Atelectasis occurs in the dependent parts of the lungs of most patients who are anesthetized. Development of atelectasis is associated with decreased lung compliance, impairment of oxygenation, increased pulmonary vascular resistance, and development of lung injury. The adverse effects of atelectasis persist into the postoperative period and can impact patient recovery. This review article focuses on the causes, nature, and diagnosis of atelectasis. The authors discuss the effects and implications of atelectasis in the perioperative period and illustrate how preventive measures may impact outcome. In addition, they examine the impact of atelectasis and its prevention in acute lung injury.

Pulmonary atelectasis: a pathogenic perioperative entity.

BACKGROUND Morbidly obese patients, during anesthesia and paralysis, experience more severe impairment of respiratory mechanics and gas exchange than normal subjects. The authors hypothesized that positive end-expiratory pressure (PEEP) induces different responses in normal subjects (n = 9; body mass index 40 kg/m2). METHODS The authors measured lung volumes (helium technique), the elastances of the respiratory system, lung, and chest wall, the pressure-volume curves (occlusion technique and esophageal balloon), and the intraabdominal pressure (intrabladder catheter) at PEEP 0 and 10 cm H2O in paralyzed, anesthetized postoperative patients in the intensive care unit or operating room after abdominal surgery. RESULTS At PEEP 0 cm H2O, obese patients had lower lung volume (0.59 +/- 0.17 vs. 2.15 +/- 0.58 l [mean +/- SD], P < 0.01); higher elastances of the respiratory system (26.8 +/- 4.2 vs. 16.4 +/- 3.6 cm H2O/l, P < 0.01), lung (17.4 +/- 4.5 vs. 10.3 +/- 3.2 cm H2O/l, P < 0.01), and chest wall (9.4 +/- 3.0 vs. 6.1 +/- 1.4 cm H2O/l, P < 0.01); and higher intraabdominal pressure (18.8 +/-7.8 vs. 9.0 +/- 2.4 cm H2O, P < 0.01) than normal subjects. The arterial oxygen tension was significantly lower (110 +/- 30 vs. 218 +/- 47 mmHg, P < 0.01; inspired oxygen fraction = 50%), and the arterial carbon dioxide tension significantly higher (37.8 +/- 6.8 vs. 28.4 +/- 3.1, P < 0.01) in obese patients compared with normal subjects. Increasing PEEP to 10 cm H2O significantly reduced elastances of the respiratory system, lung, and chest wall in obese patients but not in normal subjects. The pressure-volume curves were shifted upward and to the left in obese patients but were unchanged in normal subjects. The oxygenation increased with PEEP in obese patients (from 110 +/-30 to 130 +/- 28 mmHg, P < 0.01) but was unchanged in normal subjects. The oxygenation changes were significantly correlated with alveolar recruitment (r = 0.81, P < 0.01). CONCLUSIONS During anesthesia and paralysis, PEEP improves respiratory function in morbidly obese patients but not in normal subjects.

Positive end-expiratory pressure improves respiratory function in obese but not in normal subjects during anesthesia and paralysis.

ObjectiveTo evaluate the effects of a 48-hr neuromuscular blocking agents (NMBA) infusion on gas exchange over a 120-hr time period in patients with acute respiratory distress syndrome.DesignMultiple center, prospective, controlled, and randomized trial.SettingFour adult medical or mixed medical-sur

Effect of neuromuscular blocking agents on gas exchange in patients presenting with acute respiratory distress syndrome.

The effects of different levels of positive end-expiratory pressure (PEEP) (zero to 15 cm H2O) on the static inflation volume-pressure (V-P) curve of the respiratory system and on gas exchange were studied in eight patients with the adult respiratory distress syndrome (ARDS). Alveolar recruitment with PEEP was quantified in terms of recruited volume, i.e., as difference in lung volume between PEEP and zero end-expiratory pressure (ZEEP) for the same static inflation pressure (20 cm H2O) from the V-P curves obtained at the different PEEP levels. In addition, static compliance of the respiratory system at fixed tidal volume (0.7 L) was determined at the different PEEP levels. The results suggest that: (1) in some patients with ARDS the V-P curves determined on ZEEP exhibit an upward concavity reflecting progressive alveolar recruitment with increasing inflation volume, and PEEP results in alveolar recruitment (range of recruited volume at 15 cm H2O of PEEP: 0.11 to 0.36 L); (2) in other patients with ARDS the V-P curves on ZEEP are characterized by an upward convexity, and PEEP results in a volume displacement along this curve without alveolar recruitment and with enhanced risk of barotrauma; (3) the PEEP-induced increase in arterial oxygenation is significantly correlated to the recruited volume but not to the changes in static compliance. The shape of the static inflation V-P curves on ZEEP allows the prediction of alveolar recruitment with PEEP.

Effects of Positive End-expiratory Pressure on Alveolar Recruitment and Gas Exchange in Patients with the Adult Respiratory Distress Syndrome

The Inspiratory Work of Breathing during Assisted Mechanical Ventilation

Effects of anesthesia and muscle paralysis on respiratory mechanics in normal man.

Effects of anesthesia and muscle paralysis on respiratory mechanics in normal man

The intrapulmonary distribution of inspired gas (ventilation/unit lung volume, VI), functional residual capacity (FRC), closing capacity (CC), and the slope of phase III were determined in five awake and five anesthetized-paralyzed volunteers who were in the prone position with the abdomen unsupported. After induction of anesthesia-paralysis, FRC was less in four of five subjects and CC was consistently less. At FRC there was no difference in the vertical gradient of regional lung volumes between the awake and anesthetized-paralyzed prone subjects. Also, there was no difference in VI between the two states. The normalized slope of phase III decreased consistently with induction of anesthesia-paralysis, but the vertical distribution of a 133Xe bolus inhaled from residual volume was not different between the two states. The data of the study are compatible with 1) a pattern of expansion of the respiratory system during anesthesia-paralysis and mechanical ventilation different than that during spontaneous breathing and 2) a more uniform intraregional distribution of inspired gas and/or a different sequence of emptying during anesthesia-paralysis.

Regional Intrapulmonary Gas Distribution in Awake and Anesthetized-Paralyzed Prone Man

Single-breath oxygen (SBO2) tests for individual lungs were performed in five healthy awake volunteers. In the supine position, all subjects consistently showed a closing volume (CV) for both lungs. In the lateral position, CV was demonstrated in all analyzable SBO2 tests for the dependent and in 20 of 28 for the nondependent lung. CV consistently occurred in the dependent before it occurred in the nondependent lung (asynchronous onset of CV). After CV had been reached in the nondependent lung, a mean of 77 ml of gas was still expired from the dependent lung. No changes in CV for individual lungs were demonstrated with changes in body position. Expiratory flow limitation consistently occurred first in the dependent lung (asynchronous onset of flow limitation). In the lateral position, at high lung volumes, the dependent lung achieved higher flow rates; later in expiration, the nondependent lung contributed progressively more gas to the total expirate (asynchronous emptying). In general, our findings are consistent with both the “asynchronous flow limitation” and the “airway closure” interpretations of CV. However, some of the results are not consistent with either theory.

A. D. Sessler

Papers

Effects of anesthesia and muscle paralysis on respiratory mechanics in normal man.

Effects of anesthesia and muscle paralysis on respiratory mechanics in normal man

Regional Intrapulmonary Gas Distribution in Awake and Anesthetized-Paralyzed Prone Man

Single-breath oxygen tests for individual lungs in awake man