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.

Abstract and Introduction

Introduction

Obesity is a pathologic increase in the body adipose tissue that is associated with an augmented incidence of chronic health-threatening conditions, such as diabetes, cardiovascular diseases, and cancer.^[1] In 2015, 12% of the world adult population and 5% of the world pediatric population were obese, with the highest prevalence among women aged 60 to 64 yr living in high-income countries.^[2] Since 1980, the incidence of obesity has globally increased across all age classes and sociodemographic levels.^[2] The higher rates of this increase were observed among children. In the United States, one of the countries most affected by the "obesity pandemic," the prevalence is approaching 40% among adults, 20% among adolescents, and 14% among children, and it is higher in women, Hispanic whites, and black Americans.^[3]

In the adult, obesity is classically defined as a body mass index greater than 30 kg/m² (normal range: 20 to 25 kg/m², with subjects between 26 and 30 considered overweight). Body mass index is the weight (expressed in kilograms) divided by the square of height (expressed in meters). The body mass index is easy to use, and high values have been shown to correlate with an increased incidence of comorbidities.^[1] However, there is only an indirect correlation between body mass index and total body fat, and this correlation is variable among different ethnicities.^[4] Body mass index expresses an excess in body weight, and factors other than the overall body fat, such as the muscular, bone, and connective tissue mass, can influence weight.

Moreover, the distribution of the excessive adipose tissue, as well as its absolute amount, should be considered when evaluating the detrimental effects of obesity. For example, the amount and distribution of body fat show some sex-related differences: men have less adipose tissue than women for the same body mass index,^[5] but are more likely to be affected by central obesity, where the adipose tissue accumulates around visceral organs, particularly in the abdominal cavity, whereas women follow more often a gynoid pattern, with fat accumulation around the hips and the proximal extremities.^[6] The central subtype has been linked to a higher cardiovascular risk than gynoid subtype.^[7] In this context, complementary tools to classify obesity could help to stratify the clinical risk. For example, waist circumference has been pointed out as a marker of central obesity, with higher values predicting a greater cardiovascular risk for the same body mass index.^[8] This concept would be particularly useful to the field of anesthesia since what impairs respiratory physiology is the visceral fat pushing against the diaphragmatic muscle, causing a reduction in expiratory reserve and decreased ventilation/perfusion ratio^[9,10] (see Figure 1 and following paragraph). However, except for a few retrospective studies underlying the contribution of waist circumference, rather than absolute body mass index, to worsened perioperative outcomes,^[11,12] current literature in anesthesia and intensive care still rely on body mass index to identify and categorize obesity.^[13] This focused review will discuss how abdominal fat influences airway management and the mechanics of respiration during spontaneous breathing and artificial ventilation. On the other hand, specific topics such as use of neuromuscular blocking agents, extracorporeal lung support, and tracheostomy will not be covered.

(Enlarge Image)

Figure 1.

Changes induced by central obesity during spontaneous ventilation in upright position. Differences in classical lung volumes between the lean and the obese patient. Expiratory reserve volume (ERV) is greatly reduced in the obese, leading to a reduction of functional residual capacity (FRC) and total lung capacity (TLC). Residual volume (RV) is unchanged. Tidal volume (TV) and inspiratory capacity (IC) are only slightly reduced. The main mechanism for this pattern is the cephalic displacement of the diaphragm by the abdominal content (thick red arrow on the "obese" side), which leads to an increase in pleural pressure (high-density red spots, as compared to scarce red spots on the "lean/gynoid obesity" side). *Intended as a pattern where adipose tissue distributes mainly around the hips and the proximal extremities, whereas abdominal fat is relatively lower. EELV, end-expiratory lung volume; IRV, inspiratory reserve volume; VC, vital capacity; WOB, work of breathing.

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Next Section

Table 1. Management during Preinduction Period
Pathologic Mechanisms	Consequence	Phase	Interventions
(1) Preexisting hypoxemia due to ventilation-perfusion mismatch (2) ↓FRC →↓EELV available for denitrogenation (3) Collapse of the upper airways	Rapid desaturation during even short periods of apnea	Preinduction	• Positioning in reverse Trendelenburg
			• Use of noninvasive positive pressure ventilation while delivering preoxygenation
			• Use of high fractions of inspired oxygen (between 85 and 100%; 100% if difficult laryngoscopy predicted)
			• Monitor for end-tidal oxygen >80% or end-tidal nitrogen <5%
		Ventilation after induction	Consider obese patients as full stomach. Rapid sequence intubation with succinylcholine or high-dose rocuronium allows fast securing of the airway avoiding difficult mask ventilation. Sellick maneuver (cricoid pressure to occlude the esophagus) can be applied.
		Intubation	• Return in supine position but with the patient previously fixed in ramp position or attempt laryngoscopy directly in reverse Trendelenburg
			• When available and if confident with the technique, use directly videolaryngoscopy
			• Laryngeal masks as a rescue device should always be ready for use
			• When coexisting factors of difficult intubation, consider awake intubation with flexible fibroscopy

EELV, end-expiratory lung volume; FRC, functional residual capacity.

Table 2. General Rules and Recommended Settings
Recommended Settings in Mechanically Ventilated Obese Patients in ICU • Mode: PC or VolC. • Tidal volume: 6 ml/kg PBW (4–8 ml/kg PBW). • Inspiratory time: 0.6–1.0 s. • Plateau pressure: <28 cm H₂O, or end-inspiratory transpulmonary pressure ≤20 cm H₂O as measured with esophageal manometry. • Driving pressure ≤15 cm H₂O. • Minute volume 10 l/min. • PEEP setting in the context of hypoxemia: perform lung recruitment maneuvers and decremental PEEP titration as described by Kacmarek et al.⁵⁵ • Monitor transpulmonary pressure. End-expiratory transpulmonary pressure should be slightly positive at the PEEP level coinciding with best compliance. • Set FIO₂ to maintain Spo₂ ≥88%. • Keep head elevated 30°. • Noninvasive ventilation up to 48 h after extubation. Recommended Settings in Mechanically Ventilated Obese Patients in the Operating Room • Mode: PC or VC. Note: with Trendelenburg position and pneumoperitoneum PC could result in hypoventilation in airway closure present. In this setting VC preferable. • Tidal volume: 6 ml/kg PBW (4–8 ml/kg PBW). • Inspiratory time: 0.6–1.0 s. • Recruitment maneuvers⁵⁵ after any event that could cause atelectasis (example: disconnection from the circuit). • If refractory hypoxemia: perform recruitment maneuver and decremental PEEP titration as described by Kacmarek et al. ⁵⁵ • Extubate with the head elevated at 30°. • NIV after extubation, up to 48 h after, depending on patient condition.

FIO₂, fraction of inspired oxygen; ICU, intensive care unit; NIV, noninvasive ventilation; PBW, predicted body weight (PBW_MEN = 50.0 + 0.905 ([height in cm] – 152.4); PBW_WOMEN = 45.5 = 0.905 ([height in cm] – 152.4); PC, pressure control; PEEP, positive end-expiratory pressure; Spo₂, oxygen saturation measured by pulse oximetry; VolC, volume control.