New Developments in Mechanical Ventilation

Mechanical ventilation of patients with acute lung injury/acute respiratory distress syndrome (ARDS) should commence with low tidal volume (VT), low stretch and adequate positive end-expiratory pressure (PEEP), as proposed by the first ARDSnet trial. The majority of patients with ARDS will achieve their goals of oxygenation and plateau pressure, utilising the lung protective strategy. In the remaining minority of patients, these end-points may not be achieved. Such patients have a significantly high mortality and should be considered for rescue strategies relatively early on. If the patients respond positively to lung recruitment trials, using rescue strategies may open atelectactic alveoli and allow oxygenation or plateau pressure targets to be achieved. None of these rescue strategies have been shown to reduce mortality, although short-term objectives of improvement in oxygenation or reduction in plateau pressures may be achieved. Therefore, the selection of these strategies should be based on availability and level of comfort of the operators.

Acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) occur after diverse pulmonary or systemic insults. Although necessary for keeping the patient alive, mechanical ventilation has been implicated in the associated high morbidity and mortality. New therapies have attempted to improve oxygenation and ventilation, while providing lung protection. A multitude of causes can lead to ALI/ARDS. However, the dysfunction of the endogenous surfactant system is a shared characteristic. It has been suggested to use surfactant replacement therapy in patients with ARDS in order to overcome the ongoing inactivation of endogenous surfactants by plasma proteins entering the alveolar spaces. The evidence regarding surfactant replacement therapy in ARDS patients is discussed in this chapter.

Although some small studies have shown beneficial effects of surfactant replacement therapy, larger studies failed to establish this effect. Therefore, exogenous surfactants are not recommended for routine use in patients with ALI/ARDS.

Biphasic positive airway pressure (BIPAP)/airway pressure release ventilation (APRV) is a mechanical ventilation mode based on a high flow or demand-valve continuous positive airway pressure (CPAP) system, in which the level of CPAP is switched between a higher and a lower pressure in a time-dependent manner, generating time-cycled, pressure-limited ventilation. Supported and unsupported spontaneous breathing is possible at both levels of airway pressure, but the optimal level of spontaneous breathing has yet to be determined. BIPAP/APRV has proved efficient in increasing oxygenation compared with controlled mechanical ventilation in acute respiratory distress syndrome (ARDS). In addition, BIPAP/APRV with spontaneous breathing waives the need for muscle paralysis, and allows reduced sedation, as well as cardiovascular support with drugs. BIPAP/APRV with spontaneous breathing seems to be a useful ventilatory strategy for patients with less severe hypoxaemic respiratory failure, but caution is required in those patients with more severe lung injury, as reflected by an arterial oxygen tension(Pa,O₂)/inspiratory oxygen fraction (FI,O₂) of <120 mmHg.

Neurally adjusted ventilatory assist (NAVA) is a form of ventilator support in which the electrical activity of the diaphragm (EAdi) drives the ventilator. The signal, obtained from the crural portion of the diaphragm via a nasogastric or orogastric feeding tube, is transformed into a waveform that is utilised to regulate the inspiratory assistance. The ventilator applies pressure to the airway throughout inspiration in proportion to EAdi, which is multiplied by an adjustable gain constant (NAVA level). With NAVA, both the timing and the magnitude of ventilator delivered assistance are controlled by the EAdi. In contrast with the conventional pneumatically-driven modes, NAVA has been shown to improve patient–ventilator interaction and yield a remarkable reduction in any asynchronies. Furthermore, with NAVA the patient retains control of her/his breathing pattern, which is more variable in nature; this variability might determine improved oxygenation in patients with acute lung–volume reduction. Because tidal volume is not remarkably increased when augmenting the NAVA level, this novel mode has the potential to maintain protective ventilation in spontaneously breathing patients.

For the last three decades, extracorporeal lung assist (ECLA) has been employed as a life-saving therapy in few highly-specialised centres. A deeper understanding of acute respiratory distress syndrome (ARDS) pathophysiology, improved technology and the positive results of recent trials have led to a reassessment of ECLA in the clinical setting. The referral and transfer of sicker patients to specialised extracorporeal membrane oxygenation (ECMO) centres has been shown to improve clinical outcome. The CESAR (conventional ventilator support versus extracorporeal membrane oxygenation for severe adult respiratory failure) trial was the first positive randomised controlled trial to investigate ECMO use in adult patients with ARDS. In 2009, many healthcare systems worldwide successfully faced the influenza A (H1N1) pandemic, instituting networks of specialised intensive care units (ICUs), for transfer of the sickest patients and management with ECMO. There is also an increasing interest in new and less invasive extracorporeal techniques, primarily aimed at carbon dioxide removal, which may be more widely applied in combination with a strictly protective ventilatory strategy.

Ventilator associated pneumonia (VAP) is associated with increased morbidity, mortality and burden for the healthcare system. Oropharyngeal secretions, pooled above the endotracheal tube (ETT) cuff, are the primary source of pathogens in this iatrogenic infection. Improvements in the ETT cuff design to achieve tracheal sealing and maintaining the internal cuff pressure within the recommended range (25–30 cmH₂O) have a pivotal role in the prevention of pulmonary aspiration of colonised oropharyngeal secretions and VAP. Additionally, ETTs coated with antimicrobial agents prevent colonisation of their internal lumen and biofilm formation; however, further evidence is necessary to assess the role of biofilm in the pathogenesis of VAP. The semirecumbent position is universally recommended; yet, laboratory studies challenge the benefits of such a position. Finally, during positive pressure ventilation, the ventilatory parameters that influence the inspiratory flow, i.e. the duty cycle, have a significant effect on retention of mucus and, potentially, on risks of lung infections. Further clinical evidence is necessary to assess benefits and limitations of ventilatory settings on VAP prevention.

Patients with chronic airflow obstruction and difficult or prolonged weaning are at increased risk for prolonged invasive mechanical ventilation. Several randomised controlled trials, mainly conducted in patients who had pre-existing lung disease, have shown that the use of noninvasive ventilation (NIV) in order to advance extubation in patients with difficult and prolonged weaning can result in reduced periods of endotracheal intubation, complication rates and improved survival. Patients in these studies were haemodynamically stable, with a normal level of consciousness, no fever and a preserved cough reflex. The use of NIV in the management of mixed populations with respiratory failure after extubation, including small proportions of chronic respiratory patients, did not show clinical benefits. By contrast, NIV immediately after extubation is effective in avoiding respiratory failure following extubation and improving survival in patients at risk for this complication, particularly those with chronic respiratory disorders, cardiac co-morbidity and hypercapnic respiratory failure. Finally, both continuous positive airway pressure and NIV can improve clinical outcomes in patients with post-operative acute respiratory failure, particularly abdominal and thoracic surgery.

Automated mechanical ventilation using advanced closed loops is anticipated to assume a larger role in supporting critically ill patients in intensive care units (ICU) in the future, for several reasons. They have the potential to improve knowledge transfer by continuously implementing automated protocols while improving patient outcomes. Additionally, closed-loop systems may provide a partial solution to address forecasted clinician shortages by reducing ICU-related costs, time spent on mechanical ventilation, and staff workload. At present, few systems that automate medical reasoning with advanced closed loops are commercially available. Preliminary studies evaluating first generation automated weaning systems and fully automated ventilation are promising. These closed-loop programmes will be refined as the technology improves and clinical experience with these products increases.