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. Three of the most important are: (1) supporting gas exchange without causing injury from applied pressure or FiO2; (2) providing comfortable interactive support; (3) facilitating the discontinuation process as the lung recovers.
Innovations need to focus on addressing clinical challenges and improve important clinical outcomes.
Over the past quarter century, innovations that have generated considerable clinical interest include airway pressure release ventilation, high-frequency ventilation, 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 has convincing randomized control trial data, demonstrating improved clinical outcomes.
The overarching goal of positive pressure mechanical ventilation (MV) is to provide adequate gas exchange support without 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 apply pressure and gas flow to the lungs through either masks or artificial airways. Timing and magnitude of pressure and flow application tend to mimic the normal ventilatory pattern using modes of support incorporating assist/control breath triggering mechanisms, pressure- or flow-targeted gas delivery patterns, and various breath cycling algorithms. This support also includes positive end-expiratory pressure (PEEP) and supplemental oxygen (Chap. 48).
For novel or unconventional MV approaches to be considered of value, it is prudent they address important clinical challenges and be shown to improve important clinical outcomes (eg, mortality, duration of ventilation, sedation needs, complications). From our perspective, there are three major ventilator management challenges facing clinicians today: minimizing ventilator-induced lung injury, optimizing patient-ventilator synchrony, and facilitating the discontinuation process. This chapter will focus on these challenges and assess several novel approaches introduced over the last two decades in the context of these challenges.
CHALLENGES IN PROVIDING MECHANICAL VENTILATION
Ventilator-Induced Lung Injury
While patients in respiratory failure need adequate tissue oxygenation and acid/base balance, the lungs are fragile structures easily injured by the application of positive alveolar pressures, alveolar collapse-reopening, and high oxygen exposure. This challenge is made more difficult because the underlying lung injury is usually heterogeneous and what MV strategies may benefit gas exchange in one region (eg, higher pressure) may cause worse injury in another.1
Lung injury from MV is often termed ventilator-associated lung injury, or more commonly, ventilator-induced lung injury (VILI)2–7 (Chap. 51). Pathologically, VILI resembles the inflammatory response seen in other forms of ...