Obesity has become a major public health problem, with recent data demonstrating that about a third of the US population is obese [1]. The economic impact of obesity is significant because of the associated comorbidities and the increased health-care utilization of obese subjects [2]. This increasing proportion of obesity among the overall population is reflected by the high proportion of obese subjects who are admitted to intensive care units (ICUs). Some series have estimated that 18–25 % of critically ill subjects are obese [3, 4]. In this particular population, the most common reason for admission to an ICU is respiratory failure, and up to 55 % of obese subjects who are admitted to an ICU require mechanical ventilatory support [5, 6]. This emphasizes the importance of understanding the mechanisms of respiratory failure, ventilatory strategies, and the approach to weaning and extubation to decrease the risk for reintubation and improve the outcomes in critically ill obese patients. This last aspect is the focus of this chapter.

Obese subjects have an increased oxygen demand as a consequence of the increased body mass from excess adipose tissue [7]. Meeting this high oxygen demand is challenging because obesity affects the respiratory system at different levels (lung mechanics, lung ventilation/perfusion, respiratory muscles, and upper airways obstruction) [8, 9]. Lung mechanics and lung volumes are significantly affected by obesity. There is consensus that lung volumes are significantly reduced in obese individuals, particularly functional residual capacity and expiratory reserve volume, leading to a restrictive ventilatory defect [10–12]. This restrictive defect is worsened by a generalized stiffness and low compliance of the respiratory system, which has been attributed to increased blood volume within the lungs (caused by the augmented cardiac output from increased oxygen demand from excess adiposity), low lung volumes, and an overall low chest wall compliance [8]. These changes are exacerbated by a high airway resistance caused by parapharyngeal fat deposition in the upper airway and low forced expiratory volume in 1 sec (FEV1) in these subjects, suggesting small airway remodeling.

Obese subjects also have a ventilation/perfusion mismatch. Although, in obesity, the lower lung zones are well perfused, because of the aforementioned reasons, there is closure of small airways in this region, making the lower zones relatively hypoventilated. This closure of the small airways in the bases causes the ventilation to be redistributed to the upper lobes, which are usually hypoperfused, resulting in a ventilation-perfusion mismatch with subsequent hypoxemia. Lastly, obese subjects have respiratory muscle insufficiency from lack of endurance and fatigue from increased work of breathing and relatively high oxygen consumption. These factors all lead to increased risk of respiratory failure resulting from low respiratory reserve.

Noninvasive ventilation (NIV) helps overcome some of the aforementioned effects of obesity by (a) “stenting” the upper airway; (b) unloading the respiratory muscles, reducing the work of breathing; and (c) increasing the lung volume, augmenting alveolar ventilation and reversing atelectasis [13].

International guidelines and consensus statements recommend liberating patients from mechanical ventilation as soon as possible while making every effort to decrease the chances for reintubation [14, 15]. The steps required to achieve these goals include assessment of readiness for extubation, performance of a spontaneous breathing trial, and consideration of NIV after extubation. These basic principles should be applied to obese critically ill subjects with some modifications.

Once the underlying triggering factor for respiratory failure has been identified and adequately treated, clinical assessments are needed to determine the patient’s readiness for discontinuation of ventilator support and extubation. Although there is no consensus about the parameters that need to be met to deem a patient “ready” for extubation, in general, there should be adequate oxygenation (SpO₂ > 90 % or PaO₂ > 60 mmHg), adequate acid base status (pH of 7.40 ± 0.05), hemodynamic stability, and adequate mental status. The first (and probably most important) consideration in obese subjects is that, because of their relatively high intrapleural pressure, the level of positive end-expiratory pressure (PEEP) that is needed to keep the alveoli recruited may be higher than the level recommended by the guidelines to deem the patient ready (usually the guidelines recommend a PEEP between 5 and 8 cmH₂O) [14]. In general, because of the altered lung mechanics in obese subjects, we advocate the measurement of esophageal pressures as a surrogate for the pleural pressure, with titration of PEEP to maintain the transpulmonary pressure gradient (pressure in the airway – pleural pressure) between 0 and 5 cm H₂O [16]. In our experience, the mean end-expiratory esophageal pressure of severely morbidly obese subjects is approximately 17 cmH₂O [17], which means that these patients should be kept on a relatively high PEEP even when considering extubation. This requirement is one of the justifications to extubate these patients to NIV. If the esophageal manometry is not available, it is reasonable to keep the PEEP between 10 and 15 cmH₂O based on previously published observational studies [8]. These data suggest that obese subjects can be deemed ready for extubation if they meet the aforementioned criteria, even if the PEEP is relatively high.

Once the subject is ready for extubation, then the next step is to proceed with a spontaneous breathing trial (SBT). Although the literature recommends the use of either a T-piece trial or a trial under PEEP and some pressure support ventilation, based on the lung mechanics of the obese subjects it is recommended that they undergo a SBT maintaining the same level of PEEP used during the acute phase with some pressure support. The criterion for failure of SBT in obese subjects does not differ from the general population and includes tachypnea, hypoxemia, tachycardia, hemodynamic instability, or signs of respiratory distress (thoracoabdominal paradox, use of accessory muscles for respiration). If none of the criteria for failure are present during the SBT, then extubation should follow. If a high level of PEEP has been maintained during the weaning process, it is reasonable to extubate to NIV. Even if the PEEP is at a low level (<8 cmH₂O), NIV should still be considered in obese subjects being liberated from invasive mechanical ventilation.