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 the Periinduction Period

Breathing at low respiratory volumes impacts gas exchange, in particular oxygenation, which worsens as expiratory reserve volume approaches the residual volume.[21] Indeed, an impaired expiratory capacity seems to be the primary determinant of a decreased ventilation/perfusion ratio leading to shunting and hypoxemia. Redistribution of ventilation toward the apices with the bulk of perfusion delivered to the bases has been observed in spontaneously breathing obese patients at the end of exhalation, especially in those whose expiratory reserve volume was reduced less than 300 ml.[22] The hyperperfusion of poorly ventilated alveolar units in the most dependent parts of the lungs could explain the increased alveolar to the arterial ratio for oxygen and the consequent hypoxemia in otherwise healthy obese individuals. Of note, the decreased ventilation/perfusion ratio improves after weight loss after bariatric surgery, probably reflecting a restoration in end-expiratory volumes.[10]

Moreover, obesity may be associated with hypoventilation, and hence hypercapnia.[23] As a result, the patient with obesity may present to the preinduction period with blood gases values already altered. In addition, the mass loading by cervical fat facilitates the collapse of the upper airways,[24] and a high body mass index is a predictive factor for difficult mask ventilation.[25] Last, but not least important, the decrease in functional respiratory capacity translates into a lower volume available for denitrogenation (time to safe apnea), decreasing the safe apnea time with faster desaturation during laryngoscopy, similar to what happens in children.[26] In this setting, the importance of providing adequate oxygenation before intubation is emphasized. When technically feasible, the best way to deliver oxygen to the patient with obesity during elective intubation is by providing pressure support and PEEP utilizing noninvasive positive pressure ventilation.[27] Noninvasive positive pressure ventilation is particularly helpful since it recruits alveolar units closed at low values of FRC and prevents atelectasis after induction of general anesthesia, an occurrence particularly relevant in the obese.[28] By maintaining alveolar recruitment, positive pressure at end inspiration increases the nonhypoxic apnea time during the induction maneuvers.[29] It has been suggested to keep a 25° head position while delivering preoxygenation.[30] The combination of upright positions and positive airway pressure should be effective in improving the expiratory volume available for denitrogenation.

There is no consensus about the proper fraction of oxygen to deliver. A high percentage of inspired oxygen (greater than 80%) would denitrogenate the functional residual capacity faster and more efficiently but at the cost of reabsorption atelectasis formation during the intubation time, with consequent rebound hypoxemia.[31]

On the other hand, lower inspired oxygen (as low as 60%) prevents atelectasis but is associated with a shorter safe apnea time.[31] The problem of atelectasis after preoxygenation is accentuated in patients with obesity since the mechanism is reabsorption in alveoli upstream from a closed airway, and closed airways are at the center of obese respiratory pathophysiology, as assessed in the preceding paragraph (see also Figure 2B). As noted above, however, this could yield a very short time for laryngoscopy before desaturation starts. A good practical compromise would be to target inspired oxygen of 80%, to limit atelectasis formation while not shortening safe apnea time too much.

Contrasting data exist about laryngoscopy in the population with obesity. The incidence of difficult airway management appears to be higher in the intensive care unit,[32] most likely because of the urgency of most of the cases, while during elective surgery, the body mass index is a poor indicator of difficult endotracheal intubation.[25] However, as the concept of central obesity is a predictor of comorbidities more accurate than the body mass index itself, so the distribution of cervical fat should be taken into consideration when evaluating a possible difficult airway. In this regard, an index such as a neck circumference greater than 42 cm should be complementary to the body mass index.[33] When attempting a direct laryngoscopy, cushions and blankets should be put under the patient's head and neck, to achieve a "ramped position."[34] Alternatively, the head can be elevated between 25° and 40° by tilting the bariatric table. These approaches facilitate the alignment of oral, pharyngeal, and laryngeal axes, simplifying laryngoscopy. If facing coexisting predictors of both difficult intubation and mask ventilation (body mass index greater than 50 kg/m2, neck circumference greater than 42 cm, male sex, age greater than 45 y, poor dentition, a thyromental distance of less than 6 cm, the presence of a beard, and so forth), video laryngoscopy or awake fiberoptic intubation should be considered. Table 1 summarizes maneuvers that can help improve the management of the obese patient before induction.