2023 édition
Les patients obèses présentent une physiologie pulmonaire et des caractéristiques mécaniques spécifiques et ont un risque accru de complications pulmonaires postopératoires.
According to the World Health Organization (WHO), in 2016, more than 1.9 billion adults were overweight; of these, over 650 million were obese. In the same year, 41 million children under five and over 340 million 5 to 19 years old were overweight or obese. In addition, a recent report shows a prevalence of obesity in 32% of males and 34% of females in the U.S.
Obesity leads to anatomical and physiological peculiarities that can use threatening complications under general anesthesia, thus worsening the results of an otherwise successful surgery. As an increasingly large proportion of obese people will need general anesthesia at least once, obesity is a significant challenge for anesthetists.
How the anatomy and physiology of obesity affect intraoperative ventilation
The additional fat present in cases of obesity increases the thoracic and intra-abdominal pressures. The latter hinders diaphragm excursions and leads to stiffening of the thoracic wall, decreased lung volume due to compression atelectasis, and reduced chest wall and lung compliance. Compression atelectasis occurs when the local pleural pressure is higher than the airway pressure in the alveoli. The high abdominal pressure may also increase the pressure on the lower vena cava and thus shift blood from the abdomen to the thorax, further increasing pressure on the alveoli. Overall, atelectasis can be detected during general anesthesia in up to 90% of all patients. Even if the patient is only moderately overweight, the above mentioned circumstances can reduce functional residual capacity (FRC) and, during mechanical ventilation, reduce end-expiratory lung volume (EELV). In morbidly obese patients, the FRC can be reduced by as much as 50% after induction of general anesthesia.
Additional factors that can influence intraoperative ventilation and the postoperative phase in obese patients.
Supine position in the OR.
While obese patients already have around 20% less lung volume than expected before induction of anesthesia in a prone position, their lung volume is further reduced by about 50%. Patients should therefore remain upright as long as possible, at least upright for as long as possible, or when in a post-anesthesia or intensive care unit. In a post-anesthesia care or intensive care unit, if clinically and clinically appropriate. If the type of surgery allows, all patients with a BMI > 40 kg/m2 should be kept upright for as long as possible and at least when in PACU or ICU and medically feasible, and if surgery allows, all patients with BMI > 40 kg/m2 should be ventilated in an elevated upper body position. Another way to relieve the pressure on the diaphragm and optimize intraoperative ventilation is to maintain reverse Trendelenburg positioning from the induction of anesthesia until immediately after extubation.
However, intraoperative lung protective ventilation is associated whit a reduced risk of these complications. It consists mainly of the following adaptations:
“Vt should be limited to 6 – 8 ml/kg predicted body weight (PBW), not actual body weight, as lungs do not grow whit body fat. This Vt can usually be achieved by driving pressures below 13 cm H20 in obese patients with healthy lungs. That applies especially to morbidly obese patients. In order to be able to apply the above lung protective pressures during intraoperative ventilation, the atelectasis may need to be dissolved with a recruitment manoeuvre, so the lung compliance increases or even normalizes.
Perioperative lung protective ventilation consists of low Vt, low driving pressures and also an initial RM. In obese patients, this combination can reduce the risk of postoperative pulmonary complications. An RM means that the lung is ventilated with intermittent higher plateau pressure. When properly performed, an RM can increase FRC by opening the atelectatic lung areas and thus prevent hypoxemia, improve oxygen saturation and airway compliance, as well as reduce respiratory work.
Performing an RM with the manual ventilation bag is widespread, although high pressure peaks can lead to severe hemodynamic and pulmonary side effects. Recruitment with the bag means that uncontrolled high pressures are applied because the pressure exerted with the bag cannot be controlled easily. The ventilator, on the other hand, offers full pressure control and no PEEP drop. Thus, the bag method is not recommended any longer.
A stepwise RM is a ventilator-controlled procedure that may cause less hemodynamic, inflammatory, and barotraumatic complications. To recruit the lung, and thus determine the optimal PEEP, the pressure is stepwise increased under control of the ventilation parameters and hemodynamics. The delta between inspiratory pressure and PEEP is kept constant whereby the maximum pressure must be at least equal to the alveolar opening pressure. The stepwise manoeuvre can be individually adapted to the patient if changer in compliance and hemodynamics are observed in the RM process.
Obese patients present with specific lung physiology and mechanical characteristics and have an increased risk of postoperative pulmonary complications. Intraoperatively, lung protective ventilation with a low Vt, And RMs with much higher PEEP levels than currently used are recommended. In this context, a stepwise RM has been shown to be advantageous for obese patients, provided that an individualizes patient-specific PEEP is set.
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