Initial ventilator settings for critically ill patients

The lung-protective mechanical ventilation strategy has been standard practice for management of acute respiratory distress syndrome (ARDS) for more than a decade. Observational data, small randomized studies and two recent systematic reviews suggest that lung protective ventilation is both safe and potentially beneficial in patients who do not have ARDS at the onset of mechanical ventilation. Principles of lung-protective ventilation include: a) prevention of volutrauma (tidal volume 4 to 8 ml/kg predicted body weight with plateau pressure <30 cmH2O); b) prevention of atelectasis (positive end-expiratory pressure ≥5 cmH2O, as needed recruitment maneuvers); c) adequate ventilation (respiratory rate 20 to 35 breaths per minute); and d) prevention of hyperoxia (titrate inspired oxygen concentration to peripheral oxygen saturation (SpO2) levels of 88 to 95%). Most patients tolerate lung protective mechanical ventilation well without the need for excessive sedation. Patients with a stiff chest wall may tolerate higher plateau pressure targets (approximately 35 cmH2O) while those with severe ARDS and ventilator asynchrony may require a short-term neuromuscular blockade. Given the difficulty in timely identification of patients with or at risk of ARDS and both the safety and potential benefit in patients without ARDS, lung-protective mechanical ventilation is recommended as an initial approach to mechanical ventilation in both perioperative and critical care settings.

Introduction of positive pressure ventilation during a polio epidemic in 1952 resulted in a large reduction of mortality in patients with respiratory failure (87% to less than 15%) and marked the birth of modern intensive care medicine [2].
Better understanding of the eff ects of positive pressure ventilation on respiratory physiology and mechanics has led to an appreciation of potential side eff ects of positive pressure ventilation, in particular ventilator-associated lung injury [3]. Th e key determinants of ventilator-associated lung injury are cyclic alveolar distension (volutrauma) and recruitment/derecruitment (atelectrauma), the size of available lung ('baby lung'), with an additional contribution from preexisting sepsis, vascular pressures, respiratory rate and inspiratory fl ow [3]. Avoiding high tidal volume ventilation is the only intervention with convincing survival benefi t in patients with ARDS [4]. More recently, observational studies and a randomized clinical trial suggested a benefi t of avoiding conventional high tidal volume ventilation in all critically ill patients [5,6].
Th e systematic review by Fuller and colleagues [1] highlights the importance of the low tidal volume ventilation strategy in patients without ARDS at the onset of mechanical ventilation. Th e results from 8 out of 13 studies included in the fi nal analysis of this systematic review show that lower tidal volumes at initiation of mechanical ventilation reduce progression to ARDS. Similar fi ndings were reported in another recent systematic review that combined observational studies and clinical trials in both ICUs and perioperative settings [7]. Neither of these systematic reviews raised concerns about the safety of low tidal volume ventilation in patients without ARDS.
Given the diffi culty of identifying patients with ARDS in a timely fashion and both the safety and potential benefi t of low tidal volume ventilation in patients without ARDS, the question arises whether conventional high tidal volume ventilation should ever be used in critical care or perioperative settings [8,9].
High tidal volume ventilation was recommended in operating rooms in the early 1970s to prevent atelectasis [10]. However, later studies did not support this approach and the focus has shifted towards the role of positive

Abstract
The lung-protective mechanical ventilation strategy has been standard practice for management of acute respiratory distress syndrome (ARDS) for more than a decade. Observational data, small randomized studies and two recent systematic reviews suggest that lung protective ventilation is both safe and potentially benefi cial in patients who do not have ARDS at the onset of mechanical ventilation. Principles of lung-protective ventilation include: a) prevention of volutrauma (tidal volume 4 to 8 ml/kg predicted body weight with plateau pressure <30 cmH 2 O); b) prevention of atelectasis (positive end-expiratory pressure ≥5 cmH 2 O, as needed recruitment maneuvers); c) adequate ventilation (respiratory rate 20 to 35 breaths per minute); and d) prevention of hyperoxia (titrate inspired oxygen concentration to peripheral oxygen saturation (SpO 2 ) levels of 88 to 95%). Most patients tolerate lung protective mechanical ventilation well without the need for excessive sedation. Patients with a stiff chest wall may tolerate higher plateau pressure targets (approximately 35 cmH 2 O) while those with severe ARDS and ventilator asynchrony may require a short-term neuromuscular blockade. Given the diffi culty in timely identifi cation of patients with or at risk of ARDS and both the safety and potential benefi t in patients without ARDS, lungprotective mechanical ventilation is recommended as an initial approach to mechanical ventilation in both perioperative and critical care settings.
end-expiratory pressure, recruitment maneuvers, and the avoidance of a high fraction of inspired O 2 (FiO 2 ) as safer and more eff ective ways to prevent atelectasis than high tidal volume [11,12].
Th e second concern with regards to low tidal volume ventilation is the increase of the carbon dioxide partial pressure (PCO 2 ), but acidosis is usually easily corrected by increasing respiratory rate except in patients with severe ARDS, where permissive hypercapnia may actually be desirable [13]. Another concern regarding low tidal volume ventilation is the potential increase in the need for sedation [14]. However, there is little evidence to Kilickaya and Gajic Critical Care 2013, 17:123 http://ccforum.com/content/17/2/123 support this claim, particularly in patients without ARDS [15].
Although limited, the current evidence, including the current report by Fuller and colleagues [1], suggests that the risk/benefi t ratio of low tidal volume ventilation in patients with or without ARDS is on the side of benefi t. In Figure 1 we provide a pragmatic approach to lung protective mechanical ventilation in patients with and without ARDS.

Competing interests
The authors have no fi nancial or other potential confl icts of interest to disclose.