Brian K. Parker, MD, MS; Sara Manning, MD; Michael E. Winters, MD, MBA

Disclosures

Western J Emerg Med. 2019;20(2):323-330. 

In This Article

Mechanical Ventilation

Initiation of mechanical ventilation in the intubated obese ED patient can be challenging. Improper ventilator settings can lead rapidly to respiratory or hemodynamic deterioration and increased morbidity and mortality. Similar to the mechanical ventilation of non-obese patients, important ventilator settings for the obese patient include ventilator mode, respiratory rate, positive end-expiratory pressure (PEEP), and, in volume-controlled modes, tidal volume.

The two most common modes of mechanical ventilation used in the obese patient are volume-controlled ventilation (VCV) and pressure-controlled ventilation (PCV).[36] To date, the superiority of one mode over the other has not been demonstrated in the literature.[13] Notwithstanding, some clinicians prefer PCV, as the decelerating waveform may improve distribution of airflow to the alveoli.[13]

The benefits of a low-tidal-volume (6-8 milliliters per kilogram [ml/kg]) ventilation strategy in patients with acute respiratory distress syndrome (ARDS) have been well established.[37,38] In recent years, the use of low-tidal-volume ventilation has also been recommended for patients without ARDS.[39–41] Importantly, the tidal volume must be calculated using ideal body weight rather than total body weight. This is especially important for the intubated obese patient, for whom the use of total body weight to determine the tidal volume can lead to injurious lung volumes, barotrauma, and ventilator-induced lung injury.

As previously discussed, obese patients produce excessive amounts of carbon dioxide due to increased metabolic demand, increased oxygen consumption, and increased WOB.[11–14] As a result, they adopt a rapid, shallow breathing pattern and have a normal respiratory rate that ranges from 15–21 breaths per minute.[13,14] When setting the ventilator, it is important to account for this altered physiology and initially set a higher respiratory rate than for the non-obese patient.[13]

Obese patients demonstrate improved respiratory mechanics and alveolar recruitment when provided with PEEP.[1,13,42] PEEP reverses airflow limitations and helps to prevent alveolar derecruitment caused by the decrease in FRC.[1,13] Importantly, the optimal level of PEEP in ventilated obese patients remains uncertain.[13] They might benefit from a higher initial PEEP setting (i.e., 10 cm H2O) in contrast to non-obese patients, who are commonly started on lower levels of PEEP (i.e., 5 cm H2O).[13,43,44] The initial PEEP setting in the individual obese patient should also take into account the anticipated hemodynamic effects when PEEP exceeds extant intrathoracic pressure, including decreases in venous return, right ventricular output, and pulmonary perfusion. Expiratory flow limitation observed in the obese can result in an auto-PEEP phenomenon. In that event, extrinsic PEEP should be set at two-thirds intrinsic PEEP.[13]

Finally, the ventilated obese patient should be placed in a reverse Trendelenburg or sitting position.[1] Similar to optimal patient positioning for preoxygenation and RSI, the reverse Trendelenburg or sitting position reduces intrathoracic pressure, reduces atelectasis, improves V/Q mismatch, decreases the incidence of hypoxemia, and may improve the laryngoscopic view.[27,28]

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