Ventilatory Mechanics in the Patient With Obesity

Luigi Grassi, M.D.; Robert Kacmarek, Ph.D.; Lorenzo Berra, M.D.


Anesthesiology. 2020;132(5):1246-1256. 

In This Article

Mechanics During Weaning From Mechanical Ventilation

When extubating a patient with obesity, especially after prolonged mechanical ventilation in a critical care setting, it can be useful to consider the concept of work of breathing. The work of breathing can be defined as the effort made by the respiratory muscles to overcome the elastic and resistive forces that oppose the expansion of the respiratory system.[61] In the periextubation period, muscular weakness and residual activity of sedatives and muscle relaxants can impair the action of respiratory muscles, negatively impacting the weaning process. In patients with obesity, the situation is further complicated by the elevated pleural pressure. High pleural pressure by diaphragmatic displacement results in a negative transpulmonary pressure at the end of exhalation, leading to atelectasis and small airways collapse. As a result, the work of breathing is increased,[18,62] given that a significant part of it is spent to overcome the closing pressure of the airways to generate a flow. This mechanism can prolong the weaning phase.

In this context, a PEEP sufficient to contrast airway collapse during exhalation might be beneficial even if, as discussed above, standard levels of end-expiratory pressure are usually too low to meet the needs of mechanically ventilated obese patients.[37,38] A strategy providing higher levels of end-expiratory pressure at extubation (in the range of 15 to 30 cm H2O) in order to achieve a positive transpulmonary pressure throughout the respiratory cycle can significantly facilitate the weaning process.[63] Positive pressures in the airways should be maintained along the whole periextubation period. After invasive ventilation has been removed, positive pressure in the small airways can be maintained by means of noninvasive ventilation. The role of simple systems to create a continuous positive airway pressure such as the Boussignac mask has been investigated in the elective postoperative period, with established benefits in terms of oxygenation and improved lung volumes.[64] Noninvasive positive pressure is physiologically attractive because it keeps the small airways inflated and counteracts the collapse of the soft tissue at the level of the upper airways. Obstruction of the upper airways is a common phenomenon in the obese, which is magnified by sedative agents. So far, there is a lack of large studies assessing the benefits of noninvasive positive pressure in the critically ill obese population after prolonged ventilation. Preferably, the subject should be positioned with the upper part of the body upright (sitting position), in order to improve lung volumes by releasing the pressure caused by abdominal content.[65]

When weaning a patient from mechanical ventilation, it would be ideal to determine the work of breathing, which indicates the level of effort, the patient is required to produce during spontaneous ventilation. Good clinical practice suggests gradually decreasing the pressure support while monitoring variables such as the tidal volume and the respiratory rate. In this context, a spontaneous breathing trial through a T piece (the patient is still intubated but disconnected from the ventilator, and breathes through a system that provides a humidified oxygenated flow of air) can accurately simulate the patient's ventilatory effort.[66]