical standards of today. A question to Ms Moss is why she summarizes the outcome of the well performed physiological study by Delisle et al² by the statement “What is the clinical importance of the study; the ‘So what?’ if you will.”

There is clear evidence that conventional modes frequently fail to assist inspiration during both invasive^3–5 and noninvasive ventilation.^6,7 Opposite to traditional views, increased sedation seems to worsen asynchrony.⁸ Though it is debatable whether adverse effects such as prolonged time of ventilation and ventilator-induced diaphragm dysfunction are directly related to patient-ventilator asynchrony, it is clear that the search for methods to monitor and prevent the patient from fighting the ventilator has been a key issue of mechanical ventilation in spontaneously breathing patients for at least 5 decades.⁹

Thus, is it not in the best interest of respiratory therapists to reduce this problem? NAVA is a scientifically documented mode of mechanical ventilation; it has regulatory approval by the United States Food and Drug Administration. Since NAVA allows monitoring of diaphragm electrical activity, you can—for the first time—reliably detect patient-ventilator asynchrony at the bedside.¹⁰ Twenty studies (involving a total of 280 patients) have compared conventional modes to NAVA and showed improved patient-ventilator interaction during both invasive and noninvasive ventilation, in patients of all ages. Nineteen studies (involving a total of 291 patients) have reported that physiological parameters are equivalent or improved during NAVA, compared to conventional modes. For a recent review and book chapter see Sinderby and Beck.^11,12

Thus, I am not sure that Ms Moss's statement, “I believe the use of NAVA has not yet been adequately justified in the literature” is fully justifiable.

Finally, I agree in principle with Ms Moss that evidence-based medicine and “well designed” randomized controlled trials are important. However, the approach of the randomized controlled trial in the critical care setting is very “young”^13,14 and is still encountering major methodological challenges in both design and outcome.¹⁵ For example, meta-analyses of PEEP trials have shown beneficial outcome only in patients with ARDS.¹⁶ Yet physiology can teach us that applying external PEEP overcomes intrinsic PEEP and improves patient triggering and synchronizes ventilator assist to inspiratory effort. Should we apply PEEP or not in COPD?

Is it not that, despite decades of research, we have no universally accepted clinical evidence that any mode of ventilation is superior to any other mode? To suggest that only outcome data from randomized controlled trials are required before new modes can be used simply perpetuates a myth and blinds us to other approaches to rationally selecting the best treatment options.¹⁷

Until having reached the point (if ever) where both new and conventional modes of mechanical ventilation have been sufficiently validated, it is the skills of the practitioner that ensure the safety of the patient. If treatments are based on adequate information, innovations that improve monitoring and delivery of ventilatory assist could strengthen personalized care, without affecting protocolized care. Is it now that evidence-based medicine has become an excuse to avoid the disruption of “normal” clinical activities that follow the introduction of medical innovations?

“Anyone who has never made a mistake has never tried anything new.” — Albert Einstein

I welcome further productive discussions on the topic of how best to integrate new technologies of mechanical ventilation into the clinical arena.

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