Background: The use of 100% oxygen during induction of anesthesia may produce atelectasis. The authors investigated how different oxygen concentrations affect the formation of atelectasis and the fall in arterial oxygen saturation during apnea. Methods: Thirty-six healthy, nonsmoking women were randomized to breathe 100, 80, or 60% oxygen for 5 min during the induction of general anesthesia. Ventilation was then withheld until the oxygen saturation, assessed by pulse oximetry, decreased to 90%. Atelectasis formation was studied with computed tomography. Results: Atelectasis in a transverse scan near the diaphragm after induction of anesthesia and apnea was 9.8 +/- 5.2 cm2 (5.6 +/- 3.4% of the total lung area; mean +/- SD), 1.3 +/- 1.2 cm2 (0.6 +/- 0.7%), and 0.3 +/- 0.3 cm2 (0.2 +/- 0.2%) in the groups breathing 100, 80, and 60% oxygen, respectively (P < 0.01). The corresponding times to reach 90% oxygen saturation were 411 +/- 84, 303 +/- 59, and 213 +/- 69 s, respectively (P < 0.01). Conclusion: During routine induction of general anesthesia, 80% oxygen for oxygenation caused minimal atelectasis, but the time margin before unacceptable desaturation occurred was significantly shortened compared with 100% oxygen.

This individual patient meta-analysis of 2,127 patients ventilated under general anesthesia for surgery from 15 randomized controlled trials shows that intraoperative ventilation with low tidal volume protects against postoperative pulmonary complications, but further trials are necessary to define the role of intraoperative higher positive end–expiratory pressure to prevent postoperative pulmonary complications after major abdominal surgery. Background: Recent studies show that intraoperative mechanical ventilation using low tidal volumes (V _T) can prevent postoperative pulmonary complications (PPCs). The aim of this individual patient data meta-analysis is to evaluate the individual associations between V _T size and positive end–expiratory pressure (PEEP) level and occurrence of PPC. Methods: Randomized controlled trials comparing protective ventilation (low V _T with or without high levels of PEEP) and conventional ventilation (high V _T with low PEEP) in patients undergoing general surgery. The primary outcome was development of PPC. Predefined prognostic factors were tested using multivariate logistic regression. Results: Fifteen randomized controlled trials were included (2,127 patients). There were 97 cases of PPC in 1,118 patients (8.7%) assigned to protective ventilation and 148 cases in 1,009 patients (14.7%) assigned to conventional ventilation (adjusted relative risk, 0.64; 95% CI, 0.46 to 0.88; P < 0.01). There were 85 cases of PPC in 957 patients (8.9%) assigned to ventilation with low V _T and high PEEP levels and 63 cases in 525 patients (12%) assigned to ventilation with low V _T and low PEEP levels (adjusted relative risk, 0.93; 95% CI, 0.64 to 1.37; P = 0.72). A dose–response relationship was found between the appearance of PPC and V _T size (R ² = 0.39) but not between the appearance of PPC and PEEP level (R ² = 0.08). Conclusions: These data support the beneficial effects of ventilation with use of low V _T in patients undergoing surgery. Further trials are necessary to define the role of intraoperative higher PEEP to prevent PPC during nonopen abdominal surgery.

Background: Morbidly obese patients show impaired pulmonary function during anesthesia and paralysis, partly due to formation of atelectasis. This study analyzed the effect of general anesthesia and three different ventilatory strategies to reduce the amount of atelectasis and improve respiratory function. Methods: Thirty patients (body mass index 45 +/- 4 kg/m) scheduled for gastric bypass surgery were prospectively randomized into three groups: (1) positive end-expiratory pressure of 10 cm H2O (PEEP), (2) a recruitment maneuver with 55 cm H2O for 10 s followed by zero end-expiratory pressure, (3) a recruitment maneuver followed by PEEP. Transverse lung computerized tomography scans and blood gas analysis were recorded: awake, 5 min after induction of anesthesia and paralysis at zero end-expiratory pressure, and 5 min and 20 min after intervention. In addition, spiral computerized tomography scans were performed at two occasions in 23 of the patients. Results: After induction of anesthesia, atelectasis increased from 1 +/- 0.5% to 11 +/- 6% of total lung volume (P < 0.0001). End-expiratory lung volume decreased from 1,387 +/- 581 ml to 697 +/- 157 ml (P = 0.0014). A recruitment maneuver + PEEP reduced atelectasis to 3 +/- 4% (P = 0.0002), increased end-expiratory lung volume and increased Pao2/Fio2 from 266 +/- 70 mmHg to 412 +/- 99 mmHg (P < 0.0001). PEEP alone did not reduce the amount of atelectasis or improve oxygenation. A recruitment maneuver + zero end-expiratory pressure had a transient positive effect on respiratory function. All values are presented as mean +/- SD. Conclusions: A recruitment maneuver followed by PEEP reduced atelectasis and improved oxygenation in morbidly obese patients, whereas PEEP or a recruitment maneuver alone did not.

To evaluate whether the shape of the airway pressure-time (Paw-t) curve during constant flow inflation corresponds to radiologic evidence of tidal recruitment or tidal hyperinflation in an experimental model of acute lung injury. Prospective randomized laboratory animal investigation. Department of Clinical Physiology, University of Uppsala, Sweden. Anesthetized, paralyzed, and mechanically ventilated pigs. Acute lung injury was induced by lung lavage. During constant inspiratory flow, the Paw-t curve was fitted to a power equation: airway pressure =a · timeb+c,where coefficient b (stress index) describes the shape of the curve:b = 1, straight curve; b < 1, progressive increase in slope; and b > 1, progressive decrease in slope. Tidal volume (Vt) was 6 mL/kg, and positive end-expiratory pressure was set to obtain a b value between 0.9 and 1.1 before (b = 1) and after (b = 1 after recruiting maneuver) application of a recruiting maneuver. Positive end-expiratory pressure was decreased and Vt increased to obtain 0.9 >b > 0.8 and 0.8 >b > 0.6, whereas positive end-expiratory pressure and Vt were both increased to obtain 1.3 >b > 1.1 and 1.5 >b > 1.3. Experimental conditions sequence was random. Pulmonary computed tomography was obtained during end-expiratory and end-inspiratory occlusions. Tidal recruitment was quantified as nonaerated (between −100 and +100 Hounsfield units) lung area at end-expiration minus end-inspiration. Tidal hyperinflation was quantified as hyperinflated (between −900 and −1000 Hounsfield units) lung area at end-inspiration minus end-expiration. Computed tomography images showed that tidal recruitment and tidal hyperinflation corresponded to b < 1 and b > 1, respectively. Stress index values and tidal recruitment and tidal hyperinflation values were significantly correlated (R2 = .917 and R2 = .911, p < .0001, respectively). Shape of the Paw-t curve detects tidal recruitment and tidal hyperinflation.

We tested whether the continuous monitoring of dynamic compliance could become a useful bedside tool for detecting the beginning of collapse of a fully recruited lung. Prospective laboratory animal investigation. Clinical physiology research laboratory, University of Uppsala, Sweden. Eight pigs submitted to repeated lung lavages. Lung recruitment maneuver, the effect of which was confirmed by predefined oxygenation, lung mechanics, and computed tomography scan criteria, was followed by a positive end-expiratory pressure (PEEP) reduction trial in a volume control mode with a tidal volume of 6 mL/kg. Every 10 mins, PEEP was reduced in steps of 2 cm H2O starting from 24 cm H2O. During PEEP reduction, lung collapse was defined by the maximum dynamic compliance value after which a first measurable decrease occurred. Open lung PEEP according to dynamic compliance was then defined as the level of PEEP before the point of collapse. This value was compared with oxygenation (Pao2) and CT scans. Pao2 and dynamic compliance were monitored continuously, whereas computed tomography scans were obtained at the end of each pressure step. Collapse defined by dynamic compliance occurred at a PEEP of 14 cm H2O. This level coincided with the oxygenation-based collapse point when also shunt started to increase and occurred one step before the percentage of nonaerated tissue on the computed tomography exceeded 5%. Open lung PEEP was thus at 16 cm H2O, the level at which oxygenation and computed tomography scan confirmed a fully open, not yet collapsed lung condition. In this experimental model, the continuous monitoring of dynamic compliance identified the beginning of collapse after lung recruitment. These findings were confirmed by oxygenation and computed tomography scans. This method might become a valuable bedside tool for identifying the level of PEEP that prevents end-expiratory collapse.