CHAPTER 50: Novel Modes of Mechanical Ventilation

KEY POINTS

• Mechanical ventilators are support devices, not therapeutic devices. The clinical goal is thus to support gas exchange without causing harm.

• A number of challenges face clinicians in providing safe and effective mechanical ventilatory support. Two of the most important are (1) supporting gas exchange without causing injury from applied pressure or $FiO2$; (2) providing comfortable interactive support as the lung recovers.

• Innovations need to focus on addressing clinical challenges. Moreover, to be accepted as “standard of care,” an innovation must be shown to improve an important clinical outcome.

• Recent innovations focusing on supporting gas exchange in a “lung protective” fashion include airway pressure release ventilation, high frequency ventilation, and adaptive support ventilation.

• Recent innovations focusing on improving patient-ventilator synchrony include various feedback controls on variable flow-pressure-targeted breaths, proportional assist ventilation, and neutrally adjusted ventilatory assistance.

• While all of these innovations have conceptual appeal and supporting observational data, none as yet have convincing randomized control trial data demonstrating improved clinical outcomes.

INTRODUCTION

The overarching goal of positive pressure mechanical ventilation is to provide adequate gas exchange support while not causing harm. Indeed, positive pressure mechanical ventilators are only support technologies, not therapeutic technologies. As such they cannot be expected to “cure” disease; they can only “buy time” for other therapies (including the patient’s own defenses) to work.

Conventional approaches to positive pressure ventilation involve applying ventilatory patterns mimicking normal through either masks or artificial airways. This is usually done with modes of support incorporating assist/control breath triggering mechanisms, gas delivery patterns governed by either a set flow or pressure, and breath cycling based on either a set volume, a set inspiratory time or a set flow. Often this support includes positive end expiratory pressure (PEEP) and supplemental oxygen. In recent decades a number of novel or unconventional approaches to providing mechanical ventilatory support have been introduced. For these to be considered of value, however, it would seem reasonable that they address important clinical challenges and be shown to improve important clinical outcomes (eg, mortality, duration of ventilation, sedation needs, complications). The remainder of this chapter will focus on challenges facing clinicians in providing mechanical ventilatory support and assess several novel approaches introduced over the last two decades in the context of these challenges.

CLINICAL CHALLENGES FACING CLINICIANS PROVIDING MECHANICAL VENTILATORY SUPPORT

VENTILATOR INDUCED LUNG INJURY

Probably the most important challenge facing clinicians providing mechanical ventilatory support today is managing the balance between providing adequate gas exchange and avoiding lung injury associated with positive airway pressure and oxygen exposure. On the one hand, patients in respiratory failure need adequate tissue oxygenation and acid/base balance; on the other hand, the lungs are fragile structures easily injured by excessive stretch, alveolar collapse-reopening and high oxygen exposure. This challenge is made more difficult by the fact that lung ...