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Background: Neurally adjusted ventilatory assist (NAVA) is a pneumatically-independent mode of mechanical ventilation controlled by diaphragm electrical activity (EAdi), and has not yet been implemented in very small species. Objectives: The aims of the study were to evaluate the feasibility of applying NAVA in very small species and to compare this to pressure support ventilation (PSV) in terms of ventilatory efficiency and breathing pattern, and evaluate the impact of instrumental dead space on breathing pattern during both modes. Methods: Nine healthy rats (mean weight 385 8 4 g) were studied while breathing on PSV or NAVA, at baseline or with added dead space. Results: A clear difference in breathing pattern between NAVA and PSV was observed during both baseline and dead space, where PSV – despite similar EAdi and tidal volume as during NAVA – caused shortened inspiratory time (p < 0.05) and increased the respiratory rate (p < 0.05). A higher minute ventilation (p < 0.05) in order to reach the same arterial CO2 was observed. Ineffective inspiratory efforts occurred only during PSV and decreased with the dead space. Conclusion: This study demonstrates, in a small group of animals, that NAVA can deliver assist in very small species with a higher efficiency than PSV in terms of eliminating CO2 for a given minute ventilation. [ABSTRACT FROM AUTHOR]