Ventilatory management during routine general anaesthesia.

Intraoperative hypoxaemia and postoperative respiratory complications remain the challenges of modern anaesthetic practice. Anaesthesia causes both depression of respiratory centres and profound changes of respiratory mechanics. Most anaesthetized patients consequently require mechanical ventilation and supplemental oxygen. Recent data suggest that intraoperative respiratory management of a patient can affect postoperative outcome. In this review, we briefly describe the mechanisms responsible for the impairment of intraoperative gas exchange and provide guidelines to prevent or manage hypoxaemia. Moreover, we discuss several aspects of mechanical ventilation that can be employed to improve patients' outcome.

Pressure-controlled ventilation does not improve gas exchange in morbidly obese patients undergoing abdominal surgery.

BACKGROUND: Morbid obesity results in marked respiratory pathophysiologic changes that may lead to impaired intraoperative gas exchange. The decelerating inspiratory flow and constant inspiratory airway pressure resulting from pressure-controlled ventilation (PCV) may be more adapted to these changes and improve gas exchanges compared with volume-controlled ventilation (VCV). METHODS: Forty morbidly obese patients scheduled for gastric bypass were included in this study. Total intravenous anesthesia was given using the target-controlled infusion technique. During the first intraoperative hour, VCV was used and the tidal volume was adjusted to keep end-tidal PCO(2) around 35 mmHg. After 1 h, patients were randomly allocated to 30-min VCV followed by 30-min PCV or the opposite sequence using a Siemens Servo 300. FiO(2) was 0.6. During PCV, airway pressure was adjusted to provide the same tidal volume as during VCV. Arterial blood was sampled for gas analysis every 15 min. Ventilatory parameters were also recorded. RESULTS: Peak inspiratory airway pressures were significantly lower during PCV than during VCV (P < 0.0001). The other ventilatory parameters were similar during the two periods of ventilation. PaO(2) and PaCO(2) were not significantly different during PCV and VCV. CONCLUSION: PCV does not improve gas exchange in morbidly obese patients undergoing gastric bypass compared to VCV.

Consequences of under- and over-humidification.

Respiratory mucosal and lung structures and functions may be severely impaired in mechanically ventilated patients when delivered gases are not adequately conditioned. Although under- and over-humidification of respiratory gases have not been defined clearly, a safe range of temperature and humidity may be suggested. During mechanical ventilation, gas entering the trachea should reach at least physiologic conditions (32 degrees C-34 degrees C and 100%relative humidity) to keep the ISB at its normal location. Clinicians must keep in mind that relative humidity is more important than absolute humidity: the warmer the gas, the higher the risk of tracheal mucosa dehydration and proximal airway obstruction. Practical assessment of the adequacy of the humidification system in use is not easy. The consistency (thin, moderate, or thick) of the patient's sputum should be evaluated regularly [47]. Full saturation of inspiratory gases is likely when water condensation is observed in the flex tube [91,92]. Nevertheless, no clinical parameter is accurate enough to detect all the effects of inadequate conditioning [45]. When mechanical ventilation is extended beyond several days, adequate conditioning of respiratory gases becomes increasingly crucial to prevent retention of secretions and to maximize mucociliary function; a requirement that respiratory gases reach at least physiologic conditions is appropriate.