New modes of neonatal ventilation: let there be light.

Neonatal mechanical ventilation has a long history of change by trial and error. From intubated, then nasal CPAP to IMV to high frequency to synchronization and various types of volume targeting, we have adopted changes in our practice based more on the types of technology available than on data generated from large, randomized controlled trials. The reasons are easy to understand. First, during the early years of neonatology as a recognized specialty, mortality was so high as to overwhelm our thoughts, giving us the zeal and desperation to justify trying new things about which we knew little. Then, as we saw improved mortality, we realized that morbidity in our patients was a major problem, and we enthusiastically adopted approaches which held promise in small trials, sometimes without waiting for the larger ones, and sometimes, such as in the case of high-frequency ventilation, in spite of them. We were dazzled by the microprocessor, and by our new ability to measure aspects of pulmonary function at the bedside in critically ill neonates. This new technology was rapidly incorporated into the ventilator platforms and became a part of daily management. At the same time, this new technology allowed the ventilator to sense the presence of the patient. Again, this seemed a true advance, though one which has yet to be proven superior by large RCTs. Today, we find ourselves surrounded by technologically advanced equipment with a veritable alphabet soup of neonatal ventilatory modes F IMV, synchronized intermittent mandatory ventilation (SIMV), assist/control (A/C), PSV, volume guarantee (VG), PCV, BiPAP, APRV, PRVC F the list goes on. Though there are data to help us, we often find ourselves struggling to integrate what we can potentially do with these devices into what we know of newborn physiology and coming up short. In this issue of the Journal, two papers shed some light on these problems, one from Georgetown by Abubakar and Keszler, another from Vanderbilt by Guthrie et al. While neither are large RCTs, both evaluate new modes of neonatal ventilatory support carefully and in such a way to both inform the users, who must continue to treat in the absence of more complete data, and to provide a basis for larger trials. Neither study has large numbers, but both use a crossover design in which each enrolled patient serves as his or her own control, greatly increasing the ability of these small trials to detect change. Abubakar and Keszler compare SIMV to A/C in 12 infants, both using a proprietary volume-targeting system called VG available on the Draeger 8000þ platform. Their main findings are increased work of breathing, lower SpO2, higher pressure requirements and greater variability of tidal and minute volumes during SIMVþ VG as compared to A/Cþ VG. The major difference between these modes is that A/Cþ VG potentially provides some support above the set PEEP level to all patient-initiated breaths, while SIMVþ VG only provides such support for the number of breaths determined by the set SIMV rate. These findings complement and extend earlier studies of these modes with volume-targeting without VG, and add support to the general concept that more uniform support of patient respiratory effort may be beneficial. Guthrie et al. evaluate a different mode of ventilation called mandatory minute ventilation (MMV), and compared this to SIMV, again using a crossover design in 20 patients. MMV is a mode which requires the operator to determine what the appropriate minute ventilation for the patient should be, and the ventilator then monitors the patient’s ability to generate this volume every 7.5 seconds. If the calculation suggests the volume target will not be met, SIMV breaths are delivered at the targeted volume to achieve the desired minute ventilation. A Draeger ventilator, the Evita 4, also provided this mode. They found that, with MMV, fewer mechanical breaths were required as compared to SIMV, without any change in CO2 values or oxygen levels. In fact, because MMV is a mode in which spontaneous breaths are augmented with pressure support F that is, the infant’s spontaneous initiation of a breath has an added positive pressure component to support tidal volume delivery F what the authors actually showed was a decrease in SIMV breaths, not mechanically supported breaths in general. This difference is a subtle one. Pressure-supported breaths are flow cycled, and allow the patient to modulate inspiratory flow and determine the actual duration of inspiration. An SIMV breath has a set inspiratory time, which is not altered by the mechanical characteristics of the infant’s lung. Thus, SIMV breaths essentially ‘‘dial the patient out’’. The elimination of such breaths then allows the patient to assume another level of respiratory control. The authors suggest that this mode might allow us to more effectively utilize the infant’s endogenous respiratory drive to support ventilation, eliminating the ‘‘heavy hand’’ of the mechanically imposed breath. Address correspondence and reprint requests to Mark C. Mammel, MD, Children’s Hospital F St Paul, University of Minnesota, 347 N. Smith Avenue, Room 505, St Paul, MN 55102, USA. Children’s Hospital F St Paul, University of Minnesota, St Paul, MN, USA.