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434 PART 4: Pulmonary Disorders

Breathing Controlled trial): a randomized controlled trial. Lancet. INTRODUCTION
2008;371:126-134. The overarching goal of positive pressure mechanical ventilation is to
• Labeau SO, Van de Vyver K, Brusselaers N, et al. Prevention of provide adequate gas exchange support while not causing harm. Indeed,
ventilator-associated pneumonia with oral antiseptics: a sys- positive pressure mechanical ventilators are only support technologies,
tematic review and meta-analysis. Lancet Infect Dis. 2011;11: not therapeutic technologies. As such they cannot be expected to “cure”
845-854. disease; they can only “buy time” for other therapies (including the
• Levine S, Nguyen T, Taylor N, et al. Rapid disuse atrophy of dia- patient’s own defenses) to work.
phragm fibers in mechanically ventilated humans. N Engl J Med. Conventional approaches to positive pressure ventilation involve
2008;358:1327-1335. applying ventilatory patterns mimicking normal through either masks
• Neto AS, Cardoso SO, Manetta JA, et al. Association between use or artificial airways. This is usually done with modes of support incor-
porating assist/control breath triggering mechanisms, gas delivery
of lung-protective ventilation with lower tidal volumes and clini- patterns governed by either a set flow or pressure, and breath cycling
cal outcomes among patients without acute respiratory distress based on either a set volume, a set inspiratory time or a set flow. Often
syndrome: a meta-analysis. JAMA. 2012;308:1651-1659. this support includes positive end expiratory pressure (PEEP) and
• Ranieri VM, Suter PM, Tortorella C, et al. Effect of mechanical supplemental oxygen. In recent decades a number of novel or uncon-
ventilation on inflammatory mediators in patients with acute ventional approaches to providing mechanical ventilatory support have
respiratory distress syndrome: a randomized controlled trial. been introduced. For these to be considered of value, however, it would
JAMA. 1999;282:54-61. seem reasonable that they address important clinical challenges and be
• The Acute Respiratory Distress Syndrome Network. Ventilation shown to improve important clinical outcomes (eg, mortality, duration
with lower tidal volumes as compared with traditional tidal of ventilation, sedation needs, complications). The remainder of this
volumes for acute lung injury and the acute respiratory distress chapter will focus on challenges facing clinicians in providing mechani-
syndrome. N Engl J Med. 2000;342:1301-1308. cal ventilatory support and assess several novel approaches introduced
• Tobin MJ, Lodato RF. PEEP, auto-PEEP, and waterfalls. Chest. over the last two decades in the context of these challenges.
1989;96:449-451.
CLINICAL CHALLENGES FACING CLINICIANS
PROVIDING MECHANICAL VENTILATORY SUPPORT

REFERENCES ■ VENTILATOR INDUCED LUNG INJURY
Probably the most important challenge facing clinicians providing
Complete references available online at www.mhprofessional.com/hall 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/
CHAPTER Novel Modes of Mechanical base balance; on the other hand, the lungs are fragile structures easily
injured by excessive stretch, alveolar collapse-reopening and high oxygen
50 Ventilation exposure. This challenge is made more difficult by the fact that lung
injury is usually heterogeneous and thus what may benefit gas exchange
Mashael Al-Hegelan in one region (eg, higher pressure) may cause worse injury in another. 1
Neil R. MacIntyre Lung injury from mechanical ventilatory support is often termed
ventilator associated lung injury, or more commonly, ventilator-induced
lung injury (VILI). Pathologically, VILI resembles in many ways the
2-7
KEY POINTS inflammatory response seen in other forms of acute lung injury and
2,3
• Mechanical ventilators are support devices, not therapeutic devices. the acute respiratory distress syndrome (ALI/ARDS). The principal
The clinical goal is thus to support gas exchange without causing harm. cause of VILI is alveolar injury induced by alveolar overstretch at end
inspiration (overdistension), extended periods of tidal breath delivery
• A number of challenges face clinicians in providing safe and effec- above normal physiologic values, and cyclic atelectasis-recruitment that
tive mechanical ventilatory support. Two of the most important are occur during positive pressure ventilation (Fig. 50-1). In general, the
2-7
(1) supporting gas exchange without causing injury from applied
; (2) providing comfortable interactive support as
pressure or Fi O 2
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 Volume
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 Pressure
neutrally adjusted ventilatory assistance. FIGURE 50-1. Ventilator-induced lung injury during positive pressure ventilation comes
• While all of these innovations have conceptual appeal and support- from several factors. Depicted is the sigmoidal-shaped pressure-volume relationship seen in
ing observational data, none as yet have convincing randomized the acutely injured lung. Injury can occur from end inspiratory overdistention (upper right
control trial data demonstrating improved clinical outcomes. region), repetitive excessive tidal breath delivery (middle region), and repetitive collapse-
reopening of alveolar units (lower left region).

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