Abstract and Introduction
Abstract
Background Pathophysiological changes due to obesity may complicate mechanical ventilation during general anaesthesia. The ideal ventilation strategy is expected to optimize gas exchange and pulmonary mechanics and to reduce the risk of respiratory complications.
Methods Systematic search (databases, bibliographies, to March 2012, all languages) was performed for randomized trials testing intraoperative ventilation strategies in obese patients (BMI ≥30 kg m), and reporting on gas exchange, pulmonary mechanics, or pulmonary complications. Meta-analyses were performed when data from at least three studies or 100 patients could be combined.
Results Thirteen studies (505 obese surgical patients) reported on a variety of ventilation strategies: pressure- or volume-controlled ventilation (PCV, VCV), various tidal volumes, and different PEEP or recruitment manoeuvres (RM), and combinations thereof. Definitions and reporting of endpoints were inconsistent. In five trials (182 patients), RM added to PEEP compared with PEEP alone improved intraoperative PαO2/FIO2 ratio [weighted mean difference (WMD), 16.2 kPa; 95% confidence interval (CI), 8.0–24.4] and increased respiratory system compliance (WMD, 14 ml cm H2O; 95% CI, 8–20). Arterial pressure remained unchanged. In four trials (100 patients) comparing PCV with VCV, there was no difference in PαO2/FIO2 ratio, tidal volume, or arterial pressure. Comparison of further ventilation strategies or combination of other outcomes was not feasible. Data on postoperative complications were seldom reported.
Conclusions The ideal intraoperative ventilation strategy in obese patients remains obscure. There is some evidence that RM added to PEEP compared with PEEP alone improves intraoperative oxygenation and compliance without adverse effects. There is no evidence of any difference between PCV and VCV.
Introduction
The number of obese patients undergoing surgery, either bariatric or non-bariatric, is steadily increasing. These patients have a priori healthy lungs. However, the pathophysiological changes induced by obesity make these patients prone to perioperative complications, such as hypoxaemia, hypercapnia, and atelectasis. Immediately after the induction of general anaesthesia, atelectasis develops, leading to a reduction in both ventilation–perfusion ratio and pulmonary compliance, even in non-obese patients. Obesity is characterized by several alterations in the mechanics of the respiratory system that tend to further exaggerate impairment of gas exchange. It has been demonstrated that in anaesthetized patients, arterial partial pressure of oxygen (PαO2) is inversely related to BMI. Finally, intraoperative respiratory changes may extend to the postoperative period and may subsequently necessitate the use of supplementary oxygen. It may also delay discharge from the post-anaesthesia care unit, increase the need for respiratory physiotherapy or non-invasive ventilation, and also increase the probability of intensive care unit admission. Furthermore, it has been demonstrated that obesity is a risk factor for postoperative tracheal re-intubation, morbidity, and mortality.
Several trials have tested different intraoperative ventilation strategies in these patients, for instance, various ventilation modes, PEEP, or recruitment manoeuvres (RM) which is the application of a sustained increase in positive pressure to the airway in order to reopen collapsed alveoli. However, the ideal ventilation strategy in obese patients undergoing surgery under general anaesthesia has not yet been established. This ventilation strategy would be expected to optimize gas exchange and pulmonary mechanics, and to minimize the risk of postoperative respiratory complications. We have performed a systematic review and meta-analysis to determine the impact of different intraoperative ventilation strategies on these endpoints in obese patients undergoing surgery.