Retention of airway secretions is a frequent complication in critically ill patients requiring invasive mechanical ventilation (MV). This complication is often due to excessive secretion production and ineffective secretion clearance. One of the main causes is the presence of an endotracheal tube (ETT) which has been shown to decrease mucociliary clearance and hinders the generation of adequate peak expiratory flows when coughing. Other factors such as suboptimal airway humidification, inspiratory flow bias, semi-recumbent position, prolonged immobilization and respiratory muscles weakness further impair sputum clearance. Mucus retention may impede optimal gas exchange, and lead to atelectasis, increased work of breathing, bacterial colonization and development of pulmonary infections, prolonging the need for MV. These conditions, added to initial factors, increase morbidity and mortality in critically ill patients, making secretion clearance an essential factor for patients' prognosis.

Secretion removal techniques, such as, manual or mechanical hyperinflations, chest vibrations or expiratory rib cage compressions, prior to suctioning, are commonly used by physiotherapists in intensive care units (ICU). However, the evidence assessing respiratory physiotherapy techniques in critically ill patients is scant and sometimes inconsistent, making it difficult to extrapolate the results and standardize the clinical practice. Moreover, the execution of these techniques often differs among professionals based on their experience, training, and resources availability.

Mechanical insufflator-exsufflator (MI-E) is a respiratory physiotherapy technique that aims to assist or simulate a normal cough by using an electro-mechanical dedicated device. A positive airway pressure is delivered to the airways, in order to hyperinflate the lungs, followed by a rapid change to negative pressure that promotes a rapid exhalation and enhances peak expiratory flows. MI-E is commonly used in patients with ineffective cough mainly due to respiratory pump failure (i.e: neuromuscular patients), and has been proposed in recent years as a technique with great potential to non-invasively clear secretions in the critically ill. Indeed, recent studies have evaluated safety and efficacy of MI-E in intubated critically ill patients with promising results and no associated adverse events. However, there is no consensus on the best MI-E settings to facilitate secretion clearance in these patients. Inspiratory and expiratory pressures of ±40 cmH2O and inspiratory-expiratory time of 3 and 2 seconds, respectively, are often used as a standard for MI-E programming in the daily routine practice, but recent laboratory studies have shown significant benefits when MI-E setting is optimized to promote an expiratory flow bias. For instance, Volpe et al. achieved significant differences in artificial mucus displacement when inspiratory flows were lowered, inspiratory time was increased to 4 seconds, and expiratory flow bias was enhanced by increasing the expiratory pressure over the inspiratory pressure. More recently, evidence from a swine model confirmed the improvement in mucus movement velocity when expiratory pressure was enhanced to increase the difference between inspiratory and expiratory pressures (i.e: +40/-70cmH2O). Importantly, increased inspiratory pressures should be avoided to prevent movement of mucus toward the lungs and potential associated detrimental effects such as alveolar damage or hemodynamic impairment.

The investigators designed this study to compare the effects of MI-E with an optimized setting versus a standard setting on the wet volume of suctioned sputum in intubated critically ill patients on invasive MV for more than 48 hours.