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Objectives

  1. Describe the obesity epidemic in the United States.

  2. Describe the pulmonary pathophysiologic changes that occur in healthy obese patients.

  3. Describe the relationship between body mass and lung size in the obese patient as applies to selection of tidal volume.

  4. Discuss the use of lung recruitment maneuvers and decremental positive end-expiratory pressure (PEEP) in obese patients.

  5. Discuss the limitations of lung protective ventilation in the obese patient.

  6. Discuss the ventilator liberation process for the obese patient.

  7. Discuss the use of continuous positive airway pressure (CPAP) and noninvasive ventilation (NIV) for obese patients post extubation.

Introduction

Normal body mass index (BMI) is 18.5 to 24.9 kg/m2. A BMI greater than 25 kg/m2 is overweight and a BMI greater than 30 kg/m2 is obese (Table 27-1). Obesity is epidemic in the United States, as 38% of the population is obese and 8% has a BMI greater than 40 kg/m2. Of concern when defining approaches to ventilatory support for this growing population is that almost every study protocol for ventilator support and liberation has excluded patient with a BMI greater than 30 to 35 kg/m2. Thus, evidence for ventilator management of these patients is limited.

Table 27-1Classification of Obesity by BMI

Overview

The BMI of obese patients has a marked effect on their respiratory system even when those patients are healthy. The larger the subject, the lower the lung volume, with the primary volume affected being the expiratory reserve volume. As a result, positional atelectasis and airway closure is common even when patients are healthy. Most of these patients have sleep apnea and require as much as 20 cm H2O nocturnal CPAP. It is most likely that the high levels of CPAP are needed not just to overcome their upper airway obstruction but to prevent atelectasis and airway closure during sleep. The BMI increases metabolic rate, resulting in increased oxygen consumption and carbon dioxide production. These patients may also have obesity hypoventilation syndrome (OHS), contributing to hypercapnia and the need for noninvasive ventilation (NIV) when asleep. Patients with OHS might require high levels of both expiratory and inspiratory pressures with NIV.

In the critically ill patient, these pathophysiologic changes markedly alter the position of the respiratory system pressure-volume (PV) curve. The PV curve is shifted downward and to the right (Figure 27-1). This results in a reduced functional residual capacity. The position of the PV ...

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