New modes of mechanical ventilation: proportional assist ventilation, neurally adjusted ventilatory assist, and fractal ventilation Paolo Navalesi, MD,* and Roberta Costa, MD † Increased knowledge of the mechanisms that determine respiratory failure has led to the development of new technologies aimed at improving ventilatory treatment. Proportional assist ventilation and neurally adjusted ventilatory assist have been designed with the goal of improving patient–ventilator interaction by matching the ventilator support with the neural output of the respiratory centers. With proportional assist ventilation, the support is continuously readjusted in proportion to the predicted inspiratory effort. Neurally adjusted ventilatory assist is an experimental mode in which the assistance is delivered in proportion to the electrical activity of the diaphragm, assessed by means of an esophageal electrode. Biologically variable (or fractal) ventilation is a new, volume-targeted, controlled ventilation mode aimed at improving oxygenation; it incorporates the breath-to-breath variability that characterizes a natural breathing pattern. Keywords mechanical ventilation, patient–ventilator interaction, alveolar recruitment, neuromechanical coupling, breath-to-breath variability Curr Opin Crit Care 9:51–58 © 2003 Lippincott Williams & Wilkins, Inc. In the last two decades, ventilator manufacturers have proposed several “new” modes of mechanical ventila- tion, which have been carefully investigated by ICU re- searchers [1]. Some of these became popular and widely used in clinical practice, whereas others, despite encour- aging preliminary results, often did not reach enough popularity to have a clinical role. Surprisingly, quite of- ten these “unsuccessful” modes produced a large num- ber of studies that more often than not exceeded the number of ICU patients actually treated. Nevertheless, because the main reason to admit a patient to the ICU is to provide him or her with ventilatory assistance, the continuous recognition of previously neglected physi- ologic aspects and advancements in the knowledge of disease mechanisms are still leading to the development of novel technologies aimed at improving the outcome of patients receiving ventilatory treatment in the ICU. Mechanical ventilation is essentially instituted to buy time, allowing the patient to recover from the underlying disease causing respiratory failure. The main difference between modes is essentially the manner in which posi- tive pressure is applied to the airway, and, therefore, a “new” mode generally introduces a novel approach to support delivery [2]. Controlled modes of mechanical ventilation are often needed in patients with severe acute hypoxemic respiratory failure and acute respiratory distress syndrome (ARDS). In this case, the principal purpose of a new mode is to improve oxygenation with- out further damaging the lung. When a controlled mode is not necessary, forms of partial ventilatory assistance are preferred, in which the delivery of mechanical support is triggered by the patient’s own spontaneous breathing [3]. Accordingly, with these modes, the main objective is to enhance the coordination between the patient’s own spontaneous breathing and mechanical assistance [3]. Matching mechanical support with patient demand to improve patient–ventilator interaction: proportional assist ventilation and neurally adjusted ventilatory assist Forms of partial ventilatory assistance have been devel- oped to avoid some of the adverse effects related to con- trolled mechanical ventilation, such as excessive ventila- tion [4,5], use of sedatives and muscle relaxants [6], and muscle atrophy resulting from disuse [7]. With these modes, the ventilator interacts with the patient to as- *Pneumologia e Terapia Intensiva Respiratoria, Fondazione S. Maugeri, Pavia, and † Istituto di Anestesia e Rianimazione, Universita ` Cattolica del Sacro Cuore, Rome, Italy. Correspondence to Paolo Navalesi, MD, Fondazione S. Maugeri, Pneumologia e Terapia Intensiva Respiratoria, Via Ferrata 4, 27100 Pavia, Italy; e-mail: pnavalesi@fsm.it Current Opinion in Critical Care 2003, 9:51–58 Abbreviations ARDS acute respiratory distress syndrome BVV biologically variable ventilation EAdi electrical activity of the diaphragm NAVA neurally adjusted ventilatory assist PAV proportional assist ventilation PEEP positive end-expiratory pressure PEEPi end-expiratory elastic recoil pressure PSV pressure support ventilation ISSN 1070–5295 © 2003 Lippincott Williams & Wilkins, Inc. 51